All the elements of sub-group A in the periodic table are called
representative elements. They have only one incomplete shell,
i.e. outermost shell. These elements have their outermost orbit
incompletely filled while the inner orbits are completely filled.
Advantages of modern periodic table
Modern periodic table corrected the drawbacks (demerits) of
Mendeleev's periodic table and made the position of elements
clear. There are several advantages of modern periodic table over
Mendeleev's periodic table. Some of them are:
1. Position of metal, metalloid and non-metal
There are separate positions for metal, metalloid and non-metals
in the modern periodic table. The metals are placed on the left
side of periodic table. The non metals lie on the right side of
periodic table which include inert gases, halogens and other
elements. The elements B, C, Si, Ge, As and Te are metalloid
and lie between metals and non-metals.
2. Position of hydrogen
Hydrogen is an element with one valence electron. It can lose
that electron to acquire a positive charge similar to that of alkali
metals. So, it is placed in group IA (group 1). It is the explanation
for placing hydrogen in group IA. Even though the justification
has been given for keeping hydrogen in group IA, the position
of hydrogen is still controversial due to its similarities with
halogens in some other properties.
3. Improvement in the position of elements
The faulty position of elements like argon, potassium, etc is
solved itself while arranging elements as a function of atomic
number.
4. Position of lanthanides and actinides
All the lanthanides have similar properties so they belongs to
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group 3 and 6th period. Similarly, all the actinides belong to
group 3 and 7th period. But two series of each of these elements
are placed at the bottom of the periodic table to avoid the undue
sidewise expansion of the periodic table. All lanthanides have
similar chemical properties. Similarly, all actinides have similar
chemical properties.
5. Position of isotopes
The isotopes are the atoms of same element having same atomic
number but different mass number. The isotopes of elements are
due to the difference in number of neutrons in the atoms of same
element. For example: Protium, deuterium and tritium are three
isotopes of hydrogen.
1n 1p 1p
1n 2n
Protium Deuterium Tritium
Isotopes of hydrogen
Since all the isotopes have same number of protons, they have same
atomic number. So all isotopes occupy same position in the modern
periodic table. Thus, the modern periodic table solved the problem of
position of isotopes.
Fact &Reason
What is the position of metals, non metals and metalloids in the modern periodic
table?
Metals are placed at the left part of the modern periodic table whereas non
metals are placed at the right. Metalloids are placed in the middle between
metals and non metals.
Valence electrons and valency of elements
The electrons that are present in the outermost shell of an atom are
called valence electrons because they determine the valency of an
element. The valence electrons take part in chemical reactions and
the reactivity of elements depends upon them. A part of periodic
table is given below in which the valence electrons of some elements
are shown:
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Group/ 1 2 13 14 15 16 17 0
Period
1 H Be B C N O F He
1 2, 2 2, 3 2, 4 2, 5 2, 6 2, 7 2
2 Li Mg Al Si P S Cl Ne
2, 1 2, 8, 2 2, 8, 3 2, 8, 4 2, 8, 5 2, 8, 6 2, 8, 7 2, 8
3 Na Ca Ar
2, 8, 1 2, 8, 8, 2 2, 8, 8
4 K
2, 8, 8, 1
Valency 1 2 3 4 3 2 1 0
According to octet rule, a group of eight electrons in the outermost
shell is always stable. The elements try to gain octet either by losing,
gaining or mutual sharing of their valence electrons. The number
of electrons lost, gained or shared by an atom during a chemical
reaction is the valency of an element. Hence, the valence electrons
play an important role to determine the valency of an element.
The elements of group IA, IIA and IIIA lose all electrons of their
valence shell to attain octet. So, the valency of Na is 1, Mg is 2 and
Al is 3. The elements of group IVA (i.e, C, Si, etc) mostly share
electrons to attain octet. The elements of group VA to VIIA gain
electrons from other atoms to form octet. Hence, these elements are
the electronegative elements. But the elements of group '0' neither
lose, nor gain nor share electrons because they have their own octet
in their valence shell.
The valency of an element is determined with the help of valence
electrons. For example, Valency of elements of group IA to IVA = No.
of valence electrons.
Valency of group VA to VIIA and 'O' = 8 - No. of valence electrons.
Reactivity of the elements
The elements of a particular group have similar chemical properties
but they do not have the same properties. There is regular change
in reactivity of elements in a group. The reactivity of the elements
is largely affected by the size (radius) of their atoms. As we move
from left to right of a period, the nuclear charge (i.e, number of
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protons) increases. But the addition of electrons takes place in the
same shell. As a result, the electrons are pulled closer to the nucleus.
This decreases the size of the atom. Hence, the atomic size of Mg is
smaller than that of Na, the atomic size of Al is smaller than that Mg
and so on.
1. The reactivity of metals: The reactivity of the metals (elements
of group IA, IIA and IIIA) increases along a group on going from
top to bottom of the periodic table. As we go down from top to
bottom of a group, the size of the atom increases due to the
addition of one new shell in each successive member. Thus, the
valence electrons become farther from the nucleus and the force
of attraction between the nucleus and valence shell decreases.
This enables an atom to lose valence electrons easily. Hence, the
chemical reactivity of metals increases on moving from top to
bottom of a group in the periodic table. For example, Na is more
reactive than Li, K is more reactive than Na and so on.
Potassium (K) is more reactive than sodium (Na) because
potassium has larger size and thus less attractive force exists
between nucleus and valence electrons than that in sodium. So,
potassium loses electron easily than sodium.
2. The reactivity of non-metals: The reactivity of non-metals
(elements of group VA, VIA and VIIA) decreases from top to
bottom of a group. As we go down the group, the size of the
elements increases. Thus, the valence shell become farther from
the nucleus and force of attraction between the nucleus and
valence shell decreases. This decreases the tendency of an atom
to gain (attract) electrons. Hence, the chemical reactivity of non-
metals decreases on moving down a group in the periodic table.
For example, fluorine is more reactive than chlorine; chlorine
is more reactive than bromine and so on. Among the halogens,
fluorine is the most reactive and iodine is the least reactive.
Fluorine is more reactive than chlorine because fluorine (F) has
smaller size and more electronegativity. Due to this, fluorine
attracts electron more strongly than chlorine.
As we move from left to right in a period, the chemical reactivity
of the elements first decreases and then increases after reaching
the least reactive element.
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Fact &Reason
Fluorine is more reactive than chlorine, why?
The atomic size of fluorine is smaller than that of chlorine. Due to which, nucleus
of fluorine can attract electrons with more force and can easily gain electrons
than chlorine. So, fluorine is more reactive than chlorine.
Difference between Modern and Mendeleev's periodic table:
Modern periodic table Mendeleev's periodic table
1. It is based on the increasing
1. It is based on the increasing
atomic number of elements. atomic mass.
2. It consists of seven periods 2. It consists of seven periods
and eighteen groups. and eight groups in the
original forms.
3. The position of hydrogen,
lanthanides and actinides is 3. There is no justifiable
properly justified. position of hydrogen,
lanthanides and actinides.
4. There is proper position for
the isotopes of an element. 4. There is no idea about the
position of isotopes of an
element.
Differences between groups and periods:
Groups Periods
1. The vertical columns of 1. The horizontal rows of
elements in the periodic table elements in the periodic table
are called groups. are called periods.
2. All the elements of a group 2.The number of valence
have same number of valence electrons of elements in a
electrons. period increases from left to
right.
3. All the elements of a group 3. The elements of a period have
have same valency. different valency.
4. The elements of a group 4.The elements of the same
have similar properties. period have different
contrasting properties.
5. The size of atoms increases 5. The size of atoms decreases
from top to bottom. from left to right.
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Differences between representative and transition elements:
Representative elements Transition elements
1. The elements of sub-group B
1. The elements of sub-group
A in the periodic table and group VIII in the periodic
are called representative table are called transition
elements. elements.
2. They all are metals.
2. They include both metals
and non metals. 3. They have two shells
incompletely filled.
3. Only valence shell is
incompletely filled in these 4. They have variable valency.
elements.
5. Their salts are colourful.
4. They have a single definite
valency.
5. Their salts are mostly
colourless.
Sub shell and electronic configuration
A shell is a well defined fine circular path in which the electrons
move around the nucleus. lt is also known as orbit. The shells can be
represented as K, L, M, N ....... etc. They can also be represented in
numbers such as 1, 2, 3, 4, ……. etc.
The maximum number of electrons that can be accommodated in a
shell of an atom is given by 2n2 rule. Since the 2n2 rule of electronic
configuration could not include the electronic configuration of all the
elements, the concept of s, p, d and f sub-shells was introduced.
The sub-shells are composed of orbitals. The region around the
nucleus where the probability of finding electrons is maximum
is called orbital. The maximum number of electrons that can be
accommodated in s, p, d and f sub-shells is 2, 6, 10 and 14 respectively.
The sub-shells present in each shell is given below:
Shell Number of Sub-shells present
sub-shell
K1 s sub shell
L 2 s and p sub shells
M 3 s, p and d sub shells
N 4 s, p, d and f sub-shells
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Maximum number of electrons in sub-shell is as follows:
The stable state of having two Sub-shell Maximum
electrons in the first orbit is duplet s number of
state. For example, helium has electrons
duplet state. Similarly, a stable set
of eight electrons in the outermost 2
shell of an atom is called octet. p 6
For example, neon, argon, etc d 10
have octet. The tendency of an f 14
atom to acquire 8 electrons in the
outermost shell is called octet rule. The atoms can acquire octet by
gaining, losing or mutual sharing of electrons and it is the cause of
chemical combinations.
