Class 10 Science Solutions Nepal

MEMORY TIPS
Although the initial cost of a fluorescent lamp is more than filament bulb its long-term use
becomes cheaper than the filament lamp.

QUESTIONS
# Give two reasons why should we use a fluorescent lamp instead of a filament bulb?

Differences between a filament lamp and a fluorescent lamp.

Filament lamp Fluorescent lamp

a. It uses tungsten filament to emit a. It uses fluorescent powder to emit

light. light.

b. It is filled with inert gas. b. It is filled with mercury vapour.

c. It converts 10% of electrical energy c. It converts 30% of electrical energy

into light energy. into light energy.

d. Its average life span is about 1000 d. Its average life span is about 3000

hours. hours.

iii. Chemical effects of current

Current brings about the chemical reaction in a substance through which it passes is
called the chemical effect of current. Water undergoes decomposition to its components
when current is passed through the acidulated water (as pure water is the bad
conductor of electricity). It is called the electrolysis of water.

ACTIVITY

1. Take a potato and insert the copper wires connected to the opposite terminals
of the battery as shown in the figure.

2. Observe the potato after one hour. You will see a greenish blue spot on the potato
around the wire connected to the positive terminal of the battery. Can you explain
this phenomenon?

iv) Magnetic effect

When an electric current is passed through a conductor, it shows the magnetism. It is
called the magnetic effect of current.

ELECTROMAGNET

An electromagnet is a temporary strong magnet which is prepared for the application of
electricity.

New Creative Science, Class 10 | 97

E lect romagnet

The strength of an electromagnet depends upon,
i. The number of turn: the greater is the number of turns per unit length of the
soft iron core, the stronger will be the strength of electromagnet.
ii. The amount of current: as the amount of current through the solenoid
increases, it will increase the strength of the electromagnet.
iii. Increasing the purity of soft iron core also increases the strength of an
electromagnet.

Uses of Electromagnet

a. Electromagnets are used in electrical devices such as an electric bell, an electric
fan, telegraph, an electric train, electric motor, generators, etc.

b. For lifting and transporting large masses of iron in the form of girdles.
c. In medical practice for removing pieces of iron from wounds.

ELECTRIC BELL

It is a device which converts the electric energy to the sound energy.

It consists of a U-shaped soft iron core around which an insulated wire (electromagnet)

is wound. A soft iron plate is adjusted Contact
in front of the electromagnet. A gong is
H ammer

kept in such a way that, the hammer can Iron strip
hit the gong.

When the switch is on, the electromagnet Switch
Battery
attracts the plate, due to which the Gong
hammer hits the gong and the bell rings.
Coil of elect romagnet

As the plate is attached to the magnet,

the circuit breaks, so, the magnetic

98 | Electricity and Magnetism

property of an electromagnet is lost and the plate returns to the original position, and
again the circuit is closed and the process repeats. In this way, we hear the bell ringing.

ELECTROMAGNETIC INDUCTION

The phenomenon of generating an electric
current in a circuit by changing the magnetic
flux linked with it is called electromagnetic
induction.

Electric generator or dynamo Conduct or

A generator or dynamo is a machine used Galvanometer
for generating electric current by converting E lect romagnetic induct ion
mechanical energy into electrical energy.

Both of them produce electricity on the
basis of Faraday’s law of electromagnetic
induction.

Generally, a dynamo produces less amount of electricity than a generator.

ACTIVITY

Wound a matchbox to make about 50 turns by using an
insulated copper wire around it. Connect the galvanometer
and move the bar magnet in and out of the box. Observe the
deflection. Is the direction of deflection in both conditions
of moving inward and outward of the magnet the same?
What happens when the magnet is inserted into the coil
and kept stationary? Now increase the speed of magnet
and observe, what happens?

Bicycle dynamo Magnet

It is used in a bicycle to light the bulb. It Iron co re
produces electricity on the basis of Faraday’s with co il
laws of electromagnetic induction. It consists
of a permanent magnet and a coil of insulated Cycl e dynamo
copper wire. The coil is kept in the magnetic
field of the magnet

When the wheel of a bicycle rotates, the
magnetic flux passing through the coil
changes and an e.m.f. is induced in it.

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QUESTIONS
# The bulb glows more brightly in the bicycle when its speed in increased. Why?

ACTIVITY

Take a cycle with a dynamo. Keep the cap of the dynamo in contact with the tyre of the
bicycle. Make the wheel of the bicycle rotate first slowly and then increase the speed.
What change will you observe in the brightness of the a bulb? How can you explain this
phenomenon?

Generator

A generator is also a device which converts mechanical energy into electrical energy.

In it, an electromagnet is used instead of a permanent magnet. When the coil of the
generator is rotated in the magnetic field, the magnetic flux passing through the coil
changes and the current is induced in it. The energy required for the rotation of the
coil is obtained from the wind, power of fuels, water or steam, etc. We can increase the
current in the following ways.

i) By increasing the number of turns of the coil.
ii) By increasing the strength of the magnetic field.
iii) By using a soft iron core.
iv) By increasing the speed of the rotation of the coil.
v) By reducing the space between the magnet and the coil.

MOTOR EFFECT AND ELECTRIC MOTOR

When a current-carrying conductor is placed in the magnetic field, it experiences a

force, except when it is placed parallel to the magnetic field. Therefore the conductor

moves. This is called the motor effect. This effect is used to construct electric motors,

refrigerators, water pumps, electric fans, washing machines, vacuum cleaners, etc.

Magnetic lines
S of f orce

Current

N
Motion
Conduct ing wire

When a current carrying conductor is kept in the magnetic field, it starts to move, which is

100 | Electricity and Magnetism

called the motor effect.
The machines which are constructed on the basis of the motor effect and convert electrical
energy into mechanical energy are called electric motors. They work on the opposite of the
dynamo and generators.

MEMORY TIPS
An electric generator can be run ‘backwards’ to become a motor by passing current into it.

INVERTER

The electric device which converts alternating
current into direct current and reserves in a battery
and similarly, converts direct current of the battery
into alternating current and runs the electrical
devices is called an inverter. It is a very useful
device for the countries which have a shortage
of hydroelectricity with load shedding. It is
used in offices and houses to run electrical devices when there is no supply of the
electricity. When alternating current comes from the supply, the inverter converts this
AC into DC and stores it in the battery. Similarly, when alternating current does not
come from the supply, the inverter converts the DC of the battery into AC and runs
the electrical devices like bulbs, tubelights, computers, etc.

CHARGER

The electrical device which is used to charge the battery and the
device which contains a battery is called a charger. One end
of the charger is connected to the battery or the device
which contains the battery and another end is connected
to the supply of the current. As electric current is passed
into the device, the battery gets charged. The device has
its own turn off system. Due to this turn off system, the

New Creative Science, Class 10 | 101

charging of the battery stops itself when the battery is fully charged. It is used to
charge mobiles, radios, etc.

TRANSFORMER

It is a device which changes a low AC voltage to a high AC
voltage or vice-versa. It works on the principle of mutual
induction. It states that, when an alternating emf is
applied to a coil, it produces an alternating magnetic
flux in it. This causes to change the magnetic flux
linked with another coil kept near to it. Due to this, an
alternating emf is induced in the second coil. The coil
in which the AC current is fed is known as the primary
coil and the other is known as the secondary coil.

In a transformer, there is a rectangular soft iron core. It is made up of laminated sheets.
Two separate coils wound to the core. One of them which is connected to the AC
source is known as the primary coil and the other from which AC voltage is taken is
called the secondary coil.

Similarly, the input voltage of a transformer is called the primary voltage and the
output voltage is called the secondary voltage.

Secondary Voltage (Vs) No. of turns in secondary coils (Ns)
Primary Voltage (Vp) = No. of turns in primary coils (Np)

i.e. Vs = Ns
Vp Np

Types of transformer

There are two types of transformers. They are i) step up transformer ii) step down
transformer.

Step up transformer

The transformer which converts low AC voltage to high AC voltage i.e. (Vp < Vs) is called
the step up transformer. It has more turns in the secondary coil than in the primary
coil. It means, Np < Ns. It is used at the power station from 23000 to 400000 volts for
transmission to long distances without the loss of energy.

A C source O utput

Step up transf ormer

102 | Electricity and Magnetism

Step down transformer

The transformer which converts high AC voltage to low AC voltage i.e. (Vp > Vs) is called the
step down transformer. It has less turns in the secondary coil than in the the primary.
It that means Np > Ns. It is used in the houses to step down the high voltage current
to low voltage up to 240 or 220V. It is also used in electric appliances to get further
smaller voltage.

MEMORY TIPS

● The iron core is made from E-shaped laminators so that the magnetic flux does not
pass through air at all and the greatest flux may be obtained for a given current.

● In every transformer, there should be at least 1000 turnings in the primary coil to be
connected to 220 v mains otherwise it gets heated due to insufficient resistance.

QUESTIONS

# Why are the cores of transformers laminated?
# When are step up and step down transformers used?
# Why are stripes of iron plates joined instead of using a single block of an iron core in a

transformer?

ACTIVITY

Go to a workshop of electrical machines and observe the structure of an electric motor
and a transformer.

Differences between step up and step down transformer.

Step up transformer Step down transformer

a. It changes low AC voltage to high AC a. It changes high AC voltage to low AC

voltage. voltage.

b. The number of turns in the secondary coil b. The number of turns of the primary coil

is more than that in a primary coil. is more than that of the secondary coil.

c. It is used in power stations to transmit c. It is used in household devices.
current to long distances.

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SOLVEDNumerical

1. Calculate the number of turns in the secondary coil if a transformer is
connected to the AC main of 220 V to produce 30V. The number of turns in
the primary coil is 1000.

Solution:
Given, primary voltage (Vp) = 220V
Secondary voltage (Vs) = 30 V
No. of turns in primary coil (Np) = 1000
No. of turns in secondary coil (Ns) = ?

We have, from formula,

Vs = Ns
Vp Np

or, 30 = Ns
220 1000

or, Ns = 136.5 = 137

∴ The no. of turns in secondary coil is 137.

SAFETY MEASURES IN USING ELECTRICITY

Electricity is an important part of our life. It is used in our house, office, factory and
industry as the main source of energy. It is used for heating and lighting purposes. It
is comfortable to use. We just have to turn on and then off switches to use electricity.
Although, we hear various accidents related to the electricity, it is due to carelessness
and lack of knowledge about the structure and function of the different kinds of
electrical devices. Therefore, the following points must be considered while using
electricity and electrical devices.

a. We must use an electric wire of high quality, appropriate amperage and it
should be insulated from its outer surface.

b. Naked wires and jointed wires must be covered with insulated materials.
c. Electric plugs, switches, fuses, sockets, etc. must be connected strongly.
d. Fault electric plugs, switches, fuses, sockets, etc. must be replaced timely.
e. Plastic shoes and globes must be worn while doing work with electricity.
f. We must use a fuse of an appropriate capacity in each house, floor and room.
g. We should not touch a naked wire with our naked hands.
h. Earthing must be done for all the heavy electrical equipments.
i. Switches and fuses must be connected with a live wire.
j. If short circuiting occurs, the main switch must be immediately switched off.
k. There should be a separate power circuit and a light circuit.

