A are more reactive than the metals of groups IB to VIII. Similarly, the elements of group
IA are the most reactive metals. Lithium (Li), Sodium (Na), Potassium (K), Magnesium
(Mg), Calcium (Ca), Iron (Fe), Copper (Cu), Zinc (Zn), Mercury (Hg), Silver (Ag) and Gold
(Au) are some examples of metals.
Non-metals are placed in the right side of the modern periodic table. They are placed in
the groups V A, VIA, VII A and zero (0). The non-metals kept in group VII A are the most
reactive non-metals whereas the non-metals of zero group are chemically inert or non-
reactive metals. Fluorine (F), Chlorine (Cl), Iodine (I), Bromine (Br), Nitrogen (N), Oxygen
(O), Helium (He), Neon (Ne), Argon (Ar), etc. are examples of non-metals.
Metalloids show the properties of both metals and non-metals. So, they are placed
between metals and non-metals in the modern periodic table. Silicon, (Si), Germanium
(Ge), Arsenic (As), etc. are some examples of metalloids.
Differences between Metals and Non-metals
Metals Non-metals
1. Metals are good conductor of heat and 1. Non-metal are bad conductor of heat
electricity. and electricity.
2. Metals have more density. 2. Non-metals have less density.
Differences between Metals and Metalliods
Metals Metalloids
1. Metals are good conductor of heat and 1. Metalloids are poor conductor of heat
electricity and electricity
2. Metals are mellable and ductile. 2. Metalloids are neither melleable nor
ductile.
Differences between Nonmetals and Metalliods
Non-metals Metalloids
1. Non-metals are bad conductor of heat 1. Metalloids are semi-conductor of heat
and electricity and electricity.
2. Non-metals are placed in the right side 2. Metalloids are placed between metals
of the modern periodic table. and non-metals in the modern periodic
table.
13.5 Some Useful Metals, Non-metals and Metalloids
We use a variety of metals, non-metals and metalloids in our daily life. Among them,
some useful metals, non-metals and metalloids of our daily use are described below:
alluvial /əˈljuːvɪəl// - made of sand and earth that is left by rivers or floods
idol / ˈ a ɪ d ə l / - statue that is worshipped as a god
CHEMISTRY Oasis School Science and Environment - 8 195
1. Gold (Au)
Gold is a yellow shiny metal. It is found in sandy alluvial soil and quartz veins. Gold
is extracted from the sandy alluvial soil.
Properties of gold
1. Gold is a yellow shiny metal.
2. It is a good conductor of heat and electricity.
3. It is a non-reactive or neutral metal. So, it is
found in pure state in nature.
4. It does not react with air, water and normal Fig. 13.2 Gold
acids.
5. It is highly malleable and ductile.
Uses of gold Fig. 13.3 Sandy alluvial soil
1. Gold is used for making valuable ornaments
and jewellery.
2. It is used for making medals, coins, idols
and statues.
3. It is used for making medicines.
4. It is used for filling the cavities in teeth.
Reasonable fact-1
Gold is found in pure state in nature but not iron.
Gold is a noble metal. It does not react with air, water, acids and other chemicals in
normal condition of temperature and pressure. So, gold is found in pure state. But iron
combines with air, acids and other chemicals in ordinary condition to form various
types of compounds. So, iron is not found in pure state in nature.
2. Silver (Ag) Fig. 13.4 Silver
Silver is a shiny white metal. It is found in pure as well Fig. 13.5 Argentite ore
as combined state in nature. The main ore of silver is
argentite. An ore is the mineral found in nature from
which a metal can be extracted in a significant amount.
Silver is extracted from the argentite ore.
Properties of silver
1. Silver is a shiny white metal.
2. It is a very good conductor of heat and electricity.
3. It is not affected by air and water.
4. It is malleable and ductile.
5. It does not react with dilute acids.
196 Oasis School Science and Environment - 8 CHEMISTRY
Uses of silver Reasonable fact-2
1. Silver is used for making Gold and silver are used for making ornaments.
ornaments and jewellery.
Gold and silver are not affected by air, water and
2. It is used for making coins, normal acids. They are shiny, malleable, duclite
as well as rust free and can be converted easily
idols and statues. into the shape we desired. So, gold and silver are
3. It is used for electroplating. used for making ornaments.
4. It is used for making
medicines.
5. It is used for filling the cavities in teeth.
6. It is used for making expensive utensils and electronic devices.
3. Copper (Cu)
Copper is a reddish-brown metal which is found in
pure as well as combined state in nature. The main
ore of copper is chalcopyrite. Copper is extracted from
chalcopyrite. Copper is being used from ancient time for
making utensils.
Properties of copper Fig. 13.6 Copper
1. Copper is a reddish-brown metal.
2. It is a good conductor of heat and electricity.
3. It is malleable and ductile.
4. It does not undergo rusting but becomes dark in
cold places.
5. It changes into black oxide on heating. Fig. 13.7 Chalcopyrite ore
6. It reacts with acids and forms compounds.
Uses of copper
1. Copper is used for making Reasonable fact-3
cooking utensils.
Copper is used for making electric wires.
2. It is used for making electric
wires and other electronic and Copper is used for making electric wires
electrical devices/equipment. because it is highly ductile and one of the
best conductors of electricity.
3. It is used for making medicines,
fungicides and germicides.
4. It is used for making alloys like brass, bronze, bell metal, etc.
5. It is used for making coins, medals and statues.
6. It is used for electroplating.
CHEMISTRY Oasis School Science and Environment - 8 197
4. Iron (Fe)
Iron is a greyish-white metal. It is a reactive metal. So, it is
not found in free state in nature. The main ores of iron are
haematite and magnetite. Iron is extracted from those ores.
Iron is a very important metal for human beings. Iron is also
found in the bodies of animals and plants.
Properties of iron Fig. 13.8 Iron
1. Iron is a greyish-white metal.
2. It is a good conductor of heat and electricity.
3. It can be magnetized.
4. It undergoes rusting when
exposed to moist air.
5. It reacts with acids and other Haematite ore Magnetite ore
chemicals. Fig. 13.9
6. It is malleable and ductile.
Uses of iron
1. Iron is used for making cooking utensils and household equipment.
2. It is used for making weapons, tools, and parts of vehicles.
3. It is used for making rods, pipes, wires, bolts and materials for construction.
4. It is used in the manufacture of steel.
5. It is used for making vehicles like truck, bus, motorcylce, cycle, train, etc.
5. Aluminium (Al)
Aluminium is a bluish-white metal. It does not occur in a
free state in nature as it is a reactive metal. Aluminium is
extracted from its main ore called bauxite.
Properties of aluminium Fig. 13.10 Aluminium
1. Aluminium is a bluish-white and light metal.
2. It is a good conductor of heat and electricity.
3. It does not rust.
4. It is not affected by air and water.
5. It reacts with acids.
6. It is highly malleable and ductile.
7. It has low melting point as compared to other metals. Fig. 13.11 Bauxite ore
Uses of aluminium
1. Aluminium is used for making the bodies and parts of aeroplanes, ships, etc.
198 Oasis School Science and Environment - 8 CHEMISTRY
as it is a light and rust-free metal.
2. It is used for making electric wires and overhead electric cables.
3. It is used for making household utensils like pressure cooker, rice cooker, etc.
4. It is used for making foils and food wrappers.
5. It is used for making coins and alloys.
6. It is used for making silver paint.
Reasonable fact-4
Aluminium is used for making aeroplane.
Alumunium is a light, malleable and rusting free metal as well as it is not affected by
air and water. So, aluminum is used for making aeroplane.
6. Silicon (Si)
Silicon is a grey coloured metalloid. It does not occur
in a free state in nature but is found in the form of
compounds in a large scale. Sand or silica is the main
source of silicon. It shows the properties of both metals
and non-metals.
Properties of silicon Fig. 13.12 Silicon
1. Silicon is a grey-coloured solid substance.
2. It shows the properties of both metal and non-metal.
3. It does not occur in a free state in nature.
4. It is found in crystalline as well as amorphous forms.
5. The crystal of silicon is poor conductor of electricity but its powder is bad
conductor.
6. It does not react with air, water and acids.
Uses of silicon
1. Silicon is used for making glass.
2. It is used for making ceramic utensils.
3. It is used as a semi-conductor in electronic
equipment.
4. It is used for making polish and colours.
5. The compounds of silicon like sandstone are used Fig. 13.13 Silica
for making houses, statues, etc.
7. Sulphur (S)
Sulphur is a yellow crystalline solid. It is a non-metal having no taste and odour.