Electronic configuration
The distribution of electrons in various shells and sub shells of an
atom is called electronic configuration Aufbau principle tells us the
sequence of filling up of atomic orbitals.
According to this principle "the atomic orbitals are filled up in the
ground state in the order of increasing energy levels".
The sequence of filling the atomic sub shells or orbitals follows a
specific order which can be obtained from the following diagram.
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d Sequence of filling of
7s 7p
atomic orbitals
Thus, the correct order is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s,
4f, 5d, 6p, 7s, 5f, 6d, 7p.
The electronic configuration of some elements (atomic no. 1 to 30) in
terms of sub shells is below:
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1. Hydrogen (H)→ 1s1 2. Helium (He) →1s2
3. Lithium (Li)→ 1s2, 2s1 4. Beryllium (Be) →1s2, 2s2
5. Boron (B) →1s2, 2s2, 2pl 6. Carbon(C) →1s2, 2s2, 2p2
7. Nitrogen (N) →1s2, 2s2, 2p3 8. Oxygen (O) →1s2, 2s2, 2p4
9. Fluorine (F) →1s2, 2s2, 2p5 10. Neon (Ne) →1s2, 2s2, 2p6
11. Sodium (Na) → 1s2, 2s2, 2p6, 3s1
12. Magnesium (Mg) → 1s2, 2s2, 2p6, 3s2
13. Aluminum (Al) →1s2, 2s2, 2p6, 3s2, 3p1
14. Silicon (Si) →1s2, 2s2, 2p6, 3s2, 3p2
15. Phosphorus (P) → 1s2, 2s2, 2p6, 3s2, 3p3
16. Sulphur (S) → 1s2, 2s2, 2p6, 3s2, 3p4
17. Chlorine (Cl) →1s2, 2s2, 2p6, 3s2, 3p5
18. Argon (Ar) →1s2, 2s2, 2p6, 3s2, 3p6
19. Potassium (K) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s1
20. Calcium (Ca) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2
21. Scandium (Sc)→ 1s2, 2s2, 2p6, 3s2,3p6, 4s2, 3d1
22. Titanium (Ti) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d2
23. Vanadium (V) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d3
24. Chromium (Cr) → ls2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5
25. Manganese (Mn) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d5
26. Iron (Fe) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6
27. Cobalt (Co) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d7
28. Nickel (Ni) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d8
29. Copper (Cu) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10
30. Zinc (Zn) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10
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In chromium and copper, the actual electronic configuration differs
slightly from the expected ones, because half filled or completely
filled 'd' orbital is more stable than d4 or d9 state.
Classificationofelementsonthebasisofelectronicconfigurations
The elements in the modern periodic table are divided into four
blocks. They are:
1. s–block element 2. p–block element
3. d–block element 4. f–block element
The last electron present in the sub-shell determines the block of an
element.
1. s -block elements
The elements in which the last electron enters in s-sub-shell of
outermost shell are called s-block elements. These elements contain
1 or 2 valence electrons. The elements of group 1 and 2 are s-block
elements. Thus, the alkali metals and alkaline earth metals are
s-block elements. These are very reactive metals.
Example:
H→1s1 Be → 1s2, 2s2
Li→1s2, 2s1 Mg → 1s2, 2s2, 2p6, 3s2
Na → 1s2, 2s2, 2p6, 3s1 Ca → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2
2. p -block elements
The elements in which the last electron enters into p-sub-shell of
their outermost shell are called p-block elements. The elements of
group 13 to 18 (group O) belong to p-block elements. The p-sub-shell
of these elements is gradually filled up. Since the p-sub shell can
hold maximum of 6 electrons, the p-block elements constitute six
columns in the periodic table.
Examples:
B →1s2, 2s2, 2p1 C → 1s2, 2s2, 2p2
N→ ls2, 2s2, 2p3 O → 1s2, 2s2, 2p4
F→ ls2, 2s2, 2p5 Ne →1s2, 2s2, 2p6
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Differences between s-block and p-block elements:
s- block elements p-block element
1. The last electron of these 1. The last electron of these elements
elements enters in s-sub enterers in p-sub shell.
shell.
2. They form only 2 columns 2. They form 6 columns in the
in the periodic table. periodic table.
3. The chemical reactivity 3. The chemical reactivity decreases
of elements increases on on moving downwards in a group.
moving from top to bottom of
a group.
3. d-block elements
The elements in which the last electron enters the d–sub shell of the
outermost shell are called d-block elements. The elements of group 3
to 12 are d-block elements. The d–block elements are also known as
transition metals.
Examples:
Scandium (Sc) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d1
Titanium (Ti) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d2
Vanadium (V) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d3
Chromium (Cr) →1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5
Manganese (Mn) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d5
Iron (Fe) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6
Cobalt (Co) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d7
Nickel (Ni) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d8
Copper (Cu) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10
Zinc (Zn) → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10
4. f -block elements
The lanthanides and actinides are f-block elements. The f–sub shell
of outermost shell of these elements is gradually filled up. These
elements are called inner transition elements because two series of
these elements act as transit between the groups 3 and 4.
The series of 14 elements each are placed at the bottom of the periodic
table.
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Learn and Write
1. Elements of group 17 are highly reactive electronegative
elements. Why?
Elements of group 17 have seven valence electrons. Therefore,
they readily take one electron from other atoms to make their
outermost orbit complete and become electronegative. Thus,
elements of group 17 are highly electronegative elements.
2. Potassium is more reactive than sodium. Why?
Potassium has bigger atomic size than sodium. Hence the
valence electron of potassium is farther from the nucleus than
in sodium and is attracted with less force by the nucleus. Thus,
valence electron can be thrown easily in potassium than in
sodium. Therefore, potassium is more reactive than sodium.
3. Sodium is s-block element. Why?
According to electronic configuration of sodium i.e. 1s2, 2s22p63s1,
its last electron is present in s-sub-shell. So, it is s-block element.
Main points to remember
1. The scientific and systematic classification of elements in
definite rows and columns based on certain periodic law is
called periodic table.
2. The horizontal rows of elements in the periodic table are called
periods
3. The vertical columns of the elements having similar properties
in the periodic table are called groups.
4. Mendeleev's periodic law states that the physical and chemical
properties of elements are periodic function of their atomic
weights.
5. Modern periodic law states that the physical and chemical properties of
elements are periodic function of their atomic number.
6. There are seven periods and eighteen groups in the modern
periodic table.
7. The elements of group IA are called alkali metals because these
elements readily react with water to form alkali.
8. The elements of group IIA are known as alkaline earth metals because
they form basic hydroxides which are less soluble in water.
9. All the elements of sub-group A in the periodic table are known
as representative elements.
10. The isotopes are the atoms of same element having same atomic
number but different mass number.
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11. The region around the nucleus where the probability of finding
electron is maximum is called orbital.
Exercise
A. Very short answer questions ( 1 mark)
1. On what basis did Mendeleev periodic table differ from the
Modern periodic table?
2. What are lanthanides and actinides?
3. Write down the position of transition metals in modern
periodic table.
4. What are valence electrons ?
5. Define duplet state with one example.
6. What is meant by octet state? Give one example.
7. State the Modern Periodic Law
8. State Mendeleev's periodic law.
9. Write down the maximum number of electrons that can be
accommodated in p and d sub shells.
10. Define halogens.
11. What are alkaline earth metals?
12. What are p-block elements?
13. Write the electronic configuration of potassium.
14. To what groups do magnesium and nitrogen belong in
modern periodic table?
15. In which group of modern periodic table are alkali metals,
alkaline earth metals and halogens kept?
16. In which groups are noble gases, most reactive non-metals,
most reactive metals and metalloids placed?
17. Write down two factors that determine the reactivity of
elements.
18. What are alkali metals ?
19. To what groups do calcium and chlorine belong in modern
periodic table?
20. In what group and period does gold lie in the modern periodic
table?
21. Study the electronic configuration given below and answer
the following questions:
1s2, 2s22p6, 3s23p6, 4s13d10
Name the element and write its block
B. Short answer questions (2 marks)
1. What is the cause of placing calcium in IIA(2) group of the
modern periodic table?
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2. Elements of Group IA (1), IIA (2) and IIIA (3) are more
reactive as we go down in the group of the periodic table,
why?
3. Mention any two differences between Mendeleev's Periodic
Table and Modern Periodic Table.
4. Write any two differences between s-block and p-block
elements.
5. Fluorine is more reactive than Chlorine, why?
6. Write two characteristics of 's' block elements.
7. Write any two reasons for keeping hydrogen in group IA in
periodic table.
8. Write two differences between IA group elements and VIIA
group elements.
9. Sodium is called metal, why?
10. Atomic size of inert gases does not affect inertness, why?
11. Why are the elements of group IA called alkali metals?
12. Li, Na and K are placed in the same group of the periodic
table, why?
13. Hydrogen can also be placed in group 17 (VIIA), why?
14. Why are elements of group VIIA called halogens?
15. Write electronic configuration of calcium on the basis of
sub-shell. Among magnesium and calcium, which element is
more reactive and why?
16. Why is Bromine more active than Iodine although both lie in
the same group?
17. What happens to the reactivity of non-metal when its size
increases? Why?
18. What are alkaline earth metals? Why are they called
alkaline earth metals?
19. Write down the electronic configuration of the element
which lies in second period and group 16 (VIA) in modern
periodic table based on sub-shells.
C. Long answer questions (3 marks)
1. Write any three characteristics of Modern Periodic Table.
2. Mention any three defects in Mendeleev's periodic table.
3. On the basis of which property are the elements classified as
metals, non metals and metalloids?