104 | Electricity and Magnetism

l. Wiring should not be done is damp places.
m. We must ensure colour code of the wires, brown/red coloured insulation is for

live wires, blue/black insulation for neutral wire and green or yellow stripped
insulation for the earth wire.

ELECTRIC POWER CONSUMPTION

It is important to know the methods to calculate the electric power consumption. The
electric power consumption in our home is measured by an electric meter. The electric
meter measures the electricity in kilowatt hour (kWh). For convenience, 1 kWh is the
energy consumed by an electrical device of power 1kw for one hour.

Energy = Power × time
∴ 1 kWh = 1 kw × 1h = 1000 w × 3600 s
∴ 1 kWh =3.6 × 106 J
The electricity consumed is calculated as,
Consumption of electricity (E) = Electric power (P) × no. of devices (N) × time taken (t)
∴E=P×N×t
To calculate the total cost of electricity consumed due to the use of electrical appliances,
We use,
Cost = Electricity consumed × rate in kWh
i.e., Cost = E.C × rate

SOLVEDNumerical

1. In a house, 12 tube lights of 40W for 4 hours, 3 televisions of 40W for 4 hours
and 2 bulbs of 100W for 2 hrs in a day and 2 electric irons for 2 hrs are used
in a week. Now calculate the cost of electricity in that house in a month if the
rate of electricity is Rs 7.

Solution:

Here, if 40 W tube lights glow for 4 hours

P = 40 = 0.04 W
1000

N = 12

t = 4 hrs

Electricity consumed (E.C.) = ?

From formula,

Electricity consumed in one day = P × N × t = 0.04 × 12 × 4 = 1.92 kWh

∴ E.C. in one month by the tube light = 1.92 × 30 = 57.6 kWh

New Creative Science, Class 10 | 105

If 3 televisions of 40W are used for 4 hours
Then,

P = 40, W = 0.04 kw, N = 3, t = 4 hrs, E.C. = ?
From formula,

E.C. (consumed) in one day = P × N × t = 0.04 × 3 × 4 = 0.48 kWh
∴ E.C (consumed) in one month = 0.48 × 30 = 14.4 kWh
If two bulbs of 100W are used for 2 hrs,

P = 100W = 0.1 kw, N = 2, t = 2hrs, E.C = ?
We have,

E.C. (consumed) in one day = P × N × t = 0.1 × 2 × 2 = 0.4 kWh
∴ E.C. (consumed) in one month = 0.4 × 30 = 12 kWh
If two irons of 750W each are used for 2 hours,

P = 750W = 0.75 W, N = 2, t = 2 hrs per week
We have,

E.C. in one week = P × N × t = 0.75 × 2 × 2 = 3 kWh
∴ E.C. in one month = 3 × 4 = 12 kWh
Thus, total E.C. consumed in that house in one month

= (57.6 + 14.4 + 12 + 12) kWh
= 96 kWh
= 96 unit
∴ Total cost = 96 × Rs. 7 = Rs 672

MODEL QUESTIONS ANSWER

1. Why are electrical appliances connected parallel in domestic circuit?

Ü Electrical appliances are connected parallel in domestic circuits so that the rests
of the electrical devices would work if any one of them gets damaged.

2. Switches and fuses should be kept in the phase wire or live wire in the
electrical circuit. Why?

Ü Fuses and switches should be kept in the phase wire so that when they blow or
kept in off state then the current may not flow through and prevents us from the
probable accidents.

3. Soft iron cores are used in electromagnets and transformers. Why?

Ü Soft iron cores are used in electromagnets because soft iron can be easily
magnetized or demagnetized. So, they can be used as good electromagnets or
temporary magnets. In transformers, it increases the magnitude of the induced
emf in the secondary coil.

106 | Electricity and Magnetism

4. Why is the iron core of a transformer laminated?
Ü If the iron core of a transformer is not laminated then as we know iron is a

conductor, so current would be induced and dissipate the power as heat. Such
current is known as eddy current. So, to reduce the eddy current and to prevent
the loss of input voltage, the iron core of a transformer is laminated by varnish.

5. Coloured wires should be used in domestic electrification. Why?
Ü Colour codes help us to identify the type of the wire i.e. whether it is live, neutral

or earth wire. Hence, we would be able to connect the appliances properly, and
it saves us from the electrical hazards. Similarly, it also helps us during the
maintenance or repairing of electrical goods and house wiring.

6. Nichrome is used as a heating element. Why?
Ü Nichrome is an alloy of 60% nickel and 40% chromium. It has high resistance

and high m.p. (at 900°C). It can produce heating effect without getting damaged.
So, it is used as a heating element.

7. An electric bulb is filled with inert gases. Why?
Ü The electric bulb is filled with inert gases to prevent the damaging of tungsten

filament due to its oxidation.

8. Earthing should be done for heavy electrical appliances. Why?
Ü Earthing should be done for heavy electrical appliances for our own safety.

When current leaks in the appliance, the current would go in the earth and we
do not get an electric shock if we touch such appliances.

9. DC cannot be used in transformers. Why?
Ü The transformer works in the principle of mutual induction in which the

magnetic flux must continuously change in the primary coil to produce current
in the secondary coil. But the magnitude and polarity of DC remains constant
which would not produce magnetic flux in primary coils. Thus, DC cannot be
used in transformers.

10. A insulated wire must be used in electromagnets. Why?
Ü Insulated wires are used in electromagnets to prevent the short-circuiting.

SUMMARY
 The devices which consume electricity to convert it into other forms are called electrical

devices or loads.
 The load may be connected in series or in parallel with each other according to the requirement.
 The increase in the number of cells in their series combination increases the brightness of the

bulb, but the brightness does not increase in case of their parallel combination.
 There are two types of circuits i) open and ii) closed.

New Creative Science, Class 10 | 107

 The current having a fixed magnitude and direction is called the Direct current (DC). It is
produced by battery, DC dynamo, DC generators, etc.

 The current whose magnitude changes continuously and whose direction changes
periodically is called the Alternating current (AC). It is produced by AC generators.

 Electricity may bring i) heating effect, ii) chemical effect, iii) lighting effect and iv) magnetic effect.
 Switches and fuses should be connected to live or phase wires.
 Fluorescent lamps are more efficient than filament lamps.
 A conductor which behaves as a magnet only when electricity is passed through it is called

electromagnet.
 Whenever there is a change in magnetic flux linked with a closed coil, an emf is induced in the

wire of the coil; the phenomenon is called electromagnetic induction.
 A generator and dynamo work on the principle of electromagnetic induction and is based on

Faraday’s laws.
 When an electric current is passed through a conductor in a magnetic field, the conductor

experiences the thrust and it moves. This effect is called the motor effect.
 The devices which convert high AC voltage to low AC voltage and vice-versa are called

transformers.
 The transformer works on the principle of mutual induction.
 There are two types of transformers i.e. step up transformer and step down transformer.

EXERCISE

1. What do you mean by
(a) Series and parallel combination of cells?
(b) Series and parallel combination of loads?

2. A combination of bulbs is so done that when one of the bulbs is decreased, the
brightness of other bulbs increases. What type of combination is it? What are the
advantages and disadvantages of this type of combination of bulbs?

3. What are the advantages of
(a) parallel combination of loads? (b) parallel combination of cells?

4. Differentiate between:
(a) AC and DC
(b) Step up transformer and step down transformer
(c) filament lamp and fluorescent lamp
(d) electric motor and generator
(e) inverter and charger

5. Give reason.
(a) Electrical appliances are connected in parallel for domestic electrification.
(b) Colour coding of wires is very beneficial to us.
(c) The switch should be kept in the phase wire.
(d) An MCB is better than a fuse.
(e) The heavy electrical appliances should be connected to the earth wire.

108 | Electricity and Magnetism

(f) The fluorescent lamp is more efficient than a filament lamp.
(g) A nichrome wire is used in a heater.
(h) A tungsten filament is used inside a filament bulb.
(i) The core of the transformer should be laminated.
(j) Soft iron cores should be used in electromagnets.
(k) Birds sit safely on high voltage transmission lines.
(l) The wires in solenoid should be insulated.
6. What is an electromagnet? How can you increase its strength?
7. Write down any five points of safety measures while using electricity.
8. What do you mean by electromagnetic induction?
9. Write down the structure and function of the inverter and charger.
10. How can we increase the current produced in a generator?
11. Write three uses of an electromagnet.
12. Describe the structure and function of the bicycle dynamo.
13. Why is a step up transformer used in power stations? And write the use of step
down transformers.
14. Describe the structure and types of the transformer.

Numerical problems

15. Calculate the capacity of a fuse needed to be put in a heater of 880 W and 220 V?

16. Find the amount of the electric bill of a hostel for 30 days, if 20 bulbs of 100W and
6 fans of 75 w were used for 5 hours a day at the rate of Rs 7 per unit.

17. In a hostel, an electric bulb of 100W for 12 hours and an electric heater of 750W
are used for 4 hours everyday. Calculate the cost of using the bulb and the heater
for 30 days if the cost of one unit of electrical energy is Rs 7.

18. A step down transformer has 440V primary voltage and 1000 turns of the primary
coil. How many turns of the secondary coil will be needed to produce 220 volts
from that transformer?

19. A step down transformer has 800 primary turns and 200 secondary turns. If the
operating voltage for the load connected to the secondary is measured to be 100
V, what is the voltage supplied to the primary cell?

20. In a step down transformer, the number of turns in the secondary coil is 2 times
less. Calculate the magnitude of the output voltage for the input 440V.

A

B GLOSSARY
C

Resistance : the property that resists the flow of electricity through a conductor
Dissipate : to waste time or money or power

Conferred : to give somebody an award

Interleaved : to put thin layers of something between things

Girdle : a long strong steel or iron beam used for building bridges and houses

Momentary : lasting for a very short time

Laminated : made by sticking several thin layers together

Accommodation : the process of adopting, adjustment



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UNIT

7 Classification of Elements

About the Scientist INTRODUCTION

Dmitri Mendeleev Elements are the building blocks of all the living and
(1834) non-living matters. As the human civilization developed,
more and more elements were discovered and finally till
Dmitri Mendeleev was born this date we have 117 elements discovered so far. As the
in 8 February 1834 in Verhnie number of elements known became larger, it was felt
Aremzyani village, near Tobolsk, necessary to have some way to organize these elements.
to Ivan Pavlovich Mendeleev and Similarly, some elements showed similar properties while
Maria Dmitrievna Mendeleeva. others were completely different from each other. So, the
scientists frequently put their efforts on the classification
In 1905, Mendeleev was elected a of elements. Therefore, they had discovered several ways.
member of the Royal Swedish Among them, the periodic table is one of the greatest
Academy of sciences. The milestones in the systematic classification of elements. This
following year Nobel committee periodic classification has greatly simplified the study of
for Chemistry recommended elements and their compounds. Although not only a single
to Swedish Academy to award scientist could be given credit for it, Russian scientist Dmitri
the Nobel Prize in chemistry Mendeleev's contribution in this field is the greatest among
for 1906 to Mendeleev for his them. He had classified 63 elements discovered at that time
discovery of the Periodic system. by making a scientific periodic table.
The chemistry section of the
Swedish Academy supported this MEMORY TIPS
recommendation. The Academy
was then supposed to approve the The process of putting different elements together into different
committee choice as it has done groups or classes according to their characteristics is known
in almost every case. as the classification of elements. Antony Lavoisier was the first
scientist to classify elements into metals and non-metals.