CHEMISTRY Oasis School Science and Environment - 8 199
It is found in a free as well as combined state in Fig. 13.14 Sulphur
nature. In free state, sulphur is found on the surface
of the earth in volcanic regions and in combined state
it is found in the form of sulphides of metals like
mercury, zinc, copper, iron, silver, etc. Sulphur is also
found in hydrogen sulphide, mustard oil, onion, and
spring water. Sulphur is generally extracted from the
underground deposits.
Properties of sulphur
1. Sulphur is a yellow crystalline Reasonable fact-5
solid. Sulphur is called a non-metal.
2. It is insoluble in water. Sulphur is bad conductor of heat and electricity.
It is neither malleable nor ductile. So, sulphur is
3. It is tasteless and odourless. called a non-metal.
4. The vapour of sulphur is
poisonous for bacteria and fungi but not for human beings and animals.
5. It is a poor conductor of heat and bad conductor of electricity.
6. It burns in air and forms sulphur dioxide.
7. It does not react with acids easily.
Uses of sulphur
1. Sulphur is used for making sulphuric acid.
2. It is used for making dyes, matches, gun powder and fire crackers.
3. It is used for making germicides, fungicides, and insecticides for killing germs,
bacteria and insects in plants.
4. It is used for making skin ointments for treating skin diseases.
5. It is also used in beauty parlours to give specific shape to our hair (hair-setting).
6. It is used for hardening of rubber by heating rubber with sulphur.
Project Work
Which metals, non-metals and metalloids are being used at your home? Prepare a
list of those substances with their uses and submit your work to your science teacher.
200 Oasis School Science and Environment - 8 CHEMISTRY
SUMMARy
• Metals are the solid substances (except mercury) which are malleable, ductile
and good conductors of heat and electricity. Examples: Gold, copper, silver,
iron, aluminium, etc.
• Non-metals are generally soft, non-malleable and non-ductile substances that
can be found in solid, liquid and gaseous state. Examples: Sulphur, nitrogen,
phosphorus, oxygen, iodine, chlorine, etc.
• Metalloids are those elements that show the properties of both metals and non-
metals. Examples: Silicon, germanium, arsenic, etc.
• In modern periodic table, metals are placed in the left side except hydrogen.
Metals are placed in groups IA, II A, III A and groups IB to VIII.
• Non-metals are placed in the right side of the modern periodic table. They are
placed in the groups V A, VIA, VII A and zero (0).
• Metalloids show the properties of both metals and non-metals. So, they are
placed between metals and non-metals in the modern periodic table.
• We use a variety of metals, non-metals and metalloids in our daily life.
• Gold is a yellow shiny metal found in a free state. It is used for making expensive
jewellery, medals, statues, medicines, coins, etc.
• Silver is a shiny white metal. It is used for making ornaments, medals, statues,
coins, medicines, etc.
• Copper is a reddish-brown metal. It is used for making household utensils,
fungicides, electric wires, electronic devices, coins, etc.
• Iron is a greyish-white metal. It is used for making household utensils, weapons,
vehicles, rods, cables, pipes and various materials for construction.
• Aluminium is a bluish-white metal. It is used for making bodies and parts
of aeroplanes and ships. It is also used for making food wrappers, overhead
electric cables, utensils, etc.
• Silicon is a grey solid. It is used for making glass, ceramics, utensils, colours
and semi-conductors of electronic devices.
• Sulphur is a yellow crystalline solid. It is used for making gun powder, fire
crackers, matches, insecticides, fungicides, germicides, skin ointments, etc.
CHEMISTRY Oasis School Science and Environment - 8 201
Exercise
1. Choose the best answer from the given alternatives.
a. Which of the following is a metal?
i. Gold ii. Silicon iii. Phosphorus iv. Sulphur
b. Which of the following is a non-metal?
i. Iron ii. Aluminium iii. Sulphur iv. Silver
c. Which of the following is a metalloid?
i. Copper ii. Gold iii. Silicon iv. Iron
d. Which of the following is the ore of copper?
i. Chalcopyrite ii. Bauxite iii. Haematite iv. Argentite
e. Which of the following metals is used for making bodies and parts of
an aeroplane?
i. Gold ii. Aluminium iii. Silver iv. Iron
2. Tick () the correct statement and cross (×) the incorrect one.
a. Metals are good conductors of heat and electricity.
b. Metalloids show only the properties of non-metals.
c. Gold does not occur in a free state in nature.
d. The main ore of iron is haematite.
e. Sulphur dissolves in water.
3. Fill in the blanks using appropriate words.
a. Metals are placed in the ........................... .
b. ........................... is a shiny yellow metal.
c. The main ore of silver is ........................... .
d. ........................... is used for making glass.
e. Sulphur burns in air and forms ........................... .
f. ........................... is used for making gun powder.
4. Answer the following questions.
a. What are metals? Give any five examples.
b. What are non-metals? Give any four examples.
202 Oasis School Science and Environment - 8 CHEMISTRY
c. What are metalloids? Give any three examples.
d. Write down the position of metals, non-metals and metalloids in the modern
periodic table.
e. Where is gold found in nature?
f. Why is gold used for making ornaments?
g. Which metal is extracted from argentite ore?
h. Which metals are used for filling the cavities in teeth?
i. Which metal is used for making overhead electric cables?
j. Where is silicon found in nature?
k. What is sulphur? Where is sulphur found in nature?
5. Differentiate between:
a. Metals and Non-metals
b. Metals and Metalloids
c. Non-metals and Metalloids
6. Give reason.
a. Copper is used for making cooking utensils.
b. Aluminium is used for making bodies and parts of aeroplane.
c. Silicon is called a metalloid.
d. Iron does not occur in a free state in nature.
7. Write any three properties each of the given elements.
i. gold ii. silver iii. copper iv. iron
v. aluminium vi. silicon vii. sulphur
8. Write any three uses each of the given elements.
i. gold ii. silver iii. iron iv. silicon
v. sulphur vi. aluminium vii. copper
CHEMISTRY Oasis School Science and Environment - 8 203
14UNIt Estimated teaching periods : Th Pr
4 2
pH Colour chart
ACID, BASE AND SALT
Objectives
After completing the study of this unit, students will be able to:
• introduce acid, base and salt and explain their properties and uses.
• define indicators and identify acid, base and salt by using litmus
paper.
• prepare litmus paper by using petals of flowers.
• introduce pH and pH scale.
Course of Study
• Acid – Introduction
• Properties and uses of acids
• Some acids used in our daily life and their sources
• Bases – Introduction
• Properties and uses of bases
• Salt – Introduction
• Properties and uses of salt
• Indicators, pH and pH scale
Points to be Focused/Questions to be Discussed
• What are acids, bases and salts?
• What are properties of acids, bases and salts?
• What are the uses of acids, bases and salts?
• What are indicators?
• What is meant by pH and pH scale?
204 Oasis School Science and Environment - 8 CHEMISTRY
14.1 Introduction
We eat different types of foods, fruits and vegetables. Among them, some have sour taste
like orange, lemon, grape, apple, etc., some have bitter taste like bitter gourd, edible soda,
etc. and some have salty taste like table salt. In our surroundings, there are several kinds
of compounds which cannot be tasted due to their corrosive and poisonous nature. So, to
study all those edible and non-edible compounds, they are divided into three groups, i.e.
acids, bases and salts.
14.2 Acid
The word acid is derived from Latin word "acidus" which means sour in taste. Most of the
acids, which are edible or non-edible, are sour in taste. But it is dangerous to touch or taste
acids in laboratory. According to Swedish chemist Arrhenius, "Acids give hydrogen ions
when dissolved in water and conduct electricity." So, "acids are those chemical substances
which give hydrogen ions when dissolved in water."
HCl +H2O H+ + Cl–
HNO3 +H2O H+ + NO3–
H2SO4 +H2O 2H+ + SO4– –
14.3 Classification of Acids
1. Classification of acids on the basis of strength
a. Strong acids: Acids which undergo almost complete dissociation in aqueous solution
and produce high concentration of hydrogen ions are called strong acids. Due to more
hydrogen ion concentration, they are good conductor of electricity and have low
pH value. Examples: Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid
(HNO3), etc.
b. Weak acids: Acids which undergo partial dissociation in aqueous solution and
produce low concentration of hydrogen ions are called weak acids. Due to low
concentration of hydrogen ions, they do not conduct electricity easily and have a
high pH value. Examples: Acetic acid (CH3COOH), Carbonic acid (H2CO3), Formic
acid (HCOOH), Ascorbic acid (H2C6H6O6), etc.
Note: When the degree of dissociation is more than 30%, it is considered as a strong acid.