4. What do you mean by lanthanides and actinides? Why are
they kept separately in the periodic table?
5. Describe the significance of the modem form of periodic table
in any three points.
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6. Study the electronic configuration of elements A and B and
answer the following questions.
A = 1s2, 2s22p6, 3s23p6, 4s1
B = 1s2, 2s22p4
i) In which group of the modern periodic table do the
elements A and B lie?
ii) Write the chemical reaction between A and B.
iii) In which block of periodic table do the elements A
and B lie?
D. Very Long answer questions (4 marks)
1. How does the chemical reactivity of metals vary as we move
down in a group? Explain
2. Give the answer on the basis of the following table.
Na Mg Al Si P S Cl Ar
2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
i) In which period the elements belong to?
ii) Which blocks do Mg and Ar belongs to?
iii) Identify one active metal, one active non metal and
inert gas.
3. What variation in atomic size can be observed on moving
from top to bottom in a group of the modern periodic table?
Explain
4. What variation in atomic size can be observed on moving
from left to right in a period of the modern periodic table?
Explain the reason with example.
5. How does the chemical reactivity of non- metals vary as we
move down in a group of modern periodic table? Explain
6. Answer the following questions if atomic number of
potassium is 19.
i) Write the electronic configuration of it on the basis of
sub-shell
ii) Write its group and period in periodic table.
iii) Write the balanced chemical equation of chemical
reaction between this element and chlorine.
iv) Write its importance in plants.
7. Write down the electronic configuration of following
elements in terms of sub-shells.
Lithium, Nitrogen, Sodium, Potassium, Neon, Scandium,
Chromium, Copper, Zinc.
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8. Electronic configuration of an element is given:
1s2, 2s2, 2p6, 3s2, 3p1
To which block of the modern periodic table does the given
element belong? Write balanced equation for the reaction
between this element and chlorine. Also write two uses of the
element.
9. A small portion of modern periodic table (group IA) is given.
(i) Give two reasons for placing hydrogen along with H
metals in this group. Li
(ii) What is the common name given to the group of
Na
these elements? Why?
K
(iii) Which one is more reactive Na or K? Why?
(iv) Which element is the most reactive among these Rb
elements? Why? Cs
Project Work
Make a periodic table in a chart paper. Mark the position of s, p,
d and f-block elements. Also write the group and period numbers
of each columns and rows. Compare your work with that of your
friend.
Glossary
• Credit : praise or recognition for something done
• Contribution : the part played by somebody or something
in causing a result
• Predicated : forecast
• Octaves : an interval between two notes consisting of
eight notes
• Justification : reasoning
• Penultimate : the second last, just inner to the outermost
• Configuration : arrangement
• Successive : following in an uninterrupted sequence
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Chapter
8 Chemical
Reaction
Fritz Haber
He discovered Haber's process, Born –
Haber Cycle, Fertilizer, Chemical warfare,
explosive, etc.
Estimated Periods: 7 (5T+2P)
ObAjetctthieveesnd of the lesson, students will be able to:
• explain various types of chemical reactions;
• to write chemical equations of the chemical changes;
• to explain the factors affecting rate of chemical reactions.
We observe several changes in our daily life. The conversion of one
form of matter, energy, etc into another is called change. The change
in matter can be categorized into two types: Physical change and
Chemical change.
Physical change
A substance undergoes changes in shape, size, mass, volume, state,
etc due to physical change. For examples, tearing of paper into
pieces, melting of ice into water, dissolving salt in water, making
shirt, pants, etc from raw cloth, etc.
Thus, a physical change is a temporary change in which no new
substance is formed. The physical change can be reversed by simple
physical means.
Chemical Change
A chemical change is a permanent change which involves the change
in molecules of matter. The chemical change cannot be reversed
easily. Examples: turning of milk into curd, burning of fuel, digestion
of food, rusting of iron, growth of baby into adult, etc.
Chemical reaction
A chemical change in which combination, decomposition or exchange
of molecules of matters takes place is called chemical reaction. The
chemical reaction can be represented by word equation and formula
equation.
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All chemical reactions are represented by chemical equations. A
chemical equation is the symbolic representation of actual chemical
reaction in terms of symbols and formulae. For example, sodium
reacts with chlorine to form sodium chloride.
Sodium + Chlorine → Sodium chloride
2Na + Cl2 → 2NaCl
Product
Reactants
A chemical equation consists of two parts: reactants and products,
which are separated by an arrow (→). The substances that undergo
chemical change during a chemical reaction are called reactants.
They are written on the left hand side of the chemical equation.
Similarly, a new substance which is formed as a result of chemical
change in the reaction is called product. The products are written on
the right hand side of the equation.
Sometimes, the other conditions required for the chemical reaction
are written over an arrowhead.
For example:
Potassium Chlorate Heat → Potassium Chloride + Oxygen
Mercuric Oxide Heat → Mercury + Oxygen
Hydrogen + Chlorine Sunlight → Hydrochloric acid
A chemical equation is also known as formula equation. But, the
equation represented by the name of chemical substances in words
instead of molecular formula is called word equation. The formula
equation which holds an unequal number of atoms of one or more
elements on either side of arrow is called unbalanced equation. The
unbalanced equation is also called skeleton equation. For example:
Word Equation:
Hydrogen peroxide → Water + oxygen
Skeleton equation (formula equation):
H2O2 → H2O + O2
Balanced equation:
2H2O2 → 2H2O + O2
In a balanced chemical equation, the number of atoms of each element
is same on both sides of the arrow. In the above chemical equation,
the number of hydrogen and oxygen atoms is same in both sides of
the arrow.
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Balanced chemical equation
The chemical reactions are represented by chemical equations using
molecular formula. But these equations may not contain same
number of atoms on both sides. To make the number of atoms of each
element equal on both sides of the equations, the chemical equation
should be balanced. The unbalanced equation can be balanced by
comparing the number of atoms on two sides. Hence, a chemical
equation in which the number of atoms of each element on both sides
of the equation is equal is called a balanced chemical equation.
Balancing of chemical equation is done by writing coefficients before
the symbols or formula in such a way that the total numbers of
atoms of each element on both sides becomes equal. This method of
balancing chemical equation is called hit and trial method. The hit
and trial method is also known as hit and success method.
Method of writing balanced chemical equation
Use the following rules for writing balanced chemical equations:
(a) Write down the chemical equation in the form of word
equation. For example;
Potassium Chlorate Heat → Potassium Chloride + Oxygen
(b) Write down the symbol or molecular formula of every element
or compound involved as reactant or product. For example;
KClO3 ∆ → KCl + O2
In case of elements, symbols are written. But in case of
compounds, molecular formula is used. However, molecular
formulae are used in case of diatomic elements such as H2,
N2, O2, F2, Cl2, Br2 and I2.
(c) Write a suitable coefficient (number) to the atom or molecule
to make the number of atoms either side equal.
2KClO3 ∆ → 2KCl + 3O2
The digit used for multiplying a molecule indicates its number.
For example, 2KClO3 means two molecules of KClO3, where
there are 2 atoms of potassium, 2 atoms of chlorine and 6
atoms of oxygen.
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Examples:
1. Chemical reaction: When silver bromide is exposed to light,
silver and bromine are produced.
Word equation:
Silver bromide light → Silver + Bromine
Formula equation:
AgBr light → Ag + Br2
Balanced equation:
2AgBr light → 2Ag + Br2
2. Chemical reaction: When a mixture of nitrogen and hydrogen
under high pressure is heated in the presence of catalyst,
ammonia is produced.
Word equation:
Nitrogen + Hydrogen Heat, pressure Ammonia
Formula equation: Catalyst, promoter
N2 + H2 Heat, pressure NH3
Catalyst, promoter
Balanced equation:
N2 + 3H2 Heat, pressure 2NH3
Catalyst, promoter
3. Chemical reaction: When calcium carbonate is heated, calcium
oxide and carbon dioxide are produced.
Word equation:
Calcium carbonate ∆ → Calcium oxide + Carbon dioxide
Formula equation:
CaCO3 → CaO + CO2
The equation is balanced itself.
4. Chemical reaction: The strong heating of silver nitrate gives
silver, nitrogen dioxide and oxygen.
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Word equation:
Silver nitrate ∆ → Silver + Nitrogen dioxide + Oxygen
Formula equation:
AgNO3 ∆ → Ag + NO2 + O2
Balanced equation:
2AgNO3 ∆ → 2Ag + 2NO2 + O2
5. Chemical reaction: When a mixture of ammonium chloride
and calcium hydroxide is heated, ammonia gas, calcium chloride
and water are produced.
Word equation:
Ammonium Chloride + Calcium hydroxide → Calcium Chloride +
Ammonia + Water
Skeleton equation:
NH4Cl + Ca(OH)2 → CaCl2 + NH3 + H2O
Balanced chemical equation:
2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O
Endothermic and exothermic reactions
The reactions which require heat are called endothermic reactions.
The compounds formed by the absorption of heat are called
endothermic compounds. For example:
CaCO3 (s) ∆ → CaO (s) + CO2 (g)
2KClO3 (s) ∆ → 2KCl (s) + 3O2 (g)
N2 + O2 3000°C → 2NO (Nitric oxide)
The reactions which produce heat are called exothermic reactions.