HISTORICAL BACKGROUND OF CLASSIFICATION OF
ELEMENTS

It was John Dalton in who proposed the 'Atomic Theory'
in 1908 AD. In this theory, he postulated that the atoms of
the same elements are alike in all aspects but the atoms of
different elements are different at least in the case of their
atomic weights. Since then, atomic weight was considered
as the important property of the atom and scientists
continually tried to relate the atomic weight with the
chemical properties of the elements.

110 | Classification of Elements

Thus in 1815, Joseph Proust suggested that all elements are built up of hydrogen atoms,
and the atomic weight of elements are the multiple of atomic weight of hydrogen.
However, this hypothesis is discarded as many elements have atomic weight in
fractional numbers.

DOBEREINER’S TRIADS

German scientist Dobereiner made a group of three elements in such a way that the
atomic weight of the central element is the arithmetic mean of the other two. This
group of three elements is called triad and they have similar properties. For example,

Element Atomic mass Characteristics

Lithium 7 * All of them are monovalent metals
Sodium 23 *They form hydroxide when dissolved in water.
Potassium 39

Here, atomic mass of sodium is arithmetic mean of Lithium and Potassium.

NEWLAND’S LAW OF OCTAVES

John Newland grouped the elements in the increasing order of their atomic weight.
In this arrangement, every 8th element showed the same properties of the starting
element like eighth notes of music. So, it is also called law of octaves.

For example,

Li Be B C N O F

Na Mg Al Si P S Cl

K Ca

In the above table, Na has the same characteristics of Li and Ca has the same
characteristics of Mg.

LOTHER MEYER’S ATOMIC VOLUME CURVE

Lothar Meyer plotted a graph between Cs
atomic volume against atomic weight.
After plotting the graph he concluded A tomic volume Rb
that the elements occupying the same
positions showed similar properties. K
Na
For example: Li, Na, K, Rb and Cs Li
occupied the peaks of the curve and they
showed similar properties.

A tomic mass

New Creative Science, Class 10 | 111

MEMORY TIPS
John Newland was awarded 'Davy Award' by the Royal society of London for his law of octaves.

QUESTIONS
# Give a brief account of the history of the classification of elements.

MENDELEEV’S PERIODIC TABLE

Although several attempts had been made for the classification of elements as
described above, Dmitri Mendeleev was the one who started classifying the elements
on the basis of the periodic law. His periodic law states that 'the physical and chemical
properties of the elements are the periodic function of their atomic weight'.

Thus, Mendeleev again considered the atomic weight of elements as their fundamental
property and on this basis, he had classified 63 elements discovered so far at that time.
This table is called Mendeleev's Periodic table.

Dmitri Mendeleev classified elements on the basis of their increasing atomic weight.
As a result, he found a table which is called Mendeleev's periodic table.

A part of Mendeleev’s periodic table is given below:

Group I Group Group Group Group V Group Group Group
H II III IV VI VII VIII

Period Li Be B C N O F Fe
1 Co
Na Mg Al Si P S Cl Ni
Period
2 K Ca 1* Ti V Cr Mn Ru Rh
Cu Zn 2* 3* As Se Br Pd
Period
3 Os lr
Pt
Period
4

Period Rb Sr Y Zr Nb Mo 4*
5 Ag Cd In Sn Sb Te l

Period Cs Ba La Hf Ta W Re
6 Au Hg Th Pb Bi Po At

Name given by Mendeleev: 1* Eka - Aluminium,

2* Eka - Boron, 3* Eka - Silicon, 4* Eka - Manganese

Characteristics of Mendeleev’s periodic table

a. In this table elements were arranged in increasing order of their atomic weight.
b. This table consists of seven horizontal rows called periods and eight vertical

columns called groups.

112 | Classification of Elements

c. Some gaps were left for the undiscovered elements like scandium, gallium,
germanium, etc. which should be similar to Boron, Aluminium and Silicon,
respectively. So, he had suggested the names of those undiscovered elements as
Eka-Baron, Eka-Aluminium and Eka-Silicon

d. In this table, he had not included noble or inert gases, as they were still not
discovered at that time.

e. The groups of the Mendeleev's periodic table were further divided into sub
groups.

Advantages or merits of Mendeleev's periodic table

a. In the history of the classification of elements, Mendeleev's periodic table was
the first scientific as well as the systematic table of elements.

b. This table made the study of elements easy, fast, systematic and more convenient.
c. In this table, there were some gaps for undiscovered elements, which showed a

pathway for other scientists to discover them.
d. It helped to correct the doubtful atomic mass of some elements like gold,

platinum, uranium, etc.

Disadvantages or demerits of Mendeleev's periodic table

a. In Mendeleev's Periodic table, hydrogen was placed along with the metals like
Li, Na, K, etc. which was not properly explained.

b. He himself had dishonoured his own law by not giving separate positions for
isotopes.

c. He could not give the correct place for lanthanides and actinides.
d. He had disobeyed his own law in some cases such as Argon (having atomic mass

39.9) was placed before potassium (having atomic mass 39.1) and Co (atomic
mass=58.9) before Ni (atomic mass 58.6)
e. Highly reactive alkali metals were grouped together with less reactive coinage
metals.
f. Similar elements like Cu and Hg were placed at different groups whereas
dissimilar elements like manganese and halogens were placed at the same
group.
g. His periodic table was unable to explain the atomic properties like valency,
reactivity, etc.

MEMORY TIPS
Isotopes have the same atomic number but they have different atomic mass. For example,
Protium, Deuterium and Tritium are the three isotopes of hydrogen.

Protium 11H is the common form of hydrogen.
Duterium 21H is also known as heavy hydrogen.
Tritium 31H is the radioactive isotope of hydrogen.

New Creative Science, Class 10 | 113

QUESTIONS
# State Mendeleev's periodic law.
# List out the advantages and disadvantages of Mendeleev's periodic table.

MODERN PERIODIC TABLE

Although Mendeleev's periodic table made history on the classification of elements.
His work also had been criticized more due to the drawbacks described before. So, in
the course of finding the solution, Henry Moseley and his friends in 1913 AD proposed
another periodic law after the continuous study on the properties of elements. This
law is known as Modern Periodic law. It states that, the physical and chemical properties
of elements are the periodic function of their atomic number.

In this way, Moseley and his friends regarded atomic numbers as the fundamental
property of elements. On the basis of this periodic law, they made a scientific table
which is known as the modern periodic table. It is also known as the long form of the
periodic table.

Properties or characteristics of modern periodic table

i) In the modern periodic table, elements are arranged in the increasing order of
their atomic number.

ii) In this periodic table there are seven horizontal rows called periods and eighteen
vertical columns called groups. The first period contains only two elements, so it
is called very a short period. The second and third periods contain eight elements
each and thus called short periods. The fourth and fifth periods contain eighteen
elements each. Hence, it is called a long periods. The sixth period contains thirty
two elements. So, it is called a very long period. The seventh period contains
twenty six elements and is not completed yet. So, it is known as the long and
incomplete period.
Similarly, groups I to VII are divided into sub-groups A and B. Group VIII has
three vertical columns. There is a vertical column for zero (0) group for noble
gases. In this way, there are eighteen vertical columns in the modern periodic
table which are called groups.

iii) The 14 rare earth elements i.e., Lanthanides (atomic number 58-71) and 14
Transuranium elements i.e., actinides (atomic number 90-103) have been placed
in separate rows at the bottom of the periodic table.

iv) Metals are kept on the left hand side and non-metals are kept on the right hand
side of the table.

v) Inert gases like He, Ne, Ar, Kr, Xe and Rn are kept in zero group at extreme right
side of the table.

114 | Classification of Elements

T hePeriodic T ableof the E lements

s-Block elements p-Block elements 1 8 ( 0 - G roup)

1 ( I A)

12

H 2 ( I I A) 25 Atomic number A lk alai metals P o rs- transitio n metals 1 3 ( I I I A) 1 4 ( I V A) 1 5 ( V A) He
A lak alline earth metals M etallo ids
Hydrogen 4 Mn Element symbol L anthanides Other no n- metals 5 67 89 1 6 H4e.(0liu0Vm3 I A) 1 7 ( V I I
1.00794 Element name A c tinides H alo g ens
Be Manganese Atomic weight Transitio n metals No b le g ases 10
3 54.938049
Beryllium B C N O F Ne
Li 9.012182
Boron Carbon Nitrogen Oxygen Fluorine Neon
Lithium 12 10.811 12.0107 14.00674 15.9994 18.9984032 20.1797
6.941
13 14 15 16 17 18
11

Na Mg Al Si Pd-Block elements (Transitional elements) S Cl Ar

Sodium Magnesium 5 ( V B) 6 ( V8 (I VB) I I97I (BV) I I IB)B) 1 0 Sulfur Chlorine Argon
22.989770 24.3050 3 ( I I I B) 4 ( I V B) ( V I I I B) 1 1 2A(6l.uI9m8Bi1n5u)m38 1 22S8i.l0ic(8o5nI5 I B3P0h)o.9s7ph3o7r6u1s 32.066 35.4527 39.948

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

K Ca Sc T iV Cr MFn e Co Ni Cu Z n Ga Ge As Se Br Kr

Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
39.0983 40.078 44.955910 47.867 50.9415 51.9961 58.933200 58.6934 63.546 65.39 78.96 79.904 83.80
54.938049 55.845 69.723 72.61 74.92160
37 38 39 40 41 42 45 46 47 48 52 53 54
43 44 49 50 51

Rb Sr Y Z r Nb Mo T c Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
85.4678 87.62 88.90585 118.710 121.760 127.60 126.90447 131.29
91.224 92.90638 95.94 (98) 101.07 102.90550 106.42 107.8682 112.411 114.818
55 56 57 82 83 84 85 86
72 73 74 75 76 77 78 79 80 81

Cs Ba La H f Ta W Re Os Ir Pt Au Hg T l Pb Bi Po At Rn

Cesium Barium Lanthanum Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury Thallium Lead Bismuth Polonium Astatine Radon
132.90545 137.327 138.9055 178.49 180.9479 183.84 186.207 190.23 192.217 195.078 196.96655 200.59 204.3833
207.2 208.98038 (209) (210) (222)
87 88 89 104 105 106 107 108 109 110 111 112 113
114 115 116 117 118

New Creative Science, Class 10 | 115 Fr Ra Ac Rf D b Sg Bh H s MDt s Rg Cn Nh F l Mc L v T s Og

Francium Radium Actinium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson

(223) (226) (227) (261) (262) (263) (262) (265) (266) (269) (272) (277) (286) (289) (290) (293) (294) (294)

58 59 60 61 62 63 64 65 66 67 68 69 70 71

Ce Pr Nd Pm Sm Eu G d Tb Dy H o Er T m Y b L

Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium
140.116 140.90765 144.24 (145) 150.36 151.964 157.25 158.92534 162.50 164.93032 174.967
167.26 168.93421 173.04
90 91 92 93 94 95 96 97 98 99 103
100 101 102

T h Pa U Np Pu Am Cm Bk Cf E s F m Md No Lr

Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium

232.0381 231.03588 238.0289 (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) (262)

f-Block elements (Inner transitional elements)

Advantages or merits of modern periodic table over Mendeleev's periodic table

i) Hydrogen has atomic number one with only one electron in its k-shell. Thus,
they kept hydrogen in the first period and IA group along with Li, Na and K.

ii) The anomalies related to isotopes have automatically been solved since all
isotopes of one element have the same atomic number.

iii) When elements are arranged based on the increasing atomic number, the wrong
position of Ar and K or Ni and Co has been corrected automatically.

iv) More reactive alkali metals are kept under IA group and less reactive coinage
metals are kept under IB group of the modern periodic table.

v) Lanthanides and actinides are placed in a separate box below the main table to
avoid the undue side wise expansion of the periodic table.

vi) The representative, transitional, inner transitional and noble elements have been
separated.

vii) The elements have also been classified into s-block, p-block, d-block and f-block
according to the orbital, in which the last electron enters. Thus, the study of
elements has become more specific.