2. Classification of acids on the basis of chemical nature
a. Organic acids: Acids which are obtained from living organisms and have hydro-
carbon are called organic acids. Examples: Acetic acid (CH3COOH), Formic acid
(HCOOH), Ascorbic acid (H2C6H6O6), Maleic acid (H2C4H2O4), etc. These are weak
acids and produce less concentration of hydrogen ions in aqueous solution.
CHEMISTRY Oasis School Science and Environment - 8 205
b. Inorganic acids (Mineral acids): Acids which are obtained from minerals and do not
have hydrocarbon are called inorganic acids or mineral acids. Inorganic acids may
be strong or weak. Inorganic acids are commonly used in laboratories.
Examples: Hydrochloric acid (HCl), Nitric acid (HNO3), Sulphuric acid (H2SO4),
Carbonic acid (H2CO3), etc.
Differences between Organic acids and Inorganic acids
Organic acids Inorganic acids
1. Acids which are obtained from living 1. Acids which are obtained from
organisms and have hydrocarbon minerals and do not have hydrocarbon
are called organic acids. are called inroganic acids.
2. These are weak acids. 2. These acids may be strong or weak.
Differences between Strong acids and Weak acids
Strong acids Weak acids
1. Acids which undergo almost 1. Acids which undergo partial
complete dissociation in aqueous dissociation in aqueous solution
solution and produce high and produce low concentration of
concentration of hydrogen ions are hydrogen ions are called weak acids.
called strong acids.
2. They are good conductor of heat 2. They are poor conductor of heat and
and electricity. electricity.
3. They have low pH value. 3. They have high pH value.
14.4 Properties of Acids
a. Physical properties of acids
1. Acids possess sour taste due to the presence of hydrogen ions (H+). Most of
the fruits are sour in taste. However, it is dangerous to touch and taste acids in
laboratory as they burn our skin, tongue, etc.
2. Acids change blue litmus paper into red and methyl orange into red.
3. Strong acids are corrosive in nature.
b. Chemical properties of acids
1. Acids react with bases/ alkalis and produce salt and water.
Acid + Base Salt + Water
HCl + NaOH NaCl + H2O
H2SO4 + MgO MgSO4 + H2O
corrosive /kəˈrəʊsɪv/ - tending to destroy sth slowly
dilute / d a ɪ ˈ l u ː t / - made weaker by adding water
206 Oasis School Science and Environment - 8 CHEMISTRY
2. Dilute acids react with active metals and produce salt and hydrogen gas.
Dilute acid Metal Salt + Hydrogen
2HCl + Mg MgCl2 + H2↑
2HCl + Ca CaCl2 + H2↑
H2SO4 + Zn ZnSO4 + H2↑
3. Acids react with carbonates and bicarbonates and produce salt, water and carbon
dioxide.
Carbonate + Acid Salt + Water + Carbon dioxide
MgCO3 + H2SO4 MgSO4 + H2O + CO2↑
CaCO3 + 2HCl CaCl2 + H2O + CO2↑
Na2CO3 + 2HCl 2NaCl + H2O + CO2↑
Carbon dioxide
Bicarbonate + Acid Salt + Water +
NaHCO3 + HCl NaCl + H2O + CO2↑
Ca(HCO3)2 + 2HCl CaCl2 + H2O + CO2↑
4. Acids dissolve in water and give hydrogen ions.
HCl +H2O H+ + Cl–
H2SO4 +H2O 2H+ + SO4– –
HNO3 +H2O NO3–
H+ +
14.5 Uses of Acids
1. Sulphuric acid is used in industries for making drugs, detergents and chemical
fertilizers. It is widely used in laboratories and industries. Therefore, sulphuric acid
is also called king of chemicals or kingly water.
2. Hydrochloric acid is used in laboratories and in tanning and printing industries.
3. Nitric acid is used for making explosives, plastics and dyes.
4. Carbolic acid (Phenol) is used to kill germs.
5. Boric acid is used for washing eyes and wounds.
6. Acetic acid (vinegar) is used for preserving and flavouring foods.
7. Citric acid is used in medicines, as a source of vitamin C and flavouring drinks.
8. Carbonic acid is used in soft drinks and soda water.
9. Oxalic acid is used to remove ink-stain.
10. Tartaric acid is used in baking powder.
alkali / ˈ æ l k ə l a ɪ / - the base that dissolves in water
CHEMISTRY Oasis School Science and Environment - 8 207
Some acids of our daily use with their sources are as follows:
S.N. Name of acids Source
1. Citric acid Lemon, tomato
2. Lactic acid Milk, curd
3. Ascorbic acid Sour fruits
4. Tartaric acid Grape fruits
5. Oxalic acid Chariamilo
6. Formic acid Red ant
14.6 Base
Metallic oxides and hydroxides are called bases. Most bases or metal oxides dissolve in
water and give hydroxyl ions. The bases that dissolve in water and produce hydroxyl
(OH-) ions are called alkalis, e.g. NaOH, KOH, Ca(OH)2, Mg(OH)2, etc. Some bases like
PbO, BaO, HgO, CuO, etc. do not dissolve in water. So these compounds are bases but not
alkalis. Therefore, all alkalis are bases but all bases are not alkalis.
1. Most bases or metallic oxides dissolve in water and form metal hydroxides.
Na2O + H2O 2NaOH (Sodium hydroxide)
Mg(OH)2 (Magnesium hydroxide)
MgO + H2O 2KOH (Potassium hydroxide)
K2O + H2O
CaO + H2O Ca(OH)2 (Calcium hydroxide)
2. Alkalis give hydroxyl (OH-) ions when dissolved in water.
NaOH + +H2O Na+ + OH_
Mg(OH)2 + +H2O Mg++ + 2OH_
KOH + +H2O K+ + OH_
Ca(OH)2 + +H2O Ca++ + 2OH_
Differences between Bases and Alkalis
S.N. Bases S.N. Alkalis
1. All metallic oxides are called bases. 1. Only water soluble metallic
oxides or bases are called
alkalis.
208 Oasis School Science and Environment - 8 CHEMISTRY
2. All bases do not give hydroxyl ions 2. All alkalis give hydroxyl ions
when dissolved in water. when dissolved in water.
3. All bases are not alkalis. 3. All alkalis are bases.
Examples: NaOH,
Examples: MgO, CaO, Na2O, HgO, Mg(OH)2, NH4OH, etc. KOH,
Fe2O3, etc.
Reasonable fact-1
All alkalis are bases but all bases are not alkalis.
Bases are the substances that may or may not dissolve in water but alkalis are the bases
that dissolve in water. Insoluble bases are not alkalis. Therefore, we can say that all
alkalis are bases but all bases are not alkali.
14.7 Strong and Weak Bases
Bases (alkalis) which give more amount of hydroxyl ions in aqueous solution are called
strong bases, e.g. Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium
hydroxide [Ca(OH)2], etc. They undergo almost complete dissociation and have a high
pH value.
Bases (alkalis) which give very less amount of hydroxyl ions in aqueous solution are called
weak bases, e.g. Ferric hydroxide [Fe(OH)3], Copper hydroxide [Cu(OH)2], etc. They have
very less degree of ionization and have a low pH value.
Reasonable fact-2
Potassium hydroxide is called a strong alkali.
Potassium hydroxide is fully dissociated in water and thus produces a large number
of hydroxyl ions. So, it is called a strong alkali.
14.8 Properties of Bases or Alkalis
a. Physical properties of Bases or Alkalis
1. Bases are soapy in touch and bitter in taste.
2. Bases turn red litmus paper into blue, methyl orange into yellow and
phenolphthalein into pink.
3. Strong bases or alkalis like NaOH, KOH dissolve oil and grease.
4. Strong bases or alkalis burn our skin.
CHEMISTRY Oasis School Science and Environment - 8 209
b. Chemical properties of Bases or Alkalis
1. Bases or alkalis react with acids to form salt and water.
Base + Acid Salt + Water
2KOH + H2SO4 K2SO4 + H2O
NaOH + HCl NaCl + H2O
Mg(OH)2 + H2SO4 MgSO4 + H2O
2. Bases or alkalis react with carbon dioxide and form corresponding carbonate and
water.
Alkali + Carbon dioxide Carbonate + Water
2NaOH + CO2 Na2CO3 + H2O
2KOH + CO2 K2CO3 + H2O
Ca(OH)2 + CO2 CaCO3 + H2O
3. Alkalis react with ammonium salts and form salt, water and ammonia gas.
Alkali + Ammonium salt Salt + Water + Ammonia
NaOH + NH4Cl NaCl + H2O + NH3↑
Ca(OH)2 + 2NH4Cl CaCl2 + 2H2O + 2NH3↑
Mg(OH)2 + (NH4)2CO3 MgCO3 + 2H2O + 2NH3↑
14.9 Uses of Bases
1. Sodium hydroxide (NaOH) is used to make soaps, detergents, papers, etc. and
purification of petroleum products.