The compounds formed by exothermic reactions are called exothermic
compounds. For example,
C (s) + O2 (g) → CO2 (g) + heat
CaO (s) + H2O → Ca (OH)2 (aq) + heat
CH4 (g) + 2O2 (g) → CO2 (g) +2H2O (g) + heat
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Reversible and irreversible reactions
The reaction which occurs in forward as well as backward direction
is called reversible reaction. For example,
N2 (g) + 3H2 (g) 4500C,200–900atm 2NH3 (g)
Fe, Mo
In the above equation, the forward reaction means the combination
of nitrogen and hydrogen to give ammonia. But the decomposition of
ammonia back to nitrogen and hydrogen is the backward reaction.
A reaction which occurs in forward direction only is called irreversible
reaction. For example,
2Na + 2H2O →2NaOH + H2↑
C (s) + O2 (g) → CO2 (g)
Conditions required for chemical reaction
There are many factors which affect the rate of chemical reaction.
Some of the important factors are given below:
1. Simple contact: Some substances react with each other when
brought in contact with each other. For example: when sodium
is added to the jar containing chlorine, sodium chloride (a salt) is
formed. Similarly, when hydrogen gas is mixed with fluorine gas,
hydrogen fluoride is obtained.
2Na + Cl2 → 2NaCl
H2 + F2 →2HF
Fact &Reason
Rate of chemical reaction increases with the increase in the surface area, why?
When surface area of the reactants increases, more reacting molecules come in
contact and collide with each other. Thus, they undergo chemical reaction quickly.
2. Contact by solution: Many reactions occur in the form of their
aqueous solution. For example: Sodium chloride (NaCl) and
silver nitrate (AgNO3) do not react in their solid state. But when
aqueous solutions of these substances are mixed, they react to
form precipitate of silver chloride (AgCl).
AgNO3 (aq.) + NaCl (aq.) → AgCl↓ + NaNO3 (aq.)
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Fact &Reason
How does concentration of sodium thiosulphate affect the rate of chemical
reaction?
When the concentration of the sodium thiosulphate increases, the collision of
reacting molecules also increases. Hence, the rate of chemical reaction increases.
3. Heat: Heat increases the rate of chemical reaction. Heat gives
energy to the molecules of the reactants and makes them collide
with each other. The collision between the molecules brings
about chemical reaction. For example: When calcium carbonate
is heated, calcium oxide and carbon dioxide are formed.
CaCO3 ∆ → CaO + CO2
4. Light: Some of the reactions take place in the presence of
light. For example: When silver bromide is exposed to sunlight,
it decomposes to from silver and liquid bromine. Similarly,
hydrogen and chlorine combine to form HCl in the presence of
sunlight.
2AgBr sunlight → 2Ag + Br2
H2 + Cl2 sunlight → 2HCl
5. Pressure: High pressure is a necessary condition for many
chemical reactions. For example: Nitrogen reacts with Hydrogen
under high pressure to give ammonia.
N2 + 3H2 4500C,200–900atm 2NH3
Fe, Mo
Fact &Reason
How is rate of chemical reaction affected by pressure in gaseous reactant?
In gases reactants, pressure increases the rate of chemical reaction.
6. Electricity: Electricity brings about both combination and
decomposition reaction. When electricity is passed through
acidified water, the water decomposes to form hydrogen and
oxygen gas.
2H2O H2SO4 2H2 + O2
electricity
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Similarly, when electric spark is passed through a mixture of
hydrogen and oxygen, they unite to form water.
2H2 + O2 electric−spark → 2H2O
7. Catalyst: A catalyst is a chemical substance which increases or
decreases the rate of chemical reaction without being consumed
itself. The process of changing the rate of chemical reaction by
the use of catalyst is called catalysis. Many chemical reactions
take place in the presence of catalyst. The catalysts are classified
into two types:
a. Positive catalyst
b. Negative catalyst
a. Positive Catalyst: The catalyst which increases the rate of
chemical reaction without being consumed itself is called positive
catalyst. For example, when potassium chlorate is heated
strongly,it decomposes to produce potassium chloride and oxygen.
2KClO3 360°C → 2KCl + 3O2
(No use of catalyst, so the reaction occurs at high temperature)
But when manganese dioxide (MnO2) is added, the reaction takes
place at lower temperature.
2KClO3 240°C
MnO2 2KCl + 3O2
(Use of Catalyst, so the reaction occurs at reduced temperature)
MnO2 also acts as positive catalyst in the decomposition of
hydrogen peroxide (H2O2).
2H2O2 MnO2 → 2H2O + O2↑
b. Negative catalyst: A catalyst which decreases the rate
of chemical reaction without being consumed itself is called
negative catalyst. For example, glycerine or phosphoric acid acts
as a negative catalyst in the decomposition of hydrogen peroxide.
2H2O2 glycerine 2H2O + O2↑
(slowed rate)
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Fact &Reason
How is rate of chemical reaction affected by catalysts?
Positive catalyst like MnO2 increases the rate of chemical reaction whereas
negative catalyst like glycerin decreases the rate of chemical reaction.
Types of chemical reactions
The chemical reactions can be classified into following types on the
basis of nature of the reactions.
1. Combination or addition reaction: The chemical reaction in
which two or more substances combine to form a new substance
is called combination reaction. For example:
2H2 (g) + O2 (g) → 2H2O (l)
2Na (s) + Cl2 (g) →2NaCl (s)
N2 (g) + 3H2 (g) 4500C, 200-900 atm 2NH3 (g) + heat
Fe+Mo
Fe(s) + S (s)→ FeS(s)
2. Decomposition or dissociation reaction: The chemical
reaction in which a compound decomposes into two or more
simpler substances is called decomposition reaction. It is just
the opposite of a combination reaction. A decomposition reaction
occurs due to heat, light, electricity, catalyst, etc. Examples:
a. By heat: Heat decomposes oxides, carbonates, chlorates,
nitrates, etc of metals. Sometimes, heat as well as catalyst are
needed.
2PbO2 (s) ∆ → 2PbO (s) + O2 (g)
CaCO3(s) ∆ → CaO (s) + CO2 (g)
2KClO3(s) 240°C 2KCl (s) + 3O2 (g)
MnO2
2Pb(NO3)2 ∆ → 2PbO + 4NO2 + O2 ↑
b. By electricity: Electricity decomposes liquid or molten states
of the compounds.
2H2O (l) Electricity 2H2 (g) + O2 (g)
acid
2NaCl (molten) Electricity → 2Na (s) + Cl2 (g)
c. By light: Silver bromide is decomposed to silver and bromine
by light.
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2AgBr (s) light → 2Ag (s) + Br2 (l)
This reaction is useful in photography.
d. By Catalyst: Manganese dioxide acts as a positive catalyst in
the decomposition of hydrogen peroxide.
2H2O2 (aq) MnO2 → 2H2O (l) + O2 (g)
3. Displacement or replacement reaction: The chemical
reaction in which an element or radical of a compound is
replaced by another element or radical to form new compounds
is called displacement reaction. Generally, a more reactive
element displaces a less reactive element from its compound.
Displacement reaction can be divided into two types:
a. Single displacement reaction
b. Double displacement reaction
a. Single displacement reaction: The chemical reaction
in which an element from a compound is replaced by another
element to form a new compound is called single displacement
reaction. For example,
Zn(s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
Here, zinc displaces hydrogen from sulphuric acid.
b. Double displacement reaction: A chemical reaction
in which two reacting compounds exchange corresponding
elements or radicals to form two new compounds is called
double displacement reaction. Such reactions are shown by ionic
compounds in solution state. Examples:
AgNO3 (aq) + NaCl (aq) → NaNO3 (aq) + AgCl ↓
2AgNO3 (aq) + CaCl2 (aq) → Ca(NO3)2 (aq) + 2AgCl ↓
4. Acid-base or Neutralization reaction: A chemical reaction in
which an acid reacts with a base to form salt and water is called
acid base reaction. This reaction is also called neutralization
reaction because acid and base neutralize each other to form salt.
A salt in general is a neutral compound. Examples:
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
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acid base salt water
H2SO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + 2H2O (l)
HNO3 (aq) + KOH (aq) → KNO3 (aq) + H2O (l)
Rate of chemical reaction
The decrease in the concentration of reactants or the increase in
concentration of products per unit time is called rate of chemical
reaction or simply reaction rate. It shows the speed at which
reactants are transformed into products. The rate of reaction provides
information whether a particular reaction is slow or fast. The rate of
reaction is affected by four main factors, which are discussed below:
1. Temperature : In general, the increase in temperature increases
the rate of chemical reaction. The increased temperature
vibrates or moves the molecules of the reactants. This makes
the reacting molecules come close to each other rapidly. If the
molecules come closer, they collide with each other quickly and
the reaction becomes faster.
2. Pressure : Pressure mainly affects the chemical reaction of
gaseous reactants. The increased pressure improves the rate
of reaction of gaseous reactants. When gaseous reactants are
mixed for reaction, their molecules are far apart from each other.
If pressure is increased in the vessel containing gas molecules,
their volume decreases. This makes molecules come closer and
collide with each other rapidly. The effective collision of such
molecules results in products and the reaction becomes faster.
3. Catalyst : A catalyst has a special property to increase or decrease
the rate of chemical reaction without being consumed itself. Its
concentration and composition remains unchanged at the end of
the reaction. The use of a positive catalyst increases the rate of
reaction. A catalyst lowers the energy required to break the bond
between the molecules of reactants. Thus, even the lower energy
causes the reaction and it occurs at faster rate.
4. Surface area : The greater surface area of reactants increases the
rate of reaction. The surface area increases if the solid reactants
are crushed into their powder. In powder form, more molecules are
exposed to the other reacting molecules. The exposed molecules
collide with each other and the reaction becomes faster.
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Learn and Write
1. Heat increases the rate of chemical reactions. Why?
Heat gives energy to the molecules of the reactants and makes
them collide with each other. The collision between the molecules
brings the chemical change faster.