MEMORY TIPS

The metals from which coins are made are called coinage metals such as Cu, Ag and Au.
They occupy group IB of the modern periodic table.

QUESTIONS

# List out any four advantages of the modern periodic table over Mendeleev's Periodic
table.

# State the modern periodic law.
# Define the modern periodic table.

Differences between modern periodic table and Mendeleev's periodic table

Modern Periodic table Mendeleev’s periodic table

a. In this table, elements are arranged on the a. In this table, elements are arranged on the

basis of the increasing atomic number. basis of the increasing atomic weight.

b. It has seven periods and eighteen groups. b. It has seven periods and eight groups.

c. Hydrogen, isotopes, lanthanides and c. Proper position for hydrogen, lanthanides
actinides are given proper position in this and actinides was not given.
table with a suitable reason.

d. Highly reactive alkali metals and less d. Highly reactive alkali metals and less

reactive coinage metals are kept in reactive coinage metals are kept in one

separate groups. group.

116 | Classification of Elements

Position of elements in modern periodic table

i) Position of hydrogen: Hydrogen has only one electron and one proton. Sometimes
it loses its electron to make hydrogen ion (H+) similar to the alkali metals of IA
groups and sometimes it gains one electron to make hydride ion (H–) similar to
the halogens of VII A group. So, it shows both the characters of alkali metal of IA
group and halogens of VII A group. Due to this reason, it is very difficult to keep
it at particular place. But due to one atomic number, it is kept under IA group of
the modern periodic table.

ii) Position of metals, non-metals and metalloids: In the modern periodic table,
alkali metal (IA), alkaline earth metals (II A), and IIIA groups are kept on the left
hand side. Elements of (IA) group are called reactive metals. They react with
water to give strong alkali. So, they are also called alkali metals. Elements which
are present in IIA group of the modern periodic table are called alkaline earth
metals. They occur on the earth's crust and react with water to give hydroxide. For
example, Mg, Ca, etc. They are less reactive than alkali metals. Elements of IIIA
group are also metals except boron. They are less reactive than alkali metals and
alkaline earth metals. Elements which are present in the d-block of the modern
periodic table are called transitional elements. These elements are present between
s and p blocks or between reactive metals and non-metals. Hence, they are named
transitional elements. For example; Fe, Cu, Ni, etc.

iii) Non-metals of groups VA, VIA and VIIA are kept on the right side of the modern
periodic table. The elements of VIIA group of the modern periodic table are called
reactive non-metals or halogens. They are present in sea water in the form of salt.
They need only one electron to complete octate. So, they are reactive non-metals.

iv) Position of metalloids: The elements which show partial characteristics of metals
and non-metals are called metalloids. They are present between metals and non-
metals. For example, silicon, germanium, bismuth, etc.

v) Position of inert gases: Inert gases (zero group elements) are kept on the extreme
right hand side of the periodic table along with non-metals.

vi) Position of lanthanides and actinides: The 14 elements starting from cerium (Ce58)
to lutetium (Lu71) are called lanthanides because they show similar characteristics
of lanthanum (La57). Similarly 14 elements starting from thorium (Th90) to
lawrencium (Lr103) are called actinides because they show similar characteristics
of actinium (Ac89). These 28 elements (lanthanides and actinides) are kept below
the main table in a separate box as they have different characteristics than other
elements of the main periodic table.

New Creative Science, Class 10 | 117

PERIODS AND GROUPS

Periods

The horizontal rows of the periodic table where elements having gradual change in atomic number
and characteristics are kept are called periods. In the modern periodic table there are a total
of seven periods. They have different atomic numbers and a different atomic mass.

S.N. Period Number of elements Types of period

1. First 2 Very short period
2. Second
3. Third 8 Short period
4. Fourth 8 Short period
5. Fifth 18 Long period
6. Sixth 18 Long period
7. Seventh 32 Very long period
26 Very long but incomplete period

Characteristics of Periods

i) They are horizontal rows of the periodic table.
ii) These elements have gradual change in the atomic number as we move in a

particular period.
iii) These elements have gradual change in characteristics as we move in a particular

period.
iv) In a period, the atomic size decreases as we move from left to right in a particular

period.
v) In a period, valency first increases up to 4 and then decreases up to 0 as we move

from left to right in a particular period.

Groups

Vertical columns of the periodic table where elements having almost similar characteristics
are kept are called groups. There are a total of nine groups (recently 18 groups) in the
modern periodic table but each group is further divided into two sub-groups A and B
except the zero group. In the eighth group, there are three vertical columns but they
are not denoted by A, B and C. Nowadays, all these total 18 vertical columns are also
known as 18 groups.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

IA IIA IIIB IVB VB VIB VIIB VIIIB VIIIB VIIIB IB IIB IIIA IVA VA VIA VIIAVIIIA/0

Characteristics of Groups

i) They are the vertical column of the periodic table.
ii) All the elements in a particular group have almost the same characteristics.

118 | Classification of Elements

iii) All the elements in a particular group have the same valence electrons.
iv) The atomic size increases as we move from top to bottom in a particular group.
v) The valency remains the same for all elements in a particular group.

SUB-SHELL OR ORBITALS

According to Bohr and Burry, electrons are distributed by 2n2 formula where ‘n’ is
the number of shells. On the basis of this formula, duplet means two electrons in the
K-shell (for example helium) and octet means eight electrons in the outermost shell (for
example Ne, Ar). But the electronic configuration of all elements cannot be explained
by this formula. So, the concept of sub- shells or orbitals is assumed. According to this
concept, within the main shell, there are one or more sub-shells which are symbolized
by s, p, d and f. The shells with their sub-shells are given below.

Shell Sub shells or orbitals
K (1) s
L (2) s and p
M (3) s, p and d
N (4) s, p, d and f

The "s" sub-shell may contain a maximum two electrons, the "p" sub-shell may contain
maximum six electrons, the "d" sub-shell may contain maximum ten electrons and the
"f" sub- shell may contain a maximum fourteen electrons.

AUFBAU PRINCIPLE AND ELECTRONIC CONFIGURATION

We have already discussed that every shell contains 1s
further smaller shells which, are called sub-shells. 2s 2p
These sub-shells are represented by the symbols s, 3s 3p 3d
p, d and f. These sub-shells have different energy. 4s 4p 4d 4f
Electrons always try to enter those sub-shells which, 5s 5p 5d 5f
have less energy. So, according to Aufbau principle, 6s 6p 6d
the sub-shell which has the lowest energy will be filled 7s 7p
first with electrons. The sequence of energy levels of 8s
sub-shells is shown in the diagram below.

According to the above diagram, the sequence of
energy level is given below.

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s
< 4f < 5d < 6p < 7s ………………….

New Creative Science, Class 10 | 119

K (n = 1) 1s 2
L (n = 2) 2s, 2p 2+6=8

M (n = 3) 3s, 3p, 3d 2 + 6 + 10 = 18
N (n = 4) 4s, 4p, 4d, 4f 2 + 6 + 10 + 14 = 32

Table: Electronic configuration based on shells and sub-shells

S.N. Element Atomic Electronic Electronic Valency
no. configuration based configuration based
on shells (K, L, M, N) on sub-shells (s, p,d,f)

1 Hydrogen 1 1 1s1 1
2 1s2 0
2 Helium 2 2, 1 1s2, 2s1 1
2, 2 1s2, 2s2 2
3 Lithium 3 2, 3 1s2, 2s2 2p1 3
2, 4 1s2, 2s2 2p2 4
4 Beryllium 4 2, 5 1s2, 2s2 2p3 3
2, 6 1s2, 2s2 2p4 2
5 Boron 5 2, 7 1s2, 2s2 2p5 1
2, 8 1s2, 2s2 2p6 0
6 Carbon 6 2, 8, 1 1s2, 2s2 2p6, 3s1 1
2, 8, 2 1s2, 2s2 2p6, 3s2 2
7 Nitrogen 7 2, 8, 3 1s2, 2s2 2p6, 3s2, 3p1 3
2, 8, 4 1s2,2s2 2p6, 3s2, 3p2 4
8 Oxygen 8 2, 8, 5 1s2,2s2 2p6, 3s2, 3p3 3, 5
2, 8, 6 1s2,2s2 2p6, 3s2, 3p4 2, 6
9 Fluorine 9 2, 8, 7 1s2,2s2 2p6, 3s2, 3p5 1
2, 8, 8 1s2,2s2 2p6, 3s2, 3p6 0
10 Neon 10 2, 8, 8, 1 1s2,2s2 2p6, 3s2, 3p6, 4s1 1
2, 8, 8, 2 1s2,2s2 2p6, 3s2, 3p6, 4s2 2
11 Sodium 11

12 Magnesium 12

13 Aluminium 13

14 Silicon 14

15 Phosphorus 15

16 Sulphur 16

17 Chlorine 17

18 Argon 18

19 Potassium 19

20 Calcium 20

Classification of elements in modern periodic table

In the modern periodic table elements are classified on the basis of two principles.
i) Classification based on sub-shells in which the last electron enters
ii) Classification based on the number of incomplete shells in an atom.