2. Calcium hydroxide [Ca(OH)2] or slaked lime is used for making mortar and
bleaching powder, to reduce hardness of water and to neutralize acidity of soil.
3. Potassium hydroxide (KOH) is used in batteries and to make soft soap.
4. Aluminium hydroxide [Al(OH)3] and magnesium hydroxide [Mg(OH)2] are used to
reduce hyperacidity of stomach.
5. Ammonium hydroxide (NH4OH) is used to remove grease and stains from clothes
and to make fertilizers.
6. Calcium oxide (CaO) or quick lime is used for softening hard water, purification of
sugar and production of cement.
amorphous / əˈmɔːfəs / - having no definite shape, form or structure
210 Oasis School Science and Environment - 8 CHEMISTRY
Differences between Acids and Bases
Acids Bases
1. Acids give hydrogen ions (H+) when 1. Bases give hydroxyl ions (OH-) when
dissolved in water dissolved in water.
2. Acids are sour in taste. 2. Bases are bitter in taste.
3. Acids turn blue litmus paper into red. 3. Bases turn red litmus paper into blue.
Reasonable fact-3
We should not touch and taste acids and bases in laboratory.
Strong acids and bases burn our skin, tongue, etc. So, we should not touch and taste
acids and bases in laboratory.
14.10 Salt
Salt is a chemical substance which is formed by partial or complete replacement of
hydrogen atom by a metal or ammonium radical. In general, salts are neutral compounds
but some may be acidic or basic in nature. The process by which acid and base react
together to give salt and water is called neutralization reaction. In this reaction, H+ -ions
of the acid are completely replaced by a metal.
Acid + Base Salt + Water
HCl + NaOH NaCl + H2O
H2SO4 + 2KOH K2SO4 + 2H2O
In acid-base reaction, there may be complete or partial replacement of hydrogen atom(s)
of an acid by a metal or ammonium radical. For example,
NaOH + H2SO4 NaHSO4 + H2O
In this example, only one hydrogen from sulphuric acid is replaced by sodium atom.
2NaOH + H2SO4 Na2SO4 + 2H2O
Here, both the hydrogen atoms are replaced by sodium atoms.
14.11 Properties of Salts
1. Generally, salts are neutral but some may be acidic or basic in nature.
2. Most of the salts are water soluble but chloride salts of silver and lead and sulphate
salts of lead and barium are insoluble.
anaemia /əˈniːmɪə/ - a medical condition having too few RBCs
CHEMISTRY Oasis School Science and Environment - 8 211
3. Salts of metals like Na, K, Mg, Ca, Al and Ba are white or colourless whereas salts of
Cu, Co, Mn, Ni, Fe and Cr are colourful.
4. Some salts are salty in taste but most salts are bitter.
5. Salts conduct electricity in molten or solution state.
6. Some salts are amorphous whereas some are crystalline.
14.12 Uses of Salts
1. Table salt (NaCl) is used in our foods and also as a preservative.
2. Sodium carbonate is used for manufacture of soaps, detergents and glasses. It is also
used to reduce hardness of water.
3. Calcium sulphate is used in medical field for plastering of fractured bones and for
mixing in cement.
4. Sodium bicarbonate is used as baking powder, for reducing hyperacidity and in fire
extinguisher.
5. Copper sulphate is used for making fungicides and in copper plating.
6. Aluminium chloride is used in dry cells as an electrolyte.
7. Ammonium sulphate is used as chemical fertilizer.
8. Ferrous sulphate is used as medicine for anaemia patients.
14.13 Indicators
Indicators are those chemical substances which are used to indicate whether the
substance is acidic or basic or neutral in nature. Indicators, when treated with acidic
or basic solution, change their own colour. They remain chemically unchanged but
indicate the end point of the reaction. Litmus paper, methyl orange and phenolphthalein
are some common indicators. They are obtained from different parts of plants like roots,
flowers, leaves, etc. These are collected, crushed and mixed with organic solvent to
obtain indicators. The colour change of different indicators with acidic, basic and salt
solution is given in the table:
S.N. Indicators Colour in acid Colour in basic Colour in neutral salt
solution solution solution
1) Red litmus paper
2) Blue litmus paper No change in colour Changes into blue No change in colour
3) Methyl orange Changes into red
4) Phenolphthalein Changes into red No change in colour No change in colour
5) Red cabbage juice No change in colour
Changes into yellow No change in colour
Changes into red
Changes into pink No change in colour
Changes into green Changes into rose red
212 Oasis School Science and Environment - 8 CHEMISTRY
Reasonable fact-4
Phenolphthalein is considered as a weak indicator.
Phenolpthalein changes its colour only in basic solution but not in the solution of acids
and salts. So, phenolpthalein is considered as a weak indicator.
14.14 Universal Indicator
A universal indicator is a special kind of indicator which is used to measure the strength
of acidity or alkalinity. It is prepared by mixing several ordinary indicators of different
colour. A universal indicator changes its colour when kept in an acidic, basic or neutral
solution. The change in colour is matched with pH chart to determine the strength of the
given solution.
Differences between Ordinary and Universal indicators
S.N. Ordinary indicators S.N. Universal indicators
1. Ordinary indicators indicate only 1. Universal indicators indicate
whether the substance is acid, base whether the substance is acid,
or salt. base or salt along with their
strength.
2. They are obtained from the parts of 2. They are obtained by mixing
plant like leaves, flowers, roots, etc. different types of ordinary
indicators.
Reasonable fact-5
Universal indicator is more meaningful than an ordinary indicator.
Ordinary indicators indicate only whether the substance is acid, base or salt whereas
the universal indicator indicates whether the substance is acid, base or salt along with
their strength (pH value). So, universal indicator is more meaningful than an ordinary
indicator.
14.15 pH and pH Scale
The measure of hydrogen ion concentration present in a solution is called pH. In acidic
solution, there is high concentration of hydrogen ions and in basic solution, there is less
concentration of hydrogen ions. It is measured by using pH paper and pH meter.
The standard scale which is used to measure the strength of acidic and basic solution is
called pH scale. This scale shows the degree of acidity and alkalinity.
CHEMISTRY Oasis School Science and Environment - 8 213
pH scale consists of numbers 1 to 14 with their corresponding colours in the scale.
←Acidity increases Neutral Alkalinity increases→
pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Red
Rose Yellow Light green Green Greenish blue Blue Deep blue
Fig. 14.1 pH scale
The process of identification of acid, base or a neutral substance with the help of pH scale
is given below:
First of all, pH paper is dipped into the given solution and the change in colour is observed.
Then the colour in the pH paper is matched with the pH scale to know the value. The pH
value below 7, i.e. 1 to 6 represents acidity and above 7, i.e. 8 to 14 represents alkalinity
whereas pH value 7 represents neutrality. The substance having pH value 1 is the strongest
acid and that having pH value 14 is the strongest alkali.
Increasing Acidity (Neutral) Increasing Alkalinity
Fig. 14.2 pH colour chart
14.16 pH Meter
pH meter is a scientific instrument which is used to measure the pH value of a given
solution directly in the number. It has a box with different readings and a rod. The rod is
dipped into the solution whose pH value is to be measured. The rod has connection with
box and it directly indicates the pH value with the help of pointer.
Fig. 14.3 pH meter
214 Oasis School Science and Environment - 8 CHEMISTRY
pH value of some substances
S. N. Compounds pH
1. Hydrochloric acid (HCl) 1
2. Sulphuric acid (H2SO4) 1.2
3. Lemon juice 2.5
4. Apple, vinegar, carbonic acid 3
5. Butter 6
6. Water, salt solution, sugar solution, ethanol 7
7. Human blood 7.3
8. Baking soda 8.5
9. Ammonium hydroxide (NH4OH) 10
10. Washing soda (Na2CO3) 11.5
11. Sodium hydroxide (NaOH) 13
Activity 1
• Take three test tubes. Mark them A, B and C.
• Keep the solution of acid in test tube A, solution of base in test tube B and
solution of common salt in test tube C.
• Take red litmus paper and immerse one red litmus paper separately in each test
tube and observe the change in colour.
• Repeat above activity with blue litmus paper.
• Make a chart showing the change in colour in each case.
• Write down the conclusion of this activity.
Activity 2
• Collect petals of some flowers and crush them to get fine paste.
• Mix the paste with alcohol and filter the mixture to obtain a clear solution.
• Collect the solution in a beaker and immerse some strips of paper into the
solution.