2. Acid-base reaction is called neutralization reaction. Why?
As a result of acid-base reaction, salt and water are produced.
Generally salts are neutral. Therefore, acid-base reaction is
called neutralization reaction.
3. What happens when magnesium ribbon is burnt in air?
When magnesium ribbon is burnt in air, it gives magnesium oxide.
2Mg + O2 → 2MgO (magnesium oxide).
Main points to remember
1. A chemical reaction is a chemical change in which exchange,
combination or decomposition occurs in the molecule of substances
to produce new substance.
2. A catalyst is a chemical substance which alters the rate of
chemical reaction but itself remains unchanged at the end of the
reaction. The process of changing the rate of chemical reaction by
the use of catalyst is called catalysis.
3. There are different types of chemical reactions. They are:
combination reaction, decomposition reaction, displacement
reaction, neutralization reaction etc.
4. A reaction in which two or more substances combine together to form
a new substance is called combination reaction.
5. A reaction in which a compound decomposes into two or more simpler
substance is called decomposition reaction.
6. A chemical reaction in which an element or radical in a
compound is displaced by another element or radical to form
new compounds is called displacement reaction.
7. A chemical reaction in which an acid reacts with a base to form
salt and water is called neutralization reaction.
Exercise
A. Very short answer questions ( 1 mark )
1. Write any two factors that affect the chemical reaction.
2. What is acid-base reaction ? Give one example.
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3. Define promotor.
4. What is a catalyst?
5. What is neutralization reaction ?
6. Define positive catalyst with an example.
7. Write any two methods to increase the rate of chemical
reaction.
8. What is endothermic reaction? Write an example.
9. What is reversible reaction? Write an example.
10. What is a combination or synthesis reaction ?
11. Define negative catalyst with an example.
12. Calcium+Sulphuric acid Calcium sulphate + Hydrogen.
Mention the types of chemical reaction.
13. Define single displacement reaction.
14. Define double displacement reaction.
B. Short answer questions (2 marks)
1. What is the difference between catalyst and promoter?
2. The rate of chemical reaction increases with the increase in
the surface area of reactant. Why?
3. How does the concentration of sodium thiosulphate affect the
rate of reaction in between the hydrochloric acid and sodium
thiosulphate?
4. Write a decomposition chemical reaction caused by light.
5. What is meant by exothermic reaction? Write with examples.
6. Write down balanced chemical equation of the reaction
between sulphuric acid and ammonium hydroxide.
7. Why is it necessary to balance a chemical reaction?
8. Write any two characteristics of a catalyst.
9. Write the balanced chemical reaction between Aluminium
metal and dilute hydrochloric acid. Mention the type of
reaction with reason.
10. Write down the effect of temperature on the rate of a
chemical reaction. How does heat enhance the rate of
chemical reaction ? Write in short.
11. Why is acid-base reaction called neutralization reaction?
12. Write the following word equation into balanced formula
equation.
Nitric acid + Calcium hydroxide Calcium nitrate + Water
13. Two chemical substances A and B when mixed at room
temperature react very slowly. Suggest two different ways of
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increasing the rate of this reaction.
14. Write a balanced chemical equation in which positive catalyst
is used.
C. Long answer questions (3 marks)
1. Write any three limitations of a chemical equation.
2. Write a balanced chemical equation to show the chemical
reaction between chlorine gas and potassium iodide. Name
the type of chemical reaction.
3. List any three information that can be obtained from a
balanced chemical equation.
4. Differentiate combination and decomposition reaction.
5. Differentiate single and double displacement reaction.
6. What happens when (Write with balanced chemical
equation):
7. i) Potassium chlorate is heated?
ii) Lead nitrate is heated?
iii) Calcium reacts with water?
iv) Phosphorus is burnt in air?
v) Mercuric oxide is heated?
vi) Potassium bromide and chlorine gas are mixed together?
vii) Sodium metal comes in contact with water?
viii) Lime water is added to dilute solution of hydrochloric
acid?
ix) Silver bromide is exposed to sunlight?
D. Very long answer questions (4 marks)
1. Write down the balanced chemical equation for the following
reaction. Write a balanced equation of the chemical reaction
between Iron chloride and Ammonium hydroxide.
2. Mention the information that can be obtained from the
balanced chemical equation.
3. Explain the factors that bring out chemical change in the
chemical reaction.
Write down the balanced chemical equations for the following reactions:
1. Aluminium + Oxygen → Aluminium Oxide.
2. Calcium bicarbonate ∆ → Calcium Carbonate + Water + Carbon
dioxide.
3. Copper Carbonate ∆ → Copper oxide + Carbon dioxide.
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4. Calcium Hydroxide + Carbon dioxide → Calcium carbonate +
Water.
5. Nitric acid + Sodium hydroxide → Sodium nitrate + Water.
6. Sodium carbonate + Hydrochloric acid → Sodium chloride + Water
+ Carbon dioxide.
7. Ammonium chloride + Sodium nitrite → Sodium chloride + Water
+ Nitrogen.
8. Calcium chloride + Silver nitrate → Calcium nitrate + Silver
chloride.
Project Work
Take some dilute sulphuric acid in a test tube. Add a granule of
zinc into it. What will you observe? Cover the mouth of the test
tube with the mouth of another test tube and hold them together
in vertical position for about 2 minutes. Introduce a burning
candle to the mouth of the upper test tube. What will you observe?
Why is it so? Explain your observations and findings.
Glossary
• Symbolic : use of symbol and formula
• Reactant : substances that undergo chemical change during
the chemical reaction
• Product : the new substances which are formed as a result
of chemical change in the reactants
• Coefficient : the number placed before a letter that represents
a variable in algebra
• Catalyst : a substance that increases the rate of a chemical
reaction without itself undergoing any change
• Precipitate : to cause a solid to separate out from a solution as
a result of a chemical reaction, or separate out in
this way
176Times' Crucial Science Book - 10
Chapter
9 Acid, Base
and Salt
Soren Sorensen
The potential of Hydrogen, or pH, scale was invented by the Danish biochemist Soren
Peter Lauritz Sorensen. He introduced this scale in 1909 to measure the acidity and
basicity of substances
Estimated Periods: 8 (6T+2P)
Objectives
At the end of the lesson, students will be able to:
• define acid, base and salt;
• explain properties of acid, base and salt;
• differentiate between base and alkali;
• explain uses of acid, base and salt;
• write and balance simple acid-base reactions.
There are various types of compounds in our surrounding. They are
categorised into three categories. They are: acid, base and salt.
These compounds are obtained from natural as well as man-made
sources. For example, the acids are obtained from fruits and vegetables
such as lemon, orange, mango, amala, etc. Acetic acid (vinegar) is the
first known acid to human and is used to make pickles. Ascorbic acid
(vitamin C), found in amala is used as medicine. Similarly, caustic
soda (sodium hydroxide) is a base which is used to manufacture
soaps. Common salt and copper sulphate are salts which are very
useful. Common salt is our kitchen salt whereas copper sulphate is
used as fungicide, electrolyte, etc.
Acids
The word acid has been derived from the Latin word acidus which
means sour in taste. Initially, the sour tasting substances were
regarded as acid. But this definition could not include all the acids.
Hence, an acid is defined as a substance which produces hydrogen
ions (H+) when dissolved in water. For example, hydrochloric acid,
sulphuric acid, nitric acid, carbonic acid, acetic acid, etc.
HCl →H+ + Cl–
H2SO4 → 2H+ + SO4– – H+ + CH3COO–
HNO3 → H+ + NO3–
CH3COOH
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In fact, hydrogen ions do not exist in solution. They immediately
combine with H2O to form H3O+. The hydrated hydrogen ion that
exists in the solution of acids is called a hydronium ion (H3O+).
H+ + H2O → H3O+
Hydrogen ion Water molecule Hydronium ion
Hence, acids are defined as compounds which produce hydronium
ions (H3O+) when dissolved in water.
Fact &Reason
Sodium hydroxide is a strong base, why?
Sodium hydroxide is a strong base because it produces high concentrations of
hydroxyl ions in its solution.
Strong and weak acids
An acid which ionizes completely to give high concentration of hydrogen
ions (H+) in aqueous solution is called strong acid. The aqueous
solution of these acids have low pH value. Generally, inorganic acids
are strong acids. For example, hydrochloric acid (HCl), sulphuric acid
(H2SO4), nitric acid (HNO3), hydroiodic acid (HI) etc.
An acid which gives low concentration of hydrogen ion (H+) in
aqueous solution is called weak acid. These acids ionize partially and
are comparatively poor conductor of electricity. For example, acetic
acid, (CH3COOH), formic acid (HCOOH), carbonic acid (H2CO3), etc.
Fact &Reason
Carbonic acid is called weak acid, why?
Carbonic acid is called weak acid because it produces low concentrations of
hydrogen ions in its solution.
Organic and inorganic acid
The acids which are obtained from living organisms (plants and
animals) are called organic acids. For example: acetic acid, formic
acid, tartaric acid, lactic acid, citric acid, malic acid, ascorbic acid, etc.
The acids which are obtained from minerals and prepared in
laboratory are called inorganic acids. For example: hydrochloric acid
(HCl), sulphuric acid (H2SO4), nitric acid (HNO3), etc.
178Times' Crucial Science Book - 10
Activity 9 .1 To identify the weak and strong acid.
Materials required: Test tubes
Chemicals required:
Pieces of magnesium ribbon, dilute sulphuric acid, acetic acid, etc.