120 | Classification of Elements

i. Classification based on sub-shells in which the last electron enters

Each shell of an atom is divided into sub-shells or orbitals, which are called s, p, d
and f. The four orbitals can accommodate 2, 6, 10 and 14 electrons respectively. On
the basis of the orbital, in which the last electron enters, the elements are classified as,
s-block, p-block, d-block and f-block.

a) s-block elements: The elements whose last electron enters into "s" sub-shell
are called s-block elements. In the modern periodic table, the elements of
groups IA and IIA are the s-block elements. For example, the electronic
configuration of Na is 1s2 , 2s2 2p6 , 3s1. So, it belongs in s-block.

b) p-block elements: The elements whose last electron enters into "p" sub-
shell are called p-block elements. In the modern periodic table, the elements
of groups IIIA, IVA, VA, VIA, VIIA and zero are the p-block elements. For
example, the electronic configuration of Cl is 1s2, 2s2 2p6 , 3s2 3p5 . So, it
belongs in p-block.

c) d-block elements: The elements whose last electron enters into "d" sub-
shell are called d-block elements. The elements of groups IB, IIB, IIIB,
IVB, VB, VIB, VIIB and VIII belong to d-block. For example, the electronic
configuration of Sc is 1s2, 2s2 2p6, 3s2 3p6, 4s2, 3d1. So, it belongs in d-block.
The d-block elements are also called transitional elements.

d) f-block elements: The elements whose last electron enters into "f" sub-shell
are called f-block elements. It consists of two series of 14 elements known as
lanthanides and actinides. These elements have been placed at the bottom
of the periodic table. They are also known as inner transitional elements.

ii. Classification based on the number of incomplete shells in an atom

The elements can also be classified into four types depending upon the number of

incomplete shells in an atom. They are:

a. Representative elements b. Inner transition elements

c. Transition elements d. Noble gas

a. Representative elements: The elements having only one incompletely filled

outer-most shell are called representative elements. This includes both "s"

and "p" block elements except the inert gases.

b. Transitional elements: The elements having two incompletely filled (outer

most and second outer most) shells are called transitional elements. They

are d-block elements.

c. Inner transitional elements: The elements having three incompletely

filled (outermost, second outermost and third outermost) shells are called

inner transitional elements. They are f-block elements. They constitute

lanthanides and actinides. They have very much similar physical and

chemical properties.

New Creative Science, Class 10 | 121

Atomic size increases top toThe group of 14 elements from cerium (58) to lutetium (71) starting after
bottom in the grouplanthanum (57) is called lanthanide series and the group of 14 elements
from Thorium (90) to lawrencium (103) starting after actinium (89) is called
actinide series.
d. Noble gas: It consists of 6 elements He, Ne, Ar, Kr, Xe and Rn. They
have completely or filled valence shell with octet state except Helium (it
has completely filled K-shell with duplet state). They do not take part in
chemical reactions. So, they are known as inert or noble gas. They occupy
the group zero in the modern periodic table.

MEMORY TIPS
The elements after uranium which are synthetically prepared and belong to inner transition
elements are called trans uranium elements.

QUESTIONS
# Define the following terms with examples.

i) Transitional elements
ii) Inner transitional elements
iii)Sub-shells

General characteristics of groups and periods

1. Atomic radii
The size of atoms goes on increasing on moving from top to bottom in a group as the
number of shells goes on increasing. While going from left to right in a period, the
atomic size decreases because the number of shells remains constant but the inter
nuclear forces of attraction between nucleus and the valence electron increases. It is
due to increasing in the number of protons in the nucleus.

Li
Na

K Li Be B
Rb Atomic size decreases left to right in the period

Cs

QUESTIONS
# Which one has bigger atomic size between Na and K. Give its suitable reason.

122 | Classification of Elements

2. Valence electrons and valency

The total number of electrons in the valence shell is called valence electrons. It remains
constant is the particular group but increases along in the period as we move left
to right. Similarly, valency also remains the same for the particular group but first
increases up to four and decreases up to zero along in the period.

Li Be B C N O F Ne
1 2 34321 0

3. Chemical reactivity

In case of metals, while going from top to bottom in a group, the reactivity goes on
increasing. It is because, in case of metals, the number of shells or atomic radii goes
on increasing. Due to this, the inter nuclear force of attraction between nucleus and
valence electron decreases. As a result, the loss of electron becomes easy and reactivity
increases.

Reactivity of metals increases Li 11p
top to bottom in the group 12n
Na

K 19p
20n

Rb

Cs Sodium atom

Potassium atom

But in case of non-metals, as we go down in a group, the reactivity goes on decreasing.
It is because, the atomic radii goes on increasing and inter nuclear force of attraction
between nucleus and valence electron decreases. Hence, it will be difficult to gain
electron. Therefore, the reactivity decreases.

Reactivity of non-metals F
decreases top to bottom in
the group Cl 17p 9p
Br 18n 9n
Fluorine atom
I

Chlorine atom

In case of metals, as we go from left to right in a period, the reactivity decreases. This
is because, the atomic size decreases and it is difficult to lose electron.

Li Be B
Reactivity of metals decreases left to right in the period

New Creative Science, Class 10 | 123

In case of non-metals, as we go from left to right in a period, the reactivity increases.
This is because, the atomic size decreases and it is easier to gain electron.

NO F

Reactivity increases left to right in the period

MEMORY TIPS

Na is less reactive than K and F is more reactive than Cl.

Special names for some elements

Group IA - Alkali metals Group IIA - Alkaline earth metals

Group O - Nobel/inert gas Group VIIA - Halogens

QUESTIONS

# Which one is more reactive between Na and K? Why?
# Which is more reactive between Cl and F? Why?

Uses of periodic table

i. It makes the study of elements easy, fast, clear, systematic and scientific.
ii. It gives the correct position of elements.
iii. It helps to study the properties of elements.

MODEL QUESTIONS ANSWER

1. Define alkaline earth metals and transitional elements with examples.
Ü Alkaline earth metals: Elements which are present in IIA group of the modern

periodic table are called alkaline earth metals. They occur on the earth's crust
and react with water to give hydroxide. For example; Mg, Ca, etc.

Mg+2H2O → Mg(OH)2+H2

Ca+2H2O → Ca(OH)2+H2
Ü Transitional elements: Elements which are present in the d-block of the modern

periodic table are called transitional elements. These elements are present
between s and p blocks. Hence, they are named transitional elements. For
example; Fe, Cu, Ni, etc.

2. On the basis of the given electronic configuration, answer the following
questions.

A=1s2, 2s2 2p6, 3s1 B= 1s2, 2s2 2p6, 3s2, 3p5

(i) Predict the position of A and B in the modern periodic table.

(ii) Which compound is formed between A and B.

124 | Classification of Elements

Ü (i) Position of A in modern periodic table (ii) Position of B in modern periodic

table Period=Number of shells =3 Period = Number of shells = 3

Group = Number of valence electrons = 1A

Group = Number of valence electrons =VIIA

Block = s-block Block = p-block

Ü (ii)The compound formed between A and B in NaCl (Sodium chloride)

3. Which one is more reactive between Na and K? Why?

Ü The electronic configuration of Na is 2,8,1 while that of K is 2,8,8,1. So, the atomic
radius of K is more than Na. There is less attraction force between the valence
electron of K and its nucleus. Hence, it is easier for K to loses electron. Hence, it
is more reactive than Na.

4. Which one is more reactive between F and C? Why?

Ü Bothe fluorine and chlorine are non-metals and the size of fluorine is smaller
than that of chlorine. They gain foreign electron. To gain electron, the small size
of non-metal is more suitable. So, fluorine gains electron easily than chlorine.
Thus, fluorine is more reactive than chlorine.

SUMMARY

 There have been several attempts for the classification of elements. Among them Mendeleev's
work can be regarded as the milestone in the classification of elements.

 Mendeleev's periodic law states that, the physical and chemical properties of elements are
the periodic function of their atomic weight.

 Mendeleev's periodic table had seven periods and eight groups.
 The modern periodic law states that, the physical and chemical properties of elements are the

periodic function of their atomic number.
 Modern periodic table has less defects than Mendeleev's periodic table.
 There are seven periods and eighteen groups in the modern periodic table.
 In the modern periodic table, group IA are alkali metals, II are alkaline earth metals, VIIA are

halogen, zero are inert gases and IB are coinage metals.
 Every shell is divided into sub-shells: s, p, d and f. They can accommodate 2, 6, 10 and 14

electrons respectively.
 Reactivity increases down the group for metals and decreases down the group for non-metals.
 Metals are electron losers and non-metals are electron gainers.

EXERCISE

1. Define the following terms: (b) Actinides
(a) Alkali metals (d) Halogens
(c) Alkaline earth metals

New Creative Science, Class 10 | 125

(e) Transitional elements (f) s-block elements
(g) Inner-transitional elements (h) Lanthanides
(i) Modern periodic table (j) Mendeleev's periodic table

2. Write two differences between:

(a) Modern and Mendeleev's periodic table

(b) Periods and groups

(c) s-block and p-block elements

3. Write electronic configuration of the given elements on the basis of the sub-
shells.

(a) Na (b) Cl (c) Ca (d) K

4. Give reason:

(a) F is more reactive than Cl.

(b) K is more reactive than Na.

(c) Inert gases are kept in the zero group.

(d) Lanthanides and actinides are kept below the main block of the periodic table.

(e) Elements of group IA are called alkali metals.

(f) Lanthanides and actinides are also called inner-transitional elements.

5. Write the modern periodic law and Mendeleev's periodic law.

6. Answer the following questions based on the given electronic configuration.

A=1s2 ,2s2 2p6, 3s1 B=1s2, 2s2 2p6, 3s2

C=1s2, 2s2 2p6, 3s2 3p5 D=1s2, 2s2 2p6, 3s2 3p6

(a) Identify the elements.

(b) Predict their positions in the periodic table.

(c) What are their valency?

(d) Which one is more reactive between A and B?

(e) Which one of them is inert?

A

B GLOSSARY
C

Triad : a group of three
Penultimate : second last
Fundamental : basic or primary

Periodic table : the scientific table for the classification of elements



126 | Classification of Elements

UNIT

8 Chemical Reaction

About the Scientist INTRODUCTION

Amedeo Avogadro It is said that change is the rule of nature and nobody can
(1776) forecast the ultimate result of change because in every
moment change is going on in our nature. No scientist, no
Amedeo Avogadro was born latest and advanced spiritual or scientific invention has
in August 9, 1776 and died on become able to stop this change or reaction.
July 9, 1856 in Turin Italy.
Amedeo Avogadro was born into The changes occurring in our surroundings can be divided
a family of distinguished lawyers into mainly two types i.e.
(Piedmont family). Following
in his family's footsteps, he (i) Physical change and (ii) Chemical change
graduated in ecclesiastical law
(age 20) and began to practice Physical change is a temporary change in which only the
law. However, Avogadro was physical properties of substances such as colour, odour,
also interested in the natural taste, smell are changed. In this change, no new substances
science and in 1800 he began are formed. While chemical change is a permanent change
private studies in physics and in which both the physical and chemical properties of
mathematics. substances change with the formation of new substances.
Furthermore, physical changes can easily be reversed back
Avogadro's law states that while chemical changes are hard or impossible to reverse
equal volume of gases at the back. The melting of ice, dissolving of sugar in water, etc. are
same temperature and pressure, some physical changes whereas, the burning wood, forming
contain the same number of of water from hydrogen and oxygen are the examples of
molecules. Avogadro's hypothesis chemical change. All these chemical changes occurring are
wasn't generally accepted until called chemical reactions.
after 1858 (after his death),
when the Italian Chemist CHEMICAL REACTIONS
Stanislao Cannizzaro was able
to explain why there were some The chemical changes occurring due to addition or
organic chemical exceptions to decomposition or displacement or exchange of atoms or
Avogadro's hypothesis. molecules of the substances are called chemical reactions.
Similarly, some reactions may produce heat while some
may need heat for the reaction to proceed.