• Dry these strips of paper in the sun. In this way, litmus paper can be prepared.
Now, use these litmus papers to test acid, base and salt.
CHEMISTRY Oasis School Science and Environment - 8 215
SUMMARy
• Acids are the chemical substances that produce hydrogen ions when dissolved
in water.
• Acids which undergo almost complete dissociation in aqueous solution and
produce high concentration of hydrogen ions are called strong acids, HCl,
H2SO4, HNO3, etc.
• Acids which undergo partial dissociation in aqueous solution and produce low
concentration of hydrogen ions are called weak acids, e.g. H2CO3, CH3COOH, etc.
• Acids which are obtained from living organisms and have hydrocarbon are
called organic acids, e.g. HCOOH, CH3COOH, etc.
• Acids which are obtained from minerals and do not have hydrocarbon are
called inorganic acids or mineral acids.
• Metallic oxides and hydroxides are called bases, e.g. Na2O, MgO, Ca(OH)2, etc.
• The bases that dissolve in water and produce hydroxyl (OH-) ions are called
alkalis, e.g. NaOH, KOH, Mg(OH)2, Ca(OH)2, etc.
• Bases (alkalis) which give more amount of hydroxyl ions in aqueous solution
are called strong bases.
• Salt is a chemical substance which is formed by partial or complete replacement
of hydrogen atom by a metal or ammonium radical.
• Indicators are those chemical substances which are used to indicate whether
the substance is acidic or basic or neutral in nature.
• Litmus paper, methyl orange and phenolphthalein are some common indicators.
• The measure of hydrogen ion concentration present in a solution is called pH.
• The standard scale which is used to measure the strength of acidic and basic
solution is called pH scale.
216 Oasis School Science and Environment - 8 CHEMISTRY
Exercise
1. Choose the best answer from the given alternatives.
a. The taste of acid is ............................... .
i. bitter ii. salty iii. sour iv. sweet
b. Acids react with metals and form ............................. .
i. oxygen gas ii. nitrogen gas iii. hydrogen gas iv. chlorine gas
c. Bases convert red litmus into ................................ .
i. blue ii. green iii. yellow iv. red
d. Which of the following is not an indicator?
i. methyl orange ii. litmus paper iii. phenophthalein iv. filter paper
e. The pH value of strong acid is ......................... .
i. 1 ii. 3 iii. 4 iv. 7
2. Tick (√) the correct statement and cross (×) the incorrect one.
a. We should not touch and taste acids.
b. Sulphuric acid is an organic acid.
c. Lemon contains citric acid.
d. Bases convert methyl orange into red.
e. Sodium bicarbonate is used for making baking powder.
f. The strength of acid and base is measured in pH scale.
3. Fill in the blanks using appropriate words.
a. The acids which are found in animals and plants are called ....................... .
b. The bases that dissolve in water are called ......................... .
c. Bases react with ......................... and form carbonates.
d. ......................... is used for making baking powder.
e. The pH value of a neutral salt is ......................... .
4. Answer the following questions.
a. What are acids? Give any two examples.
b. Define inorganic acids and organic acids with any two examples of each.
c. Write any three physical and two chemical properties of acids.
CHEMISTRY Oasis School Science and Environment - 8 217
d. Write any three uses of acids.
e. Name any three acids that are used in our daily life. Also, write down the
source of each acid.
f. What are bases? Give any three examples.
g. Write any four physical properties and two chemical properties of bases.
h. What are alkalis? Give any three examples.
i. Write any three uses of bases.
j. What is a salt? Write any three properties of salt.
k. Write any three uses of salt.
l. What are indicators? Give any two examples.
m. What is meant by pH and pH scale?
5. Differentiate between:
a. Organic acids and Inorganic acids
b. Acids and Alkalis (Bases)
c. Alkalis and Bases
d. Simple indicator and Universal indicator
6. Give reason.
a. We should not touch and taste acids.
b. Acids are sour in taste.
c. Aluminium hydroxide is used to reduce hyperacidity.
d. Litmus paper is called an indicator.
7. Name any three acids, three bases and three salts that are used in our daily life.
8. Prepare a list showing the effect of various indicators in acid, base and salt.
9. Write down the major uses of the given substances.
a. Carbonic acid b. Acetic acid
c. Potassium hydroxide d. Calcium hydroxide
e. Sodium chloride f. Ammonium sulphate
10. There are two acids having pH value 1 and 5. Which one is stronger? Why?
218 Oasis School Science and Environment - 8 CHEMISTRY
15UNIt Estimated teaching periods : Th Pr
5 1
Glycerol
SOME USEFUL
CHEMICALS
Objectives
After completing the study of this unit, students will be able to:
• explain the properties of water and define hard water and soft
water.
• describe the types of hard water and explain the methods of
removing hardness of water.
• introduce sodium carbonate, sodium bicarbonate and glycerol.
Course of Study
• Water– Introduction
• Physical and chemical properties of water
• Hard and soft water
• Hardness of water – Temporary and permanent
• Removal of hardness of water
• Introduction to sodium carbonate, sodium bicarbonate and glycerol
with their uses
Points to be Focused/Questions to be Discussed
• What are the sources of water?
• What are physical and chemical properties of water?
• What is meant by hard water and soft water?
• What are the methods to remove the hardness of water?
• What are the uses of sodium carbonate, sodium bicarbonate and
glycerol?
CHEMISTRY Oasis School Science and Environment - 8 219
15.1 Introduction
We use a variety of chemical substances in our daily Face file-1
life. Some of them are soap, detergent, phenol, About 71% of the earth is covered
plastics, insecticides, water, sodium carbonate, with water.
sodium bicarbonate, glycerol, etc. Water is the
most useful substance found on the earth. It may be hard or soft. Sodium carbonate is
a compound of sodium metal. It is used for washing clothes and making soap. Sodium
bicarbonate is used for making baking powder and reducing hyperacidity. Similarly,
glycerol is used for making medicine, printing ink, ink for stamp pads, etc. In this unit,
we will learn about water, sodium carbonate, sodium bicarbonate and glycerol.
15.2 Water Face file-2
Water is one of the most useful natural resources.
About three-fourths of the earth's surface is The proportion of hydrogen and
covered by water. Of the total water on earth, oxygen in water is 2:1.
97.4% is in oceans and only less than 1 percent of
water is suitable for human use. This water is repeatedly recycled for use by the humans
and other animals. Rain is the main source of water. Oceans, rivers, lakes, ponds, ditches,
pools, etc. are other sources of water.
Water is found in all three states, viz. solid, liquid and gas. Ice is the solid state of water
which is found in high mountains called Himalayas. Cloud and water vapour are the
gaseous state of water.
Face file-3
Water does not contain acidic and basic properties. So, water is called a neutral
substance.
River Lake Ocean
Fig. 15.1 Some sources of water
Water below the surface of the earth is another source of water. We extract underground
water by digging wells. We use water for drinking,washing, bathing, irrigating,
cooking, etc.
220 Oasis School Science and Environment - 8 CHEMISTRY
15.3 Properties of Water
a. Physical properties of water
1. Pure water is colourless, odourless and tasteless.
2. It exists in solid, liquid and gaseous states.
3. Pure water is transparent.
4. It is a universal solvent.
5. Pure water is a bad conductor of electricity.
6. Its boiling point is 100 0C and freezing point is 0 0C.
Reasonable fact-1
Water is a very useful liquid.
Water is used for drinking, cooking, bathing, washing, irrigating, production of
hydroelectricity as well as in various industries as a cooling agent. So, water is a very
useful liquid.
b. Chemical properties of water
1. Water is made of two parts of hydrogen and one part of oxygen.
2. It is a neutral substance. It means that water is neither acidic nor basic in
nature.
15.4 Soft and Hard Water
Soap is used to wash clothes. Whenever we get enough amount of foam or lather by
applying a little soap, we say that the water is soft. But water from some sources like
rivers, ponds, lakes, instead of producing foam, forms sticky particles. Such water is
called hard water.
Hardness of water is caused due to the presence of soluble salts of calcium and magnesium,
viz. chloride, sulphate and bicarbonate salts of calcium and magnesium.
The water that does not contain chloride, sulphate and bicarbonate salts of calcium
and magnesium is called soft water. It produces lather with soap easily. Rain water and
distilled water are some examples of soft water.
The water that contains the chloride, sulphate and bicarbonate salts of calcium and
magnesium is called hard water. It does not produce lather with soap easily. So hard
water leads to wastage of soap. The water of wells, rivers, oceans, etc. is hard.