Procedure
1. Fill about one-third of a test tube with dilute hydrochloric acid.
2. Fill about one-third of another test tube with acetic acid.
3. Drop a small piece of magnesium ribbon in both the test tubes.
Observe what happens?
Observation:
You will observe that sulphuric acid reacts with magnesium faster
than acetic acid.
Conclusion:
Sulphuric acid is a strong acid and acetic acid is a weak acid. Repeat
the above mentioned activity taking the pieces of granulated zinc
instead of magnesium ribbon. Observe the reactions and note
which one is faster. What will you conclude from the observation?
Differences between organic and inorganic acid:
Organic acid Inorganic acid
1. It is obtained from the 1. It is obtained from minerals and
plant or animal source. can be prepared in laboratory.
2. They are weak acids. 2. They are generally strong acids.
Examples: acetic acid, lactic Examples: hydrochloric acid,
acid, formic acid etc. sulphuric acid, nitric acid, etc.
Sources of organic acid
The organic acids are found naturally in fruits and vegetables. Some
of them also are present in the body of insects, other animals, etc. All
organic acids are sour in taste.
Some of the naturally occurring organic acids and their sources are
given below:
S. N Name of Acid Source
1. Acetic acid Vinegar, sour pickles etc.
2. Citric acid Lemon, orange, mandarin, tomato etc.
3. Lactic acid Milk, curd etc.
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4. Formic acid Ant bite, nettle sting etc.
5. Oxalic acid Oxalis (Chariamilo)
6. Tartaric acid Grapes, pomelo etc.
7. Ascorbic acid (Vit. C) Amala, Harro, etc.
Properties of acids
1. Acids are generally sour in taste. But the acids such as boric
acid, salicylic acid, etc do not have sour taste.
2. Aqueous solutions of acids conduct electricity.
3. Acids change the colour of indicator as follows:
Indicators Change of colour
Blue litmus paper Red
Methyl orange (Orange) Red
Phenolphthalein (Colourless) Colourless (no change)
4. Acids give hydrogen ions when dissolved in water.
HCl → H+ + Cl–
HNO3 → H+ + NO3–
H2SO4 → 2H+ + SO4– –
5. Acids react with metals to form salt and hydrogen gas.
Acid + metal → Salt + hydrogen gas
2HCl + Zn → ZnCl2 + H2↑
H2SO4 + Mg → MgSO4 + H2↑
6. Acids react with bases (metallic oxides and hydroxides) to
form salts and water.
Acid + Base → Salt + Water
HCl + NaOH → NaCl + H2O
2HCl + CaO → CaCl2 + H2O
HNO3 + KOH → KNO3 + H2O
H2SO4 + 2NaOH → Na2SO4 + 2H2O
7. Acids react with carbonates and bicarbonates to form salt,
water and carbon dioxide gas.
Acid + carbonate or bicarbonate → Salt + water + carbon dioxide
2HCl + Na2CO3 → 2NaCl + H2O + CO2
H2SO4 + MgCO3 → MgSO4 + H2O + CO2
2HCl + Ca(HCO3)2 → CaCl2 + 2H2O + 2CO2
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Fact &Reason
Acids are not kept in metallic containers, why?
Acids are very reactive to metals. So, acids are not kept in metallic containers.
Instead, they are kept in glass containers.
Uses of acids
1. Sulphuric acid and nitric acid are used to make chemical
fertilizers, drugs, explosives, etc.
2. Carbonic acid is used to make soft drinks like soda water and
other drinks.
3. The acids like HCl, H2SO4, HNO3 etc are used in laboratory
to perform different experiments.
4. Carbolic acid or Phenol (C6H5OH) is used as germicide or
disinfectant.
5. Citric acid is a source of vitamin C.
6. Acetic acid (vinegar), tartaric acid, citric acid, etc are used for
giving taste to food.
7. Picric acid, boric acid, etc are used for washing wounds.
Precautions to be taken while using acids
It is dangerous to taste or touch strong acids. Hence, following
precautions should be remembered while using acids:
a. We should never touch or taste the strong acids because they
can burn the skin.
b. We should never fill acid in metallic vessels.
c. If any part of our body comes in contact of acid, we should
wash the affected part thoroughly with water or with dilute
solution of sodium bicarbonate. But we should never use
strong alkali like NaOH or KOH because they also corrode
skin.
Base
Bases are metallic oxides or hydroxides which react with acids to
give salt and water. For example, sodium oxide, potassium oxide,
sodium hydroxide, calcium hydroxide, iron oxide etc.
Base + Acid → Salt + Water
NaOH + HCl → NaCl + H2O
CaO + H2SO4 → CaSO4 + H2O
181 Times' Crucial Science Book - 10
The bases which are soluble in water are called alkalis. The alkalis
produce hydroxyl ions (OH–) when dissolved in water. For example,
sodium hydroxide, potassium hydroxide, ammonium hydroxide,
calcium hydroxide, etc.
KOH → K+ + OH¯
Hydroxyl ion
NaOH → Na+ + OH ¯
NH4OH is different base than other bases because it does not
contain metal in it.
Activity 9 .2 To prepare a base using magnesium (metal).
Materials required:
Beaker, a burning candle, a glass rod, funnel, filter paper, stand, etc.
Chemicals required:
A piece of magnesium ribbon, water, phenolphthalein (indicator) etc.
Procedure
1. Hold a small piece of magnesium ribbon with the help of tongs.
2. Burn the magnesium ribbon with the help of burning
candle and hold it above a white sheet of paper. Burning of
magnesium in the air produces a white powder of magnesium
oxide which is a base.
3. Now, take a beaker with little water and add the white
powder into it. Stir the mixture to dissolve completely. Add a
little more water if needed.
4. Filter the solution and take a small quantity of solution in a
test tube.
5. Add a drop of phenolphthalein into the test tube and
observe the change in colour.
Observation:
The solution of the test tube turns pink.
Conclusion:
Magnesium burns in the oxygen of air to form magnesium oxide.
This oxide dissolves in water to form an alkali, magnesium
hydroxide. An alkali (base) turns the colour of phenolphthalein
into pink.
182Times' Crucial Science Book - 10
Reactions involved:
Magnesium + Oxygen ∆ → Magnesium oxide
2Mg + O2 ∆ → 2MgO (a base)
Magnesium oxide + water → Magnesium hydroxide
MgO + H2O → Mg(OH)2 (an alkali)
Preparation of bases
The bases can be prepared in the following ways:
1. Direct combination of metal with oxygen: Sodium directly
combines with the oxygen of atmosphere to form sodium oxide.
But other metals combine with oxygen while heating.
4Na + O2 → 2Na2O
4Al + 3O2 ∆ → 2Al2O3
2Ca + O2 ∆ → 2CaO
2Cu + O2 → 2CuO
2. Action of water on soluble metal oxides: The oxides of very
active metals such as sodium, potassium, calcium, magnesium,
etc react with water to produce alkali.
Na2O + H2O →2NaOH
CaO + H2O → Ca(OH)2
K2O + H2O → 2KOH
3. Direct reaction of metal with water: Sodium, potassium and
calcium directly react with water to produce hydrogen gas and
alkali.
2Na + 2H2O → 2NaOH + H2↑
Ca + 2H2O → Ca(OH)2 + H2↑
4. Decomposition of metallic carbonates on heating: When
calcium carbonate is heated, it decomposes to calcium oxide and
carbon dioxide.
CaCO3 ∆ → CaO + CO2
All oxides and hydroxides of metal are bases. Some bases are soluble
in water while most of them are insoluble. The bases like KOH,
Mg(OH)2, NaOH, Ca(OH)2 etc are water soluble bases and are called
183 Times' Crucial Science Book - 10
alkalis. Cupric oxide, aluminium oxide, ferric oxide, silver oxide
etc. are also the bases because they are the oxides of metal. But
these oxides are insoluble in water and hence they are not alkalis.
Therefore, all alkalis are bases but all bases are not alkalis.
Strong and weak alkalis (bases)
The alkalis which completely dissociate in water to produce high
concentration of hydroxyl (OH–) ions are called strong alkalis. They
have high pH value. For example, NaOH, KOH etc.
The alkalis which dissociate partially in water to produce less
concentration of hydroxyl (OH–) ion are called weak alkalis. For
example, Cu(OH)2, Fe(OH)3, Al(OH)3, etc.
Differences between bases and alkalis:
Bases Alkalis
1. All oxides and hydroxides of 1. Only hydroxides of very
metal are called bases. active metals are alkalis.
2. They may be soluble or 2. They are soluble in water.
insoluble in water.
3. All bases are not alkalis. 3. All alkalis are bases.
Examples: CuO, NaOH, CaO, Examples: NaOH, KOH etc.
KOH, HgO, Ag2O, Fe(OH)2 etc.
Properties of bases
1. The bases are soapy (slippery) to touch.
2. They have bitter taste.
3. They turn the colour of indicators as follows:
Indicators Change of colour
Red litmus (red) Blue
Methyl orange (Orange) Yellow
Phenolphthalein (Colourless) Pink
4. Water soluble bases produce hydroxyl ion when dissolved in
water.
NaOH → Na+ + OH -
Ca(OH)2 H2O Ca+ + + 2OH -
5. They react with acids to form salt and water. The reaction
of base with acid to form neutral salt and water is called
neutralization reaction.
184Times' Crucial Science Book - 10
Base + Acid → Salt + Water
Examples:
NaOH + HCl → NaCl + H2O
Ca(OH)2 + 2HCl → CaCl2 + 2H2O
6. Alkali solution reacts with carbon dioxide to form metallic
carbonate and water.