A chemical reaction can be written in the form of an
equation called the chemical equation. Chemical equations
are the convenient ways of representing the actual chemical
reaction and they can give more information than the
sentence description of the chemical reaction.

New Creative Science, Class 10 | 127

For example: when zinc reacts with sulphuric acid, it gives zinc sulphate and hydrogen
gas as,

Zn+H2SO4 ZnSO4+H2

Terms related with chemical equations

(i) Reactants and Products

The substances which take part in a chemical reaction are called reactants. They are
written on the left hand side of an arrow. Similarly, the chemical substances which are
produced from a chemical reaction are called products. They are written on the right
hand side of the equation. For example;

HCl + NaOH NaCl + H2O

Reactants Products

(ii) Word Equation

The chemical equation which is expressed by writing full names of reactants and
products is called a word equation. For example;

Sodium + Chlorine Sodium chloride

Nitrogen + Hydrogen Ammonia

Hydrochloric acid + Sodium hydroxide Sodium chloride + Water

(iii) Formula Equation

The chemical equation which is expressed by using the symbols and molecular formula
of reactants and products is called formula equation.

For example;

2Na + Cl2 2NaCl
HCl + NaOH NaCl + H2O
2KOH + H2SO4 K2SO4 + 2H2O

Symbols in Chemical Equation

i. (s) stands for solid
ii. (l) stands for liquid
iii. (g) stands for gase
iv. (aq) stands for aqueous solution
v. (↑ ) (upward arrow) stands for gas
vi. (↓) (downward arrow) stands for precipitation
vii. ( ) shows for reversible reactions
viii. (D) stands for heat

128 | Chemical Reaction

For example: ∆ 2NH3 (g)
2NaOH (aq.) + H2 (g)
N2 (g) + 3H2 (g)
2Na (s) + 2H2O (l)

MEMORY TIPS

According to the law of conservation of mass, "mass can neither be created nor be
destroyed but it remains constant before and after the reaction".

QUESTIONS
# Define the word equation with an example.
# Define the chemical equation with an example.

Classification of Chemical Equations

Chemical equations can be classified as:

Chemical equations

Exothermic Balanced Addition

Endothermic Unbalanced Decomposition

On the basis of the absorption or production of heat, chemical equations are classified
as endothermic and exothermic reactions.

1. Exothermic reaction: The chemical reactions which produce heat during their
processing are called exothermic reactions.

For example;

CH4 + 2O2 CO2 + 2H2O + Heat

C + O2 CO2 + Heat

2. Endothermic reaction: Those chemical reactions which require heat for their

processing are called endothermic reactions.

For example;

2KClO3 ∆ 2KCl + 3O2
CaCO3 ∆ CaO+CO2

New Creative Science, Class 10 | 129

MEMORY TIPS

Exothermic reactions may occur spontaneously but for endothermic reactions they require
external energy.

On the basis of the number of atoms on reactants and products, chemical equations
may be classified as balanced equations and unbalanced equations.

1. Unbalanced (skeletal) chemical equations: The chemical equations in which
the total number of atoms of each element in reactants and products are not
equal are called unbalanced chemical equations. They do not obey the law of
conservation of mass.

For example,

N2+H2 NH3

KClO3 KCl + O2

H2O2 H2O + O2

2. Balanced Chemical Equations: The chemical equations in which the total

number of atoms of each element in reactants and products are equal are called

balanced chemical equations. They obey the law of conservation of mass.

For example;

N2+3H2 2NH3
Ca(OH)2+CO2 CaCO3+H2O
2H2O2
2H2O + O2

Ways of writing balanced chemical equations

To balance a chemical equation the following steps should be followed:

1) Write the word equation to express the chemical change

For example

Potassium hydroxide + Sulphuric acid Potassium sulphate + Water

2) Translate the given word equation into correct formula equation

KOH + H2SO4 K2SO4 + H2O

3) Now, count the total number of atoms of each element and give the suitable

coefficient to balance the equation by hit and trial method.

2KOH + H2SO4 K2SO4 + 2H2O

Examples of balanced chemical equations

1) Nitrogen + Hydrogen Ammonia (word equation)

N2 + H2 NH3 (skeletal equation)

130 | Chemical Reaction

N2 + 3H2 2NH3 (balanced equation)

2) Sodium + Chlorine Sodium chloride (word equation)

Na+ Cl2 NaCl (skeletal equation)

2Na+ Cl2 2NaCl (balanced equation)

3) Zinc + Hydrochloric acid Zinc chloride+ Hydrogen (word equation)

Zn + HCl ZnCl2 + H2 (skeletal equation)
Zn + 2HCl ZnCl2 + H2 (balanced equation)

Information obtained from a balanced chemical equation

We can obtain various information from a balanced chemical equation.
1) It shows the total number of atoms and molecules of each reactant and
product.
2) It indicates the ratio of the molecular weight of reactants and products.
3) It gives an idea about the name and symbols of reactant and product
molecules.
4) It gives information about the type of chemical reaction.

The following examples provide information about the formation of water molecule

from hydrogen and oxygen.

Word equation: Hydrogen + Oxygen Water

Skeletal equation: H2 + O2 H2O
Balanced equation: 2H2 + O2 2H2O
Molecular weight: 4 + 32
36

Ratio of hydrogen to water : 1:9

Ratio of oxygen to water : 8:9

Type of chemical equation: Addition or combination reaction.

Limitations of Chemical Equations

1) The chemical equation does not provide information about the physical state of
reactants and products.

2) It does not tell about the conditions that are required for the reaction to occur.
3) It does not tell about the concentration of reactants and products.
4) It does not provide information about the reaction path.
5) It does not tell about the speed of reaction.
6) It does not say whether the reaction is reversible or irreversible.

New Creative Science, Class 10 | 131

QUESTIONS

# Write the information obtained from a chemical equation.
# Write the limitations of the chemical equation.
# Write the chemical equation for the following word equations and balance them by hit

and trial method.

a) Calcium carbonate + Nitric acid Calcium nitrate + Carbon dioxide + Water

b) Zinc + Sodium hydroxide Sodium zincate + Hydrogen

Classification based on Reaction Mechanism

There are mainly four types of chemical reactions. They are:
a) Addition reactions
b) Decomposition reactions
c) Displacement reactions
d) Acid-base reactions

a. Addition or combination or synthesis reaction

The chemical reaction in which two or more reactants combine to form a single product
is called addition reaction. It is also termed as the combination or condensation or
synthesis reaction. For examples;

2H2 + O2 2H2O

N2 + 3H2 2NH3

C + O2 CO2

2P2 + 5O2 2P2O5

b. Decomposition or dissociation or analysis reaction

The chemical reaction in which a single reactant is decomposed into two or more
products is called decomposition reaction. For examples;

CaCO3 CaO + CO2

2KClO3 2KCl + 3O2

2H2O 2H2 + O2

2H2O2 2H2O + O2

c. Displacement Reaction

The chemical reaction in which the atom or radical of one reactant is displaced by
another is called displacement reaction. It can again be divided into two types:

132 | Chemical Reaction

i) Single Displacement Reaction In a chemical reaction, if one atom or
radical displaces another from its compound, it is called displacement
reaction. For examples;

Zn+H2SO4 ZnSO4+H2

Fe + CuSO4 FeSO4 + Cu

Mg + 2HCl MgCl2 + H2

ii) Double Displacement reaction: In this type of chemical reaction, both the

reactant molecules are decomposed into opposite ions and give the new

products after exchanging the corresponding opposite ions. Such type of

reactions usually occur in solution and give precipitate as the products.

So, this reaction is also known as precipitation reaction. For examples;

CaCl2 + 2AgNO3 Ca(NO3)2 +2AgCl

NaCl + AgNO3 NaNO3 + AgCl

Na2CO3 + MgCl2 2NaCl + MgCO3

d) Acid-base or Neutralization Reaction

In this type of chemical reaction, an acid reacts with a base to give salt and water. Here,
acid and base both lose their own properties to give neural products. The resultant
products do not resemble any of the reactants. Hence, acid-base reaction is also known
as neutralization reaction.

Acid + Base Salt + Water

H2SO4 + 2NaOH Na2SO4 + 2H2O
HCl + NaOH NaCl + H2O

2HNO3 + Ca(OH)2 Ca(NO3)2 + 2H2O
H2SO4 + 2NH4OH (NH4)2SO4 + H2O

MEMORY TIPS

The solid and insoluble substance produced during a double displacement reaction is

called precipitate.

NaCl + AgNO3 NaNO3 + AgCl ↓
Here, AgCl is the precipitate and is indicated by '↓' in a chemical reaction.

QUESTIONS

# Define: i) Addition reaction, ii) Decomposition reaction
# Why is acid-base reaction called the neutralization reaction?

New Creative Science, Class 10 | 133

Rate of Chemical Reaction

In nature, some reactions occur violently while some take a long time to complete.
The factors like heat, temperature, pressure, catalyst, electricity also affect the rate of
chemical reaction.

Rate of reaction refers to the amount of a reactant which is converted into a product in per
unit time.

Conditions for chemical reaction and factors affecting the rate of reactions

i) Contact: For any reaction to occur, there must be a contact between the
substances.

ii) Heat: Some reactions need heat to occur. They are known as endothermic
reaction. For example: when calcium carbonate is heated then only calcium
oxide and carbon dioxide is formed.

CaCO3 ∆ CaO+CO2

Heat also plays an important role in the rate of reaction. Generally, the rate
of reaction increases with the increase in temperature. With the increase in
temperature, the kinetic energy of the molecules increases hereby increasing the
chance of collision of molecules. So, the rate of the reaction increases with the
increase in temperature.

iii) Light: There are some reactions which occur in the presence of light only. For
example;

H2 + Cl2 Sunlight 2HCl
2AgBr Sunlight 2Ag + Br2
CH4 + Cl2 Sunlight CH3Cl + HCl

iv) Pressure: By the application of pressure, the volume of reactant molecules
decreases. Thus, by creating more chance of collision between the reactant
molecules the rate of reaction increases.

Example: During the manufacturing of ammonia gas by Haber's process, the
atmospheric pressure of about 200-500 is necessary.

N2 + 3H2 Fe/MnO2 2NH3
200-500 atm

v) Catalyst: The substances which alter the rate of reaction without taking part
in the reaction themselves are called catalysts. Their concentration remains
unchanged till the end of reaction. There are two types of catalysts:

a) positive catalyst b) negative catalyst
a. Positive Catalyst: They increase the rate of reaction. For example; MnO2

helps in decomposition of KClO3 and H2O2.