On the basis of salts dissolved, hardness of water is of two types:
i. Temporary hardness ii. Permanent hardness
lather /ˈlɑːðə/ - a white mass of small bubbles that is produced by mixing soap with water
CHEMISTRY Oasis School Science and Environment - 8 221
i. Temporary hardness
The hard water containing bicarbonate salts of calcium and magnesium is called
temporary hard water. Such type of water is tasty while drinking and makes our
bones healthy. This type of hardness can be removed easily.
Removal of temporary hardness of water
Temporary hardness of water can be removed easily by boiling the water or treating with
lime-water.
a. By boiling water
When the temporary hard water is boiled, the soluble bicarbonate salts of calcium
and magnesium get converted into water insoluble bicarbonate salts and settle
down at the bottom of the container. These insoluble salts can be filtered away.
Calcium bicarbonate heat
Calcium carbonate + Water + Carbon dioxide
Ca(HCO3)2 ∆ CaCO3 + H2O + CO2↑
Magnesium bicarbonate heat Magnesium carbonate + Water + Carbon dioxide
Mg(HCO3)2 ∆ MgCO3 + H2O + CO2↑
Reasonable fact-2
The hardness of water is removed when temporary hard water is bolled.
When temporary hard water is boiled, the soluble bicarbonate salts are changed into
insoluble carbonates which settle down at the bottom of the container. So, the hardness
of water is removed when temporary hard water is boiled.
b. By treating with lime-water
Temporary hardness of water can also be removed by adding lime water or slaked
lime into the hard water. Calcium hydroxide, i.e. lime water reacts with bicarbonate
of calcium and magnesium and forms insoluble calcium carbonate which makes the
water soft.
Calcium bicarbonate + Calcium hydroxide Calcium carbonate + Water
Ca(HCO3) 2 + Ca(OH)2 2CaCO3 + 2H2O
Magnesium bicarbonate + Calcium hydroxide Calcium carbonate + Water + Magnesium hydroxide
Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + 2 H2O + Mg(OH)2
ii. Permanent hardness
The hard water containing chloride and sulphate salts of calcium and magnesium is
called permanent hard water.
Removal of permanent hardness of water
Permanent hardness of water can be removed by treating the hard water with
washing soda and permutit process.
222 Oasis School Science and Environment - 8 CHEMISTRY
a. By treating hard water with washing soda
When permanent hard water is treated with washing soda or sodium carbonate,
the salts present in it change into carbonates and water becomes soft. The chemical
reactions involved in this process are given below:
Calcium chloride + Sodium carbonate Calcium carbonate + Sodium chloride
CaCO3 + 2NaCl
CaCl2 + Na2CO3
Calcium carbonate + Sodium sulphate
Calcium sulphate + Sodium carbonate CaCO3 + Na2SO4
Magnesium carbonate + Sodium chloride
CaSO4 + Na2CO3 MgCO3 + 2NaCl
Magnesium carbonate + Sodium sulphate
Magnesium chloride + Sodium carbonate MgCO3 + Na2SO4
MgCl2 + Na2CO3
Magnesium sulphate + Sodium carbonate
MgSO4 + Na2CO3
Reasonable fact-3
The permanent hardness of water gets removed when sodium carbonate is added
into the hard water.
When sodium carbonate is added into the hard water, the salts present in it change
into carbonates and water becomes soft. So, the permanent hardness of water gets
removed when sodium carbonate is added into the hard water.
b. By permutit process Hard water Sodium chloride
In permutit process, the permanent
hard water is passed through sodium Soft water
aluminosilicate (Na2Al2SiO8) or sodium Zeolite
zeolite (Na2–Z). When hard water passes Column
through the zeolite or permutit, the calcium
and magnesium ions of hard water get Finegravel
replaced by sodium ions of permutit. As a Fig. 15.2 Permutit process to remove hardness of water
result, water becomes soft.
The chemical reactions involved in permutit method are given below:
Sodium zeolite + Calcium chloride Calcium zeolite + Sodium chloride
Na2–Z + CaCl2 Ca – Z + 2NaCl
Sodium zeolite + Magnesium chloride Magnesium zeolite + Sodium chloride
Na2 – Z + MgCl2 Mg–Z + 2NaCl
Sodium zeolite + Calcium sulphate Calcium zeolite + Sodium sulphate
Na2 – Z + CaSO4 Ca–Z + Na2SO4
Sodium zeolite + Magnesium sulphate Magnesium zeolite + Sodium sulphate
Na2–Z + MgSO4 Mg–Z + Na2SO4
CHEMISTRY Oasis School Science and Environment - 8 223
Differences between Soft water and Hard water
Soft water Hard water
1. Soft water does not contain chloride, 1. Hard water contains chloride, sulphate
sulphate and bicarbonate salts of and bicarbonate salts of calcium and
calcium and magnesium magnesium.
2. It produces later with soap easily. 2. It does not produce later with soap
easily.
Differences between Temporary hardness and Permanent hardness of water
Temporary hardness Permanent hardness of water
1. It contains bicarbonate slats of 1. It contains chloride and sulphate salts
calcium and magnesium. of calcium and magnesium.
2. It can be removed easily by boiling 2. It can be removed by treating the hard
the hard water. water with washing soda and permuit
process.
Activity 1
To identify hard and soft water
• Collect water from different sources in different test tubes from your locality.
• Put 2-3 drops of shampoo or solution of soap in each of the test tubes and
stir with glass rod. Which sample of water produces lather easily? Observe
carefully.
• Identify hard water and soft water from above samples.
Activity 2
To demonstrate that temporary hardness can be removed by boiling water
• Take a beaker and put some water into it.
• Add one spoon of calcium bicarbonate or magnesium bicarbonate into the
water and stir with a glass rod.
• Now, add a few drops of shampoo or solution of soap into the beaker and
observe whether lather is formed or not.
• The water does not produce lather. It proves that the water is hard.
• Now, boil the water in the beaker and allow it to cool.
• Again, add a few drops of shampoo or solution of soap and stir with a glass
rod. Observe whether lather is formed or not. The water produces lather with
soap after boiling. It proves that temporary hardness of water can be removed
by boiling.
224 Oasis School Science and Environment - 8 CHEMISTRY
Activity 3
To demonstrate that permanent hardness of water can be removed by adding
washing soda
• Take a beaker and put some water into it. Add some calcium chloride or
magnesium chloride into it and stir.
• Now add 2-3 drops of shampoo or solution of soap into the beaker and stir it
with a glass rod. The water does not produce lather with soap as it is a hard
water.
• Now, add some sodium carbonate into the beaker and stir the mixture with a
glass rod.
• Again, put a few drops of solution of soap and stir. Observe whether the water
forms lather with soap or not.
• After adding sodium carbonate or washing soda, the water produces lather with
soap. This activity proves that permanent hardness of water can be removed by
using washing soda.
15.5 Sodium Carbonate Fig 15.3 Na2CO3 powder
Sodium carbonate is a useful chemical. It is a
compound of sodium metal. Its molecular formula
is Na2CO3. It is basic in nature and found in the form
of white powder. It is commonly known as washing
soda.
Uses
1. Sodium carbonate is used for making soap and washing clothes.
2. It is used for making paper.
3. It is used for softening hard water.
4. It is used for making glass.
5. It is used for making caustic soda.
15.6 Sodium Bicarbonate
Sodium bicarbonate is another useful compound made of sodium metal. It is a white
baking /beɪking/ - the process of cooking using dry heat in an oven
beverage /ˈbevərɪdʒ/ - any type of drink except water
lubricant /ˈluːbrɪkənt/ - a substance, e.g. oil that you put on surfaces or parts of a machine so that they
move easily and smoothly
CHEMISTRY Oasis School Science and Environment - 8 225
crystalline solid and its molecular formula is NaHCO3. It Fig. 15.4 NaHCO3 powder
is commonly known as edible soda. It is soluble in water.
It is found in the form of white powder. It is mixed with
potassium hydrogen tartate to produce baking powder.
In bakeries, baking powder is used to increase the volume
of cakes, biscuits, breads, etc.
Uses
1. Sodium bicarbonate is used for making baking
powder.
2. It is used for reducing hyper-acidity.
3. It is used for making soft drinks.
4. It is used in fire extinguishers as a source of carbon dioxide.
5. It can be used to control fungus growth.
Reasonable fact-4
Sodium bicarbonate is used in bakeries.
Sodium bicarbonate increases the volume of cakes, biscuits, breads, etc. So, it is used
in bakeries to gain more profit.
15.7 Glycerol
Glycerol is a colourless thick liquid having a sweet taste. It is
commonly known as glycerine. Its molecular formula is C3H5(OH)3.