Examples:
2NaOH + CO2 → Na2CO3 + H2O
2KOH + CO2 → K2CO3 + H2O
Mg(OH)2 + CO2 → MgCO3 + H2O
7. They react with some salts to form insoluble metallic
hydroxide.
Examples:
2NaOH + CuSO4 → Cu(OH)2 + Na2SO4
FeCl3 + 3NH4OH → Fe(OH)3 + 3NH4Cl
8. They react with ammonium salt on heating and give ammonia gas.
Examples:
NaOH + NH4Cl → NaCl + H2O + NH3↑
Ca(OH)2 + 2NH4Cl → CaCl2 + 2H2O + 2NH3↑
Ca(OH)2 + (NH4)2CO3 → CaCO3 + 2H2O + 2NH3↑
Uses of bases
1. Sodium hydroxide is used in the manufacture of soap, paper,
rayon etc. It is also used in refining of petroleum products.
2. Quick lime (CaO) is used for softening hard water, production
or purification of sugar, manufacture of washing soda etc.
3. Calcium hydroxide is used for white washing of buildings,
softening hard water etc. Calcium hydroxide is also used to
manufacture mortar and bleaching powder.
4. Ammonium hydroxide is used for the manufacture of
fertilizers, plastics, dyes etc.
5. Potassium hydroxide is used in the manufacture of soft soaps.
6. Magnesium hydroxide (milk of magnesia) and aluminium
hydroxide are used as antacid to control acidity in stomach.
7. Sodium hydroxide, potassium hydroxide and ammonium
hydroxide are very important lab reagents.
185 Times' Crucial Science Book - 10
Difference between acid and base
Acid Base
1. It gives hydrogen ion (H+) 1. Water soluble base gives
when dissolved in water. hydroxyl ion (OH–)ion when
dissolved in water.
2. It turns blue litmus in to red. 2. It turns red litmus in to blue.
3. It does not change the colour 3. It changes the colourless
of Phenolphthalein. solution of Phenolphthalein
to pink.
4. It reacts with a base to form 4. It reacts with an acid to
salt and water. form salt and water.
5. It has sour taste. 5. It has bitter taste.
6. Its pH is less than 7. 6. Its pH is more than 7.
Salt
A salt is formed by the neutralization of an acid by a base. The salts
are generally neutral to indicators. A salt is a compound which
is formed by the partial or complete replacement of one or more
hydrogen atoms of an acid by one or more metal atoms or positive
radicals.
For example, sodium chloride is a salt which is formed by the
replacement of one hydrogen atom of an acid by one sodium atom.
NaOH + HCl → NaCl + H2O
If sodium replaces one hydrogen atom from sulphuric acid (partial
replacement) sodium bisulphate is formed. But the replacement of
both hydrogen atoms from sulphuric acid by sodium produces sodium
sulphate (complete replacement).
NaOH + H2SO4 → NaHSO4 + H2O (partial replacement)
Sodium bisulphate
2NaOH + H2SO4 → Na2SO4 + H2O (complete replacement)
Sodium sulphate
Each salt molecule contains two radicals: basic radical (electropositive
radical) and acidic radical (electronegative radical). The radical that
comes from a base and carries positive charge is called basic radical
whereas the radical that comes from an acid and carries negative
charge is called acidic radical. Therefore, the basic radical is known
as metallic radical and the acidic radical is known as non-metallic
radical. For example, in the salt NaCl, Na+ is basic radical and Cl– is
acidic radical.
186Times' Crucial Science Book - 10
Preparation of salts
Salts can be prepared by the following methods:
1. Direct combination of metals with non-metals:
Some active metals such as sodium, calcium, iron, etc react
directly with non-metals to form salts.
2Na + Cl2 → 2NaCl (Sodium chloride)
2Fe + 3Cl2 → 2FeCl3 (Ferric chloride)
2. Action of acids on metals:
Metals react with acids to produce salt and hydrogen gas.
Zn + H2SO4 → ZnSO4 + H2↑
Mg + 2HCl → MgCl2 + H2↑
Cu + H2SO4 → CuSO4 + SO2 + H2O
3. Action of acids on metallic oxide: Metal oxides react with
acid to form salt and water.
Na2O + 2HCl → 2NaCl + H2O
CuO + H2SO4 → CuSO4 + H2O
4. Neutralization of an acid by an alkali (hydroxide base):
The reaction of an alkali with an acid produces salt and water.
NaOH + HCl → NaCl + H2O
Strong alkali strong acid Neutral salt
H2SO4 + Zn(OH)2 → ZnSO4 + H2O
Acidic salt
Strong acid Weak alkali
H2CO3 + 2NaOH → Na2CO3 + 2H2O
Basic salt
Weak acid Strong alkali
CH3COOH + NH4OH → CH3COONH4 + H2O
Neutral salt
Weak acid weak alkali
(Ammonium ethanoate)
Salts formed by the reaction of strong acid and strong base
are generally neutral. If the salts are formed by the reaction
of strong acid and weak base, they are acidic. If the salts are
formed by the reaction of weak acid with strong base, they are
alkaline. Similarly, salts formed from weak acid and weak base
are neutral.
5. Action of acid on metallic carbonates: The action of acid on
metallic carbonates also produces salts.
CaCO3 + 2HCl → CaCl2 + H2O + CO2↑
187 Times' Crucial Science Book - 10
Types of salts
There are three types of salts. They are:
(a) Normal salts: (Neutral) It is a salt formed by complete
replacement of hydrogen atoms of an acid by a metal. A
normal salt is generally neutral to indicators. For example
NaCl, KCl, BaSO4, Na2SO4, NH4Cl etc,
(b) Acidic salts: The salt which is formed by the neutralization
reaction between strong acid and weak base is called acidic
salt.
Example: Acidic salts are also produced by partial
displacement of hydrogen of acid by metal. etc.
CuSO4, NaHSO4, (NH4)2SO4.
Fact &Reason
Why is ammonium sulphate called acidic salt?
Ammonium sulphate is formed by the neutralization of sulphuric acid (strong
acid) and ammonium hydroxide (weak base). Thus, ammonium sulphate is called
acidic salt.
(c) Basic salts: The salt which is formed by the neutralization
reaction between weak acid and strong base is called basic
salt. Such salts are alkaline in nature. Examples: Na2CO3,
CH3COONa, CH3COOK, etc.
(d) Hydrated salts: The salts which contain water molecules in
their crystals are called hydrated salts. The water molecules
present in the crystals of salts are called water of crystallization
or water of hydration. Each salt contains a specific number
of water of crystallization. Examples of hydrated salts: Blue
vitriol (copper II sulphate pentahydrate, CuSO4.5H2O), white
vitriol (zinc sulphate heptahydrate, ZnSO4.7H2O), Epsom salt
or magnesium sulphate heptahydrate (MgSO4.7H2O), sodium
sulphate decahydrate (Na2SO4.10H2O), etc.
Although CuSO4 is a normal salt formed by the complete replacement
of hydrogen atoms of sulphuric acid by copper, it shows acidic nature
in aqueous solution due to hydrolysis. Copper sulphate ionizes into
Cu++ and SO4– –
dissociates into in its aqueous solution. At the same time, water also
H+ and OH– ions. Thus H+ ions react with SO4– – ions
to form sulphuric acid, which is a strong acid. But Cu++ ions combine
with OH– ions to form a weak base, Cu(OH)2. As a result the solution
contains more concentration of H+ ions and becomes acidic.
188Times' Crucial Science Book - 10
CuSO4 + 2H2O → H2SO4 + Cu(OH)2
Similarly, the aqueous solution of Na2CO3 is alkaline due to hydrolysis
reaction.
Na2CO3 + 2H2O → H2CO3 + 2NaOH
Properties of salts
The salts have a variety of properties, some of which are given below:
(1) Sodium chloride has a characteristic salty taste. But most of
the salts taste bitter. Some salts are tasteless too.
(2) Salts are generally neutral to indicators (e.g. NaCl, KCl,
KNO3 etc.). Some salts are acidic (CuSO4) whereas some are
alkaline (basic), e.g. Na2CO3.
(3) Salts of metals like Na, K, Al, Mg, Ca, and Ba are colourless
(white) whereas the salts of the transition metals such as Fe,
Co, Ni, Cu, Mn, Cr, etc, are coloured.
(4) Most of the salts dissolve in water. All sodium, potassium and
ammonium salts, all bicarbonates and nitrates, all chlorides
(except those of silver and lead) and all sulphates (except
those of lead and barium) are soluble in water.
(5) The salts are electro valent or ionic compounds. They conduct
electricity in their fused or aqueous solution state due to
ionization.
Uses of salts
The salts are very useful in our daily life. The uses of some of the
salts are as follows:
S Name of Mol. formula Common Uses
No. Salt name
NaCl As edible salt. In
1. Sodium Common the manufacture of
chloride salt sodium hydroxide,
washing soda, etc.
2. Sodium Na2CO3.10H2O Washing
carbonate soda In the manufacture
of glass, soap and
detergent. It is also
used to remove
hardness of water.
189 Times' Crucial Science Book - 10
3. Sodium NaHCO3 Baking It is used as
soda baking powder,
bicarbonate fire extinguisher,
Blue to reduce acidity of
4. Copper CuSO4.5H2O vitriol stomach, etc.
CaSO4.2H2O Gypsum
sulphate For electroplating,
Green preserving wood, as
5. Calcium vitriol fungicide etc.
sulphate White
vitriol To manufacture
6. Ferrous FeSO4.7H2O - chalk, in the
sulphate manufacture of
cement, etc.