134 | Chemical Reaction

2KClO3 MnO2 2KCl + 3O2
MnO2
2H2O2 2H2O + O2

Iron helps in the manufacturing of ammonia gas

3H2 + N2 Fe/MnO2 2NH3
200-500 atm

b. Negative Catalyst: They decrease the rate of reaction. For example; Glycerin

decreases the rate of decomposition of H2O2.
2H2O2 Glycerine 2H2O + O2

Characteristics of catalyst
i. The amount and chemical nature of the catalyst remain the same at the end
of the chemical reaction.
ii Catalysts do not initiate the chemical reaction but they change the rate of the
chemical reaction after initiation.

vi) Electricity: Some reactions need electricity to proceed. For example;

2H2O 2H2 + O2

N2 + 2O2 2NO2

vii) Solution: The reactions which do not occur in solid forms may take place when

the reactants are used in the solution form. For example;

NaCl + AgNO3 AgCl + NaNO3

viii) Concentration: Generally, the concentrated reactants react at a higher speed due

to more density of reactant molecules, thereby increasing the chance of collision.

It is experimentally found that the reaction between sodium thiosulphate (Na2S2O3)
and HCl takes place quickly as the concentration of Na2S2O3 goes on increasing.

MEMORY TIPS

a) In the manufacturing of vegetable ghee, Nickel (Ni) is used as a catalyst.
b) Similarly, other catalysts are Platinum (Pt), Rhodium, etc.

QUESTIONS
# Define a catalyst. How does it affect the rate of reaction.
# Explain how the concentration and temperature affects the rate of reaction.

MOLE CONCEPT

Since the reaction takes place in the atomic or molecular level, they cannot be seen
through naked eyes, and they cannot be isolated either. There is a new concept to deal
with them, known as mole concept.

New Creative Science, Class 10 | 135

Mole is defined as the collection of atoms or molecules or ions or any sub-atomic particles equal
in number of 6.023×1023. It is also known as Avogadro's number.

One mole of any substance contains 22.5 litres of substance at the normal temperature
and pressure (NTP).

One mole substance also contains one gram atomic weight or one gram molecular weight.

MEMORY TIPS

a) Normal Temperature and pressure refer to 25ºC and 1 atmospheric pressure (1 atm)
b) If the molecular weight or atomic weight of substance is expressed in terms of gram,

then it is known as gram molecular weight or gram atomic weight respectively.
i.e. Molecular weight of NaCl is 58 amu. So, gram molecular weight of NaCl is 58gm.

SOLVEDNumerical

1. How much gram of H2O is present in 2 moles of water?

Solution:
Molecular weight of H2O = 2×1+16 = 18

∴ Gram molecular weight of H2O = 18gm
Thus, 1 mole H2O contains 18 gm of H2O.
∴ 2 moles H2O contains (2×18) = 32 gm of H2O
2. How many grams of potassium chloride and oxygen is obtained when 10 gram
KClO3 is heated with MnO2?

Solution:

2KClO3 2KCl + 3O2
2(39+35+3×16)
2(39+35) 3(2×16)

244 148 96

 244 grams of KClO3 produces 148 gm of KCl
148

∴ 10 grams of KClO3 produces 244 × 10 gm of KCl = 6.06 gm of KCl
Similarly, 244 grams of KClO3 gives 96 gm of O2
∴ 10 grams of KClO3 gives 3.94gm of O2

MODEL QUESTIONS ANSWER

1) Define chemical reaction with example.
Ü The chemical change in which there is addition, dissociation or exchange of

atoms or molecules of the substance is called chemical reaction. As a result of
chemical reaction, new products are formed. For example,

HCl + NaOH NaCl + H2O

Hydrochloric acid Sodium hydroxide Sodium chloride Water

136 | Chemical Reaction

2) Write down the differences between single decomposition and double
decomposition reaction with examples.

Ü In single displacement reaction, one molecule is decomposed into two or more
products whereas in double decomposition reaction, both the reactant molecules
are decomposed into opposite ions and give new products after exchanging the
corresponding opposite ions.

Single decomposition reaction,

2KClO3 2KCl + 3O2

Here one molecule of potassium chlorate is decomposed into potassium chloride
and oxygen.

Double decomposition reaction,

NaCl + AgNO3 AgCl + NaNO3

Here, both the reactants are decomposed into opposite ions and they give new

products after exchanging the opposite ions; as shown below:

Na+ + Cl ¯ + Ag+ + NO3 ¯ AgCl + NaNO3

3) How much gram of calcium oxide (CaO) is obtained when 10g of calcium

carbonate or limestone is decomposed completely after heating?

Ü Solution:

CaCO3 ∆ CaO + CO2
40+12+48 40+16 12+32

100 56 44

Here, 100 gm of CaCO3 gives 56 gm of CaO

1 gm of CaCO3 gives 56 gm of CaO
100

10 gm of CaCO3 gives 56 × 10 gm of CaO =5.6 gm CaO
100
4) What is the function of the catalyst in chemical reaction? Write down their

types with examples.

Ü Catalysts are those chemical substances which are used to increase or decrease
the rate of chemical reaction. After changing the rate of reaction, they remain
unchanged at the end of reaction. On the basis of their action, they are of two
types:

a) Positive catalyst: They increase the rate of chemical reaction. For example,
manganese dioxide (MnO2), Iron (Fe), Nickel (Ni), etc.

b) Negative catalyst: They decrease the rate of chemical reaction. For
example: Glycerine, gypsum, etc.

New Creative Science, Class 10 | 137

5) What happens when magnesium ribbon is burnt in air?
Ü When magnesium ribbon is burnt in air, it gives magnesium oxide.

2Mg + O2 2MgO

6) What is the rate of reaction? What are the factors that affect the rate of reaction?

Ü The amount of a reactant that is converted into a product in per unit time is

called the rate of reaction. The different factors like temperature, pressure,

catalyst, contact, solution, etc. affect the rate of reaction.

SUMMARY

 All the chemical changes occurring in a substance is called chemical reaction.
 Chemical reactions can be expressed in terms of equations called chemical equation.
 Chemical reaction may be exothermic or endothermic.
 There are mainly four types of chemical reaction. They are

a) Addition reaction b) Decomposition reaction

c) Displacement reaction d) Acid-base reaction
 Catalysts are the substances which do not take part in chemical reaction but alter the rate

of reaction.
 The factors affecting the chemical reaction are heat, light, pressure, catalyst, concentration

of reactants, etc.
 The amount of reactant which is converted into a product in per unit time is called the rate of

chemical reaction.
 One mole = 6.023×1023 atoms/ molecules/particles.
 One mole = gram molecular weight = gram atomic weight.
 Generally the rate of reaction increases with the increase in temperature.

EXERCISE

1. Define the following terms with example:

(a) Physical change (b) Displacement reaction

(c) Chemical change (d) Decomposition reaction

(e) Chemical reaction (f) Acid-base reaction

(g) Balanced chemical equation (h) Rate of reaction

(i) Exothermic reaction (j) Atomic mass unit (amu)

(k) Endothermic reaction (l) Gram molecular weight

(m) Addition reaction (n) Reactant and product

2. Write two differences between:

(a) reactants and products.
(b) exothermic and endothermic reaction.
(c) word equation and symbolic equation.

(d) reversible and irreversible reaction.

138 | Chemical Reaction

3. Give reasons:

(a) Addition reaction is also known as synthesis reaction.

(b) Acid-base reaction is also known as neutralization reaction.

(c) It is necessary to balance the chemical equation.

(d) Rusting of iron is a chemical change.

4. Define catalysts and mention their types with examples.

5. What is balanced chemical equation? What information do you get from it?

6. What is the rate of chemical reaction? Describe the factors that affect the rate of
reaction.

7. What are the limitations of chemical equation?

8. Write down the modifications which have been made in the chemical equation.

9. What is Avogadro number? Write its value.

10. Define mole and gram molecular weight.

11. Translate the following word equations into balanced formula equations.

(a) Calcium chloride + Silver nitrate Silver chloride + Sodium nitrate.

(b) Potassium chlorate heat Potassium chloride + Oxygen

(c) Sulphuric acid + Sodium hydroxide Sodium sulphate + Water

(d) Ammonium chloride + Calcium hydroxide Calcium chloride + Ammonia + Water

(e) Iron + Oxygen Iron oxide

(f) Lead nitrate Lead oxide + Nitrogen dioxide + Oxygen

(g) Aluminium + Oxygen Aluminium oxide

(h) Calcium hydroxide + Carbon dioxide Calcium carbonate + Water

(i) Lead oxide + Carbon Carbon monoxide + Lead

(j) Iron + Sulphur Iron sulphide

12. Calculate the molecular weight of the following compounds.

(a) H2SO4 (b) KClO3 (c) CaCO3 (d) H2O
(h) CaO
(e) K2SO4 (f) KOH (g) HNO3

13. Solve the given numerical problems:

(a) How many grams of MgO is obtained when 5 grams of magnesium oxide is
strongly heated?

(b) 10g NaOH is treated with HCl to get NaCl. Find the weight of NaCl.

(c) How many moles of water is present in 1.8g of water.

A

B GLOSSARY
C

Collision : crash into each other

Kinetic energy : energy produced due to the motion of particles

Polymer : long or large molecule



New Creative Science, Class 10 | 139

UNIT

9 Acid, Base and Salt

About the Scientist INTRODUCTION

Svante August Arrhenius Although there are millions of compounds (both organic
(1859) and inorganic) in the world, they all can be grouped into
three classes i.e., acids, bases and salts. These substances
Svante August Arrhenius was have some more common distinguishable properties than
born on February 19, 1859, the other substances. In this chapter, we will be able to define
son of Svante Gustaf Arrhenius these compounds with their common characteristics, their
and Carolina Christina common methods of preparation, their properties and their
Thunberg. His ancestors were uses.
farmers; his uncle became
Professor of Botany and Rector ACID
of the Agricultural High school
at Ultuno near Uppsala and Robert Boyle in 1663 named a class of compounds as
later secretary of the Swedish acids, which have a sour taste, reacts with metals to evolve
Academy of Agriculture. His hydrogen and changes blue litmus paper into red.
father was a land surveyor
employed by the University of Thus, acids are the compounds that give hydrogen ions (H+)
Uppsala and in charge of its when dissolved into water, which neutralize base, turn blue
estates at Vik, where Svante litmus into red and have a sour taste.
was born. The family moved
to Uppsala in 1860. The boy For example; Hydrochloric acid (HCl), Sulphuric acid
was educated at the Cathedral (H2SO4), Nitric acid (HNO3), Acetic acid (CH3COOH),
school where the rector was a Formic acid (HCOOH), etc.
good physics teacher. From an
early age, Svante had shown Types of Acids
an aptitude for arithmetical
calculates, and at school he was Acids may be organic or inorganic according to their source.
greatly interested in mathematics And, they may be weak or strong according to their strength.
and physics. In 1876, he entered
the University of Uppsala, i) Organic Acids
studying mathematics, chemistry
and physics. The practical The acids which are present in animals' or plants' bodies or their
instruction in physics was not products are called organic acids. They are the compounds of
of the best, and in 1881 he carbon, hydrogen and oxygen. They are also called carboxylic
went to Stockholm to work acids. All organic acids are weak acids and they are a bad
under Professor E.Edlund at the conductor of electricity. Some organic acids with their sources
Academy of Sciences. are enlisted below.

140 | Acid, Base and Salt

Organic Acids Sources
Citric acid Grapes, orange, lemon
Malic acid Apple
Oxalic acid Tomatoes
Lactic acid Milk
Tannic acid Tea
Acetic acid Vinegar/pickle
Amino acid Protein
Carbonic acid Aerated drinks

ii) Inorganic Acid (Mineral acids)

The acids which are obtained from minerals present in the earth's crust are called mineral
acids. They contain elements other than carbon. They may be strong as well as weak.
They may be good or bad conductors of electricity. For example; Hydrochloric acid
(HCl), Nitric acid (HNO3), Sulphuric acid (H2SO4), etc.

MEMORY TIPS

Ants inject formic acid into the body of organisms when they bite. So their biting produces
burning sensation.

i) Strong Acids : The acids which are ionized completely into their opposite ions
when dissolved in water are called strong acids. For example; HCl, H2SO4, HNO3,
H2CO3, etc.

ii) Weak Acids : The acids which do not ionize completely into their ions when
dissolved in water are called weak acids. For example; Formic acid, acetic acid,
oxalic acid, etc.

Differences between organic and mineral acids

Organic acids Mineral acids

1. They are obtained from living 1. They are obtained from minerals

organisms (plant and animal). present on the earth’s crust.

2. They are weak acids. 2. They are strong as well as weak acids.

3. They are poor conductors of 3. They are poor as well good conductors

electricity due to less production of of electricity.

hydrogen ions.

Classification of acids on the basis of strength

On the basis of strength, acids are classified into two groups.

i. Strong acids

Those acids which undergo almost complete ionization in an aqueous solution to give hydrogen
ions are called strong acids. They are good donors of hydrogen ions and good conductors

New Creative Science, Class 10 | 141

of electricity. Hydrochloric acid, sulphuric acid, nitric acid, etc. are the examples of

strong acid.

HCl Strong H+ + Cl–
(in water)

H2SO4 Strong 2H+ + SO4– –
(in water)

HNO3 Strong H+ + NO3–
(in water)

ii. Weak acids

Those acids which do not undergo almost complete ionization in an aqueous solution to give

hydrogen ions are called weak acids. Citric acid, acetic acid, carbonic acid, tartaric acid,

etc. are weak acids. They are poor hydrogen donor as well as poor conductors of

electricity.

CH3COOH Weak CH3COO– + H+
(in water)

MEMORY TIPS

(i) Double headed arrow ( ) indicates the reversible reaction.

(ii) All organic acids are weak acids.

(iii) HCl is also present in the stomach of animals but it is an inorganic acid because it

does not have carbon atom.

(iv) H2CO3 contains carbon atom but it is obtained from mineral hence it is an inorganic acid.

Difference between strong and weak acids

Strong acids Weak acids

1. They undergo almost complete 1. They do not undergo complete

ionization to give the hydrogen ions. ionization to give the hydrogen ions.

2. They are corrosive in nature. 2. They are less corrosive in nature.

3. They are good conductors of 3. They are poor conductors of

electricity. electricity.

QUESTIONS
# What are acids? Why are organic acids poor conductors of electricity?
# Strong acids are corrosive in nature. Justify this statement.
# Acids undergo ionization to give hydrogen ions. Show it with some examples.

Characteristics of acids

Acids have the following characteristics:
1. They have a sour taste and are corrosive in nature. Strong acids are more

corrosive and burn our skin, tongue, etc.

142 | Acid, Base and Salt

2. They contain hydrogen and when they are dissolved in water, they furnish it in

the solution.

HCl H+ + Cl–

H2SO4 2H+ + SO4– –
3. They turn blue litmus paper to red, methyl orange to red and phenolphthalein

to colourless.

4. Acid solution can conduct the electricity.

5. They neutralize the base to give salt and water.

HCl + NaOH NaCl + H2O

6. They react with metals to give hydrogen gas.

Mg + 2HCl MgCl2 + H2

7. They react with carbonates and bicarbonates to give carbon dioxide.

CaCO3 + 2HCl CaCl2 + H2O + CO2

NaHCO3 + HCl NaCl + H2O + CO2

8. They react with sulphates and bisulphates to give sulphur dioxide.

CaSO3 + 2HCl CaCl2 + H2O + SO2

NaHSO3 + HCl NaCl + H2O + SO2

9. They react with sulphides to give hydrogen sulphide gas.

ZnS + 2HCl ZnCl2 + H2S

Uses of Acids

1. Nitric acid (HNO3) is used to make fertilizers like ammonium nitrate, calcium
ammonium nitrate, etc.

2. Nitric acid is used to make explosives like TNT, nitroglycerine, etc.
3. Sulphuric acid (H2SO4) is used to make fertilizers like ammonium sulphate,

superphosphate, etc.
4. Sulphuric acid is used in the batteries of cars.
5. Sulphuric acid is used as a dehydrating agent in chemical reactions.
6. Hydrochloric acid is used in tanning and printing industries.
7. Carbonic acid is used in soft drinks.
8. Boric acid is used as eyewash.
9. Acetic acid is used in flavouring food.
10. Citric acid is used in medicines as a source of vitamin – C.

MEMORY TIPS
TNT (Trinitrotoluene) is an explosive substance.

QUESTIONS

# Write down any five uses of acids.
# Mention any four characteristics of acids.

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BASE

Antacid is used to cure excessive acidity in the stomach. The antacid contains a base
magnesium hydroxide Mg(OH)2. It is also known as the milk of magnesium. Similarly,
a common household glass cleaner contains a base (ammonia). Calcium oxide is used
to neutralize the acidic nature of soil. Sodium hydroxide is used in making soap,
paper, pulp, medicine, rayon, etc. All the above bases are either metallic oxides or
hydroxides.

Metallic oxides and hydroxides are called bases.

MEMORY TIPS
All bases are not bitter, only the hydroxide of alkali metals are bitter.
Some example of bases are sodium oxide (Na2O), magnesium oxide (MgO), calcium oxide
(CaO), potassium oxide (K2O), copper oxide (CuO), etc.

Alkalis

There are many bases but only some bases which are soluble in water to give
hydroxides are called alkalis. These metallic oxides or hydroxides when dissolve in
water give hydroxyl ions.

Those metallic oxides which are soluble in water to give hydroxides are called alkalis.

MEMORY TIPS

Only 13 types of hydroxides are soluble to be known as alkali. They are: 6 hydroxides of alkali
metals (metals of group IA viz. Li, Na, K, Rb, Cs & Fr), 6 hydroxides of alkaline earth metals
(metals of group IIA viz. Be, Mg, Ca, Sr, Ba & Ra) and ammonium hydroxide (NH4OH).

Some example of alkalis are sodium hydroxide (NaOH), potassium hydroxide (KOH),

calcium hydroxide {Ca(OH)2}, aluminium hydroxide {Al(OH)3}, etc.

Na2O + H2O 2NaOH

K2O + H2O 2KOH

CaO + H2O Ca(OH)2

The above alkalis undergo ionization to give opposite ions.

NaOH Na+ + OH–

KOH K+ + OH–

Ca(OH)2 Ca++ + 2OH–

MEMORY TIPS
Ammonium hydroxide (NH4OH) is a single alkali which has no metal.

Most of the metals react with oxygen to give metallic oxides, which are called the
base but most of the bases are insoluble in water and only few bases that are soluble

144 | Acid, Base and Salt

in water to give hydroxide are called alkalis. Hence, all bases are not alkalis but all
alkalis are bases.

MEMORY TIPS

All metallic oxides are bases but only some metallic oxides which are soluble in water to
give hydroxides are called alkalis. So all bases are not alkalis but all alkalis are bases.

Differences between base and alkalis

Bases Alkalis

1. Metallic oxides are called bases. 1. Water soluble metallic oxides or

metallic hydroxides are called alkalis.

2. They may be water soluble or 2. They are water-soluble.

insoluble. Example: Na2O, K2O, CaO, Examples: NaOH, KOH, Mg(OH)2,
MgO, Al2O3, CuO, etc. etc.

Types of bases

On the basis of strength, there are two types of bases. They are:

(a) Strong base : Those bases, which undergo almost complete ionization to give
a high concentration of hydroxyl ions in an aqueous solution, are called strong
bases.

Some examples of strong bases are sodium hydroxide (NaOH), potassium
hydroxide (KOH), calcium hydroxide Ca(OH)2, etc.

(b) Weak bases : Those bases which do not undergo almost complete ionization
in an aqueous solution and give very a low concentration of hydroxyl ions are
called weak bases.

Some examples of weak bases are copper hydroxide, ferric hydroxide,
ammonium hydroxide, etc.

Preparation of bases

1. By direct combination of metals with oxygen

Metals + Oxygen Metallic oxide

2Na + O2 Na2O

2K + O2 K2O

2Ca + O2 2CaO

2. By heating metallic carbonates

Metallic carbonates heat Metal oxide + carbon dioxide

CaCO3 ∆ CaO + CO2

New Creative Science, Class 10 | 145

3. By heating metallic nitrates

Metallic nitrates heat Metal oxide + Nitrogen dioxide + Oxygen

2Cu(NO3)2 ∆ 2CuO + 4NO2 + O2

4. By the reaction of alkali metals with water

Metal + Water Metallic hydroxide + Hydrogen

2Na + 2H2O 2NaOH + H2 ↑
2K + 2H2O 2KOH + H2 ↑

Properties of Bases

1. Some of the bases are bitter in taste and soapy to touch.

2. They change red litmus paper to blue, methyl orange to yellow and phenolphthalein
to pink.

3. Alkalis absorb carbon dioxide from air and form the carbonates.

Alkalis + Carbon dioxide Carbonate + Water

2KOH + CO2 K2CO3 + H2O

Ca(OH)2 + CO2 CaCO3 + H2O

4. Bases react with acid to form salt and water (i.e. neutralization reaction)

Base + Acid Salt + Water

NaOH + HCl NaCl + H2O

2KOH + H2SO4 K2SO4 + H2O

5. Alkalis react with ammonium salts to give ammonia gas.

Alkalis + Ammonium salts Salt + Water + Ammonium

NaOH + NH4Cl NaCl + H2O + NH3

6. Heavy metal salts react with alkalis to produce insoluble metal hydroxides.

Heavy metal salt + Alkalis Hydroxide + Salt

CuSO4 + 2NaOH Cu(OH)2 + Na2SO4

Uses of Bases

1. Sodium hydroxide (NaOH) is used in the manufacturing of soap, detergents, paper,
rayon, etc.

2. Calcium hydroxide [Ca(OH)2] is used in the manufacturing of bleaching powder,
to decrease hardness of water, to neutralize the acidity of soil, washing walls, etc.

3. Potassium hydroxide (KOH) is used in batteries, to precipitate soap, in the
manufacturing of soap, etc.

4. Lime water (Ca(OH)2) is used as a laboratory reagent.

146 | Acid, Base and Salt