It is a trihydric alcohol, i.e. alcohol having three hydroxyl (OH)
groups. It dissolves in water. Glycerol is an organic compound. It
is prepared from propane by replacing three hydrogen atoms of
propane by hydroxyl (OH) groups.
H HH –3H H HH Fig.15.5 Glycerine
I II +3OH I II
H–C– C–C–H H–C– C–C–H
I II I II
H HH OH OH OH
(Propane) (Glycerol)
Uses
1. Glycerol is used for making medicines, printing ink and ink for stamp pads.
2. It is used as a sweetening agent in confectionery, beverage, medicines, etc.
3. It is used for making cosmetics and high quality soap.
4. It is used for preserving tobacco, fruits, etc.
5. It is used to prevent skin from drying.
6. It is used as a lubricant.
226 Oasis School Science and Environment - 8 CHEMISTRY
Reasonable fact-5
Human apply glycerin on hands and legs in winter.
Glycerine prevents skin from drying. So, human apply glycerin on hands and legs in
winter.
Activity 4
• Take some glycerol and taste it. Is it sweet? Now, rub it against hands and legs.
What do you feel? What is its colour? Is it colourless? Is it thicker than water?
What do you learn from this activity?
Project work
Prepare a list of chemicals that are used at your home. Also, write their uses and fill
in the table given below:
S.N. Chemicals Uses
1. Washing soda ..............................................
2. ...................................................... .............................................
3. ..................................................... .............................................
4. .................................................... .............................................
5. .................................................... .............................................
CHEMISTRY Oasis School Science and Environment - 8 227
SUMMARy
• Water is one of the most useful natural resources. About three-fourths of the earth's
surface is covered by water.
• Hardness of water is caused due to the presence of soluble salts of calcium
and magnesium, viz. chloride, sulphate and bicarbonate salts of calcium and
magnesium.
• The water that does not contain chloride, sulphate and bicarbonate salts of calcium
and magnesium is called soft water.
• The hard water containing bicarbonate salts of calcium and magnesium is called
temporary hard water.
• The hard water containing chloride and sulphate salts of calcium and magnesium
is called permanent hard water.
• Sodium carbonate is a useful chemical. It is a compound of sodium metal. Its
molecular formula is Na2CO3.
• Sodium carbonate is used for making soap, paper, glass, caustic soda, etc.
• Sodium bicarbonate is another useful compound made of sodium metal. It is a
white crystalline solid and its molecular formula is NaHCO3. It is commonly known
as edible soda.
• Sodium bicarbonate is used for making baking powder, soft drinks, reducing hyper
acidity, etc.
• Glycerol is a colourless thick liquid having a sweet taste. It is commonly known as
glycerine. Its molecular formula is C3H5(OH)3.
• Glycerol is used for making medicines, printing ink and ink for stamp pads.
228 Oasis School Science and Environment - 8 CHEMISTRY
Exercise
1. Choose the best answer from the given alternatives.
a. The boiling point of water is .......................... .
i. 0ºC ii. 100ºC iii. 0ºF iv. 100ºF
b. Which of the following is the soft water?
i. Rain water ii. Water of well
iii. Water of river iv. Water of sea
c. Which chemical substance is found in temporary hard water?
i. Calcium bicarbonate ii. Magnesium chloride
iii. Calcium chloride iv. Sodium chloride
d. The molecular formula of sodium bicarbonate is .......................... .
i. Na2CO3 ii. Na2SO4 iii. NaHCO3 iv. NaCO3
e. Which of the following substances is used for making printing ink?
i. Na2CO3 ii. NaHCO3 iii. CaSO4 iv. C3H5(OH)3
2. Tick (√) the correct statement and cross (×) the incorrect one.
a. About three-fourths of the earth's surface is covered by water.
b. The freezing point of water is 100 0C.
c. Permanent hardness of water can be removed by boiling.
d. The molecular formula of sodium carbonate is Na2CO3.
e. Glycerol does not dissolve in water.
3. Fill in the blanks with appropriate words.
a. Pure water is ...................... of electricity.
b. The water that does not produce lather with soap is called ...................... .
c. Permanent hard water contains ...................... salts of ...................... and
magnesium.
d. ...................... hardness of water can be removed by boiling.
e. ...................... is used in fire extinguisher.
4. Answer the following questions.
a. What are the three states of water?
b. Write any three physical properties and two chemical properties of water.
CHEMISTRY Oasis School Science and Environment - 8 229
c. What is meant by hard water and soft water?
d. What is temporary hardness of water? How can we remove such hardness?
Write.
e. What is permanent hard water? Name one chemical substance that is used to
remove such hardness.
f. Write down the molecular formula and three uses of sodium carbonate.
g. Which chemical is used for making baking powder?
h. Write any three uses of sodium bicarbonate.
i. What is glycerol? Write any three properties of glycerol.
j. Write any three uses of glycerol.
5. Differentiate between:
a. Hard water and Soft water
b. Rain water and Sea water
c. Temporary hard water and Permanent hard water
d. Sodium carbonate and Sodium bicarbonate
6. Prepare a list of chemicals found in temporary and permanent hard water.
7. What can be done to remove permanent hardness of water? Explain.
8. How can you identify hard and soft water? Explain.
230 Oasis School Science and Environment - 8 CHEMISTRY
UniT 16 Estimated teaching periods : Th Pr
12 2
Anton Van Leeuwenhoek
Living Beings
Objectives
After completing the study of this unit, students will be able to:
• introduce some microscopic organisms (Bacteria, Viruses and Fungi).
• describe the structure and functions of modified parts of plants, viz.
root, stem and leaf.
• explain the structure and functions of seed.
• describe germination of seed and conditions required for germination
of seed.
• explain dispersal of seeds.
• demonstrate and explain the life cycle of a flowering plant.
Course of Study
• Some microscopic organisms (Bacteria, Viruses and Fungi)
• Modification of different parts of plants (root, stem and leaf)
• Seed–Types and functions
• Dispersal of seeds
• Life cycle of a flowering plant
Points to be Focused/Questions to be Discussed
• What are microscopic organisms?
• What are bacteria, viruses and fungi?
• How are root, stem and leaf modified?
• What is a seed? What are its functions?
• What is germination of a seed?
• What is life cycle? What are different parts of a flower?
• What is meant by pollination and fertilization?
BIOLOGY Oasis School Science and Environment - 8 231
16.1 Introduction
The earth is inhabited by millions of living organisms. These organisms are broadly
classified into plants and animals. Some living beings like blue whale, elephant, etc. are
very large while some living beings like bacteria, protozoa, etc. are very small. Microscopic
organisms cannot be seen with our naked eyes. These organisms can be seen only under
the compound microscope. So, the organisms which cannot be seen with our naked eyes
are called microscopic organisms. Most of the microscopic organisms are unicellular, i.e.
having only one cell in their bodies. In this unit, you will study about bacteria, viruses
and fungi in brief.
16.2 Bacteria
Bacteria (singular-bacterium) are the smallest and most
widespread single-celled plant organisms. They were
discovered by Anton Van Leeuwenhoek in 1676 AD
and the term bacteria was coined by CG Ehrenberg in
1828 AD. Bacteria are the simplest, most primitive and
unicellular organisms. They are found everywhere in air,
water, soil, food, inside the bodies of living beings, etc.
Most bacteria are saprophytes but some are autotrophs.
Characteristics of bacteria Fig. 16.1 Bacterium
1. They have prokaryotic cell.
2. They lack well developed nucleus and many cell organelles.
3. They have a cell wall.
4. They may live alone or in colonies.
5. They reproduce asexually by fission.
6. Most bacteria are immotile but some bacteria like bacilli and spirilla can move.
7. They show autotrophic as well as heterotrophic nutrition.
8. Their size ranges between 0.5 to 5 micrometres.
9. Some bacteria can survive in ice and some in boiling water.
Types of bacteria
On the basis of shape, there are four types of Face file-1
bacteria. Bacteria and viruses cannot be seen
i. Coccus or Spherical bacteria through our naked eyes. So, they are
ii. Bacillus or Rod-shaped bacteria called microscopic organisms.
iii. Spirillum or Spiral bacteria
iv. Vibrio or Comma-shaped bacteria
primitive /ˈprɪmɪtɪv/ - very simple BIOLOGY
widespread /ˈwaɪdspred/ - existing over a large area
232 Oasis School Science and Environment - 8
i. Coccus or Spherical bacteria
Coccus (plural-cocci) or spherical-shaped bacteria are found in clusters like a bunch
of grapes or they may join end to end to from a long chain and some are found in
pairs. Examples: Staphylococci, Diplococci, Streptococci, etc.
Cocci bacteria Bacilli bacteria Spirillum Vibrio cholera
Fig. 16.2
ii. Bacillus or Rod-shaped bacteria
Bacillus (plural-bacilli) or rod-shaped bacteria may be present as single rod or in
chains. Examples: Escherichia coli, Bacillus anthracis, etc.
iii. Spirillum or Spiral-shaped bacteria
Sprillum (plural-spirilla) or spiral bacteria may have one or more flagella at the ends
of their bodies. Examples: Leptospira, Spirillum, etc.
iv. Vibrio or Comma-shaped bacteria
These bacteria are comma-shaped. In some bacteria, the body is curved and has only
one flagellum. Example : Vibrio cholera.
Structure of bacteria
The structure of bacteria is very simple. It DNA 2µm (microns)
contains the protoplast, i.e. living substance Cytoplasm
surrounded by a non-living cell wall and a Cell membrane
thin membrane called plasma membrane.
Flagella
The cell of bacteria does not contain membrane
bound cell organelles like mitochondria and
chloroplasts. There is a single molecule of
DNA attatched to the cell membrane. Golgi
complex and plastids are also absent but
ribosomes are present. Some bacteria contain Fig . 16.3 Structure of Escherichia coli bacterium
flagella.
cluster /ˈklʌstə(r)/ - a group of things of the same type that grow close together
BIOLOGY Oasis School Science and Environment - 8 233
Economic importance of bacteria
Bacteria have both useful as well as harmful effects.
a. Useful effects of bacteria
1. Some bacteria like Rhizobium, Nitrobacter, Nitrosomonas, etc. regulate nitrogen
cycle and help to increase the fertility of soil.
2. Most bacteria act on dead bodies and decompose them which helps to recycle
materials in the environment.
3. In industries, bacteria are used in curding of milk, tanning of leather, production
of vinegar, cheese making, and processing of coffee, tobacco, etc.
4. Many bacteria are used for making medicines like antibiotics.
5. Many bacteria live in our intestine and help in digestion of cellulose.
b. Harmful effects of bacteria
1. Bacteria cause many harmful diseases in plants, animals and human beings.
They cause diseases in potato, maize, paddy, etc. and damage them. In humans,
bacteria cause diseases like typhoid, tuberculosis, leprosy, cholera, tetanus,
pneumonia, dysentery, etc.
2. They spoil milk, meat, vegetables and other food items.
3. Some bacteria cause food poisoning by releasing toxins into spoiled food.
4. Some bacteria (denitrifying bacteria) reduce the fertility of soil.
16.3 Viruses
Viruses are the smallest and possibly the most primitive,
acellular, non-protoplasmic bodies. The word virus has
been derived from a Latin word venom which means poison
or poisonous fluid. Viruses are much smaller than bacteria,
and can be detected only under the electron microscope.
The size of viruses ranges from 25 nanometers (nm) to 250
nanometers [1nm = 10-9m]. On an average, viruses are about
50 times smaller than bacteria.
In 1886 AD, Meyer first described the viral disease of tobacco
and called it 'tobacco mosaic'. All viruses are totally parasitic Fig. 16.4 Flu virus
and behave like living organisms inside the living host cell,
while they are quite inactive and seem to be dead out of the
host cell. Therefore, viruses are placed on the boundary of the living world and non-living
world.
Viruses have different shapes like spherical, cylindrical, hexagonal, etc. Viruses do not
have cellular structure, i.e. they do not have cell membrane or cell organelles. They are
antibiotic / ˌ æ n t ɪ b a ɪ ˈ ɒ t ɪ k / - a substance that can destroy or prevent the growth of bacteria and cure infections
234 Oasis School Science and Environment - 8 BIOLOGY
simply made of nucleic acid (either DNA or RNA) which remains surrounded by coat of
proteins (a thin film of protein). Viruses can exist outside the living cell for a long period
of time but they cannot reproduce outside the living cell. Since viruses can reproduce
only inside the living cell of the host, they are called obligatory parasites.
Living properties of Viruses
i. Viruses contain genetic material, i.e. either DNA or RNA.
ii. Viruses can reproduce in a large number and they transmit hereditary
characteristics to their offspring.
iii. Viruses infect living organisms and cause various diseases.
Non-living properties of Viruses
i. They do not have a cellular structure.
ii. They do not perform metabolic activities.
iii. Viruses can be crystallized like non-living things.
iv. They become inert and cannot reproduce outside the living cell.
Viruses - The agents of diseases
Viruses cause a wide range of diseases among living organisms. A few hundred plant
diseases caused by viruses have been recorded so far. For example, mosaic disease of
tobacco, cabbage, cauliflower, mustard, etc; black ring spot of cabbage, leaf roll of tomato,
etc. Similarly, disease caused by viruses in human beings include AIDS, polio, mumps,
common cold, rabies, measles, chicken pox, swine flu, etc.
Classification of viruses [On the basis of host]
On the basis of types of host, viruses are of three types:
i. Plant Viruses: The viruses that attack plants
are called plant viruses. For example, Tobacco
Mosaic Virus (TMV). Most of the plant viruses
contain RNA as a genetic material. TMV is a rod
shaped RNAvirus that attacks leaves of tobacco.
An American scientist Stanley first isolated
TMV in the form of crystals in 1935 AD.
ii. Animal Viruses: The viruses that attack Fig.16.5 Structure of TMV
animal tissues are called animal viruses. For
example, Retro virus (HIV), Rhabdo virus, Paramyxo virus, Rhino virus, etc.
hereditary /həˈredɪtri/ - given to the offspring by its parents
metabolic /metəˈbɒlɪk/ - related to the chemical processes in living things that change food into energy and
materials for growth
BIOLOGY Oasis School Science and Environment - 8 235
Retro virus(HIV) Rhabdo virus Rhino virus Paramyxo virus
Fig. 16.6 Capsid
Head
iii. Bacteriophages: The viruses that attack bacteria and DNA
destroy their nuclear material are called bacteriophages
[phagein - to eat]. Actually, bacteriophages are the feeder Tail
or eater of bacteria. For example, T2 phage, T4 phage, etc.
The body of a bacteriophage virus is differentiated into
two parts, i.e. head and tail. The head is hexagonal in Tail
shape which contains a contractile protein coat called fibres
capsid. The cylindrical tail contains end plate and
tail fibres. Fig. 16.7 Bacteriophage virus
Activity 1
• Prepare a model of bacteriophage virus and tobacco mosaic virus. Label the
main parts and write down their main features.
Differences between Bacteria and Viruses
S.N. Bacteria S.N. Viruses
1.
Bacteria are very small in size. 1. Viruses are smaller than bacteria. They
2. They can be seen under a can be seen only under an electron
3. compound microscope. microscope.
4. Bacteria show metabolic 2. Viruses do not show metabolic
activities. activities.
Bacteria are living cellular 3. Viruses are acellular particles.
organisms. However, they show living properties
only inside the host.
A bacterial cell is surrounded by 4.
a distinct cell wall. Cell wall is absent in viruses.
Classification of viruses [On the basis of genetic material]
On the basis of presence of genetic material, viruses are of two types:
i. DNA Virus: The virus containing DNA as the genetic material is called DNA virus.
Examples: Bacteriophage virus, Small pox virus, Adeno virus, etc.
236 Oasis School Science and Environment - 8 BIOLOGY
ii. RNA Virus: The virus containing RNA as the genetic material is called RNA virus.
Examples: Retro virus, Polio virus, Rhabdo virus, Rhino virus, etc.
Modes of transmission of viruses
Viruses are transmitted by either of the following modes:
i. direct contact with the infected organism or the materials used by the patient.
ii. through the medium of air, water and food.
iii. by means of mouthparts, legs of flies, animal bite, etc.
iv. by means of droplets during coughing, sneezing, laughing, talking, etc.
v. from infected mother to her unborn/newly born child.
Reasonable fact-1
Viruses are called obligatory parasites.
Viruses can exist outside the living cell for a long period of time but they cannot
reproduce outside the living cell. Since viruses can reproduce only inside the specific
cell of the living host, they are called obligatory parasites.
Reasonable fact-2
Viruses are kept in the borderline of living beings and non-living things.
Viruses show the properties of both living beings and non-living things. So, viruses
are kept in the borderline of the living beings and non-living things.
16.4 Fungi
Fungi are non-green thallophytes without chlorophyll. They are distributed throughout
the world. They grow in dark and moist places on the surface of dead and decaying
organic matter. Mushroom, yeast and moulds (Mucor, Rhizopus, etc.) are some examples
of fungi. Moulds can be observed as they grow on food, bread, leather, fruits, barks of
trees, etc.
Mushroom Yeast Bread mould
BIOLOGY
Fig. 16.8
Oasis School Science and