7. Zinc ZnSO4.7H2O
sulphate Used as medicine, in
factories, etc.
8. Ammonium (NH4)2SO4
sulphate Used in eye medicine,
for white pigment etc.
As a chemical
fertilizer.
9. Calcium CaSO4.21 H2O Plaster of For plastering
sulphate paris fractured bones of
body, for making
10. Magnesium MgSO4.7H2O Epsom ceramics and statues,
sulphate salt etc.
Sal
11. Ammonium NH4Cl ammoniac Used in medicine, dye
chloride in industry etc.
Calamine
12. Zinc ZnCO3 As a good electrolyte
in dry cell, as a lab
carbonate reagent, etc.
For making ointment
for curing skin
diseases.
Indicator
An indicator is a chemical compound which indicates the acidic, basic
or neutral nature of a substance by changing its colour. The common
indicators are litmus paper, methyl orange and phenolphthalein.
Litmus paper is the most commonly used indicator in a laboratory. It
is prepared from a plant called lichen.
The change of colour of common indicator in acidic, basic and salt
solutions is given below:
190Times' Crucial Science Book - 10
S. Indicator and Colour in Colour in Colour in
N. its colour acidic soln alkaline salt soln
soln
1. Blue litmus paper (blue) Red No change No change
2. Red litmus paper (red) No change Blue No change
3. Methyl orange (orange) Red Yellow Faint
orange
4. Phenolphthalein Colourless Pink (red) No change
(colourless) (No change)
5. Red cabbage juice (red) Red Green Faint
purple
Indicators such as litmus, methyl orange and phenolphthalein
indicate whether a substance is acidic, alkaline or neutral but they
cannot measure the strength of acid and alkali.
Universal indicator
An indicator prepared by mixing several ordinary indicators of
different colours is called universal indicator. It shows different
colours in the solutions of acids or alkalis of different concentrations.
It indicates the acidic or basic character of a substance as well as the
strength of acid and base.
A red or deep red colour in the universal indicator indicates acidity
whereas blue or deep blue colour denotes alkalinity. The strength of
acid or base can be determined by matching the change in colour of
universal indicator with the colour in pH chart. pH value of neutral
substance is 7 pH value of base is more than 7 and pH value of acid
is less than 7
Application of neutralization reaction
The treatment of insect bites and acidity of stomach involves the use
of neutralization reaction. The bite of an ant or stinging of nettle
injects an acid called formic acid into our skin. The acid causes pain
with itching and swelling in the affected part. The effect of this acid is
neutralized by using calamine lotion (ZnCO3) or sodium bicarbonate.
Zinc carbonate or sodium bicarbonate reacts with the acid and
nullifies its effects. Thus, the pain is relieved. Similarly, the excess
of hydrochloric acid secreted in stomach causes hyper-acidity. To
neutralize the effect of acid, mild alkali, such as sodium carbonate is
taken. A mixture of magnesium hydroxide and aluminium hydroxide
191 Times' Crucial Science Book - 10
is also used to get relief from hyper-acidity. This mixture is commonly
called antacid.
pH of soil
Optimum pH of soil is necessary for the growth of crops, fruits and
other plants. Some plants grow well in acidic soil, some in basic
whereas some grow well in the neutral pH.
If the acidity of the soil is increased beyond limit, it can cause harms
in the growth of plants. The increased acidity of soil is removed by
adding necessary amount of calcium hydroxide in the soil.
Main points to remember
1. An acid is a substance which gives hydrogen ion when dissolved
in water.
2. The hydrated hydrogen ion that exists in the solution of acids is
called hydronium ion.
3. Bases are the metallic oxides or hydroxides which react with
acid to give salt and water.
4. The bases which are soluble in water are called alkalis.
5. A salt is a compound which is formed by partial or complete
replacement of one or more hydrogen atoms of an acid by
equivalent number of metal atoms or electropositive radical.
6. An indicator is a chemical compound which indicates the acidic,
basic or neutral nature of a substance by changing its colour.
7. An indicator prepared by mixing several organic indicators of
different colours is called universal indicator.
8. The pH of a solution is defined as the measure of hydrogen ions
present in the given solution.
Exercise
A. Very short answer questions (1 mark)
1. Define acid. Give any two examples.
2. Why is hydrogen sulphate called an acid?
3. What are bases? Give any two examples.
4. Define alkali. Give any two examples.
5. What is salt? Give any two examples.
6. Write any two physical properties of acids.
7. What is neutralization reaction?
8. What are strong acids? Give any two examples.
9. Define weak acids with any two examples.
192Times' Crucial Science Book - 10
10. Write the name and molecular formula of the salt which is
used as an electrolyte in the dry cell.
11. What is the pH value of substance formed by the chemical
reaction between sodium hydroxide and hydrochloric acid?
12. Define basic salt with two examples.
13. What type of salt is formed when strong acid reacts with
weak base? Write a molecular formula of any such salt.
14. What is pH value of neutral substance ?
15. Write down any two physical properties of alkalis.
16. Which acid is found in human stomach?
17. Give any two examples of organic acid.
18. Write down a name of alkali which is used to balance the pH
of human stomach.
19. Name any two bases with their molecular formula.
20. What is an indicator? Give any two examples.
21. Write two examples of weak acids.
22. What do you mean by universal indicator?
B. Short answer questions (2 marks)
1. Write down any two differences between acids and bases.
2. Though both sodium oxide and mercury oxide are metal
oxides, sodium oxide is called alkali but mercury oxide is not.
Why?
3. Write any two differences between neutral salt and acidic salt.
4. Acetic acid is called a weak acid, why?
5. Ammonium hydroxide is called a weak alkali, why?
6. Sodium hydroxide is called a strong alkali but ammonium
hydroxide is called a weak alkali, Why?
7. Sodium bicarbonate is called an acidic salt, why?
8. All alkalis are bases but all bases are not alkalis, why?
9. Write any two differences between bases and alkalis.
10. Write any two properties of acid.
11. The taste of acid is sour. Why?
12. Hydrochloric acid is kept in glass bottle, why?
13. Write any two methods to test alkali.
14. Why is sodium chloride called a salt?
15. Why is Phenolphthalein not much appropriate to identify
whether the given solution is acid, base or salt?
16. What is the use of finding out the pH of soil?
193 Times' Crucial Science Book - 10
17. Though hydrochloric acid is formed inside our body, it is not
organic acid. Why?
18. Write any two chemical properties of base with formula
equations.
19. Solution of ammonia is a weak alkali, why?
20. Why is acid base reaction called neutralization reaction?
22. “All the bases are not alkalis. “Justify it.
23. Why is it suggested to use soap to a person suffering from
bee sting?
24. What happens when
i. calcium carbonate is treated with dilute hydrochloric
acid?
ii. carbon dioxide gas is passed through NaOH solution?
iii. ammonium salt is treated with strong alkali?
iv. hydrochloric acid reacts with sodium hydroxide?
25. Write down any two chemical properties of alkalis with
chemical reactions.
26. Write down any two chemical properties of acid with
chemical reactions.
C. Long answer questions (3 marks)
1. In an unknown solution, red litmus paper is inserted and
colour remains unchanged then what may be contained in
vessel among acid, base and salt solution. How can it be
further tested to confirm it?
2. Write any three uses of acids.
3. Write down the uses of salts.
4. Write down the main uses of bases.
5. How can you change a magnesium ribbon into alkali?
Explain with chemical reaction.
6. A chemical substance gives hydrogen ion and chloride
ion when dissolved in water. Similarly, another chemical
substance gives sodium ion and hydroxide ion. Write their
molecular formula and also chemical reaction between them.
7. Write down the name and molecular formula of the
compound which releases hydrogen and nitrate ions in
water. What will be the colour of methyl orange in this
solution?
194Times' Crucial Science Book - 10
D. Very long answer questions (4 marks)
1. Write any four methods of preparation of salts.
2. Complete the following table:
SN. Indicator and Colour Colour Colour in
its colour in HCl in NaOH NaCl
solution solution
1. Red litmus solution
2. Blue litmus
3. Methyl orange
4. Phenolphthalein
3. Write down the name and molecular formula of the compound
which gives hydrogen and chloride ions when dissolved in
water. If few drops of methyl orange are added to this solution,
what will be its colour? Why? What will happen if the above
mentioned compound is treated with calcium oxide? Write
with balanced chemical equation.
4. Three different solutions such as caustic potash, hydrochloric
acid and salt solution are placed in three different beakers. How
would you identify each solution with the help of methyl orange
and phenolphthalein?
5. The production of gases like CO2, SO2 etc. from industries can
affect our homes and historical monuments. Explain with reasons.
Complete the following equations and balance them if necessary.
1. Zn + ............ → ZnSO4 + H2
2. Ca(OH)2 + ………… → CaCO3 + ……………
3. Cu + Conc. H2SO4 → …………….. + ………….. + ……………
4. H+ + H2O → ……………
5. Na2O + …………. → NaOH
6. CH3COOH + NH4OH → …………. + …………….
7. H2CO3 + NaOH → ...................... + .....................
195 Times' Crucial Science Book - 10
Project Work
Take solutions of acid, alkali and salt in three different beakers.
Study the change in colour of the indicators in these solutions by
taking a little of solution in different test tubes and adding the
indicators.
Glossary
• Caustic : corrosive or burning by chemical action
• Concentration : quantity of the solute present in the solution
• Partially : to a degree but not completely
• Granulated : in the form of small grains or particles
• Ointment : a balm, cream, gel, etc that is used as a medicine
• Antacid : a drug that reduces or neutralizes stomach acid
196Times' Crucial Science Book - 10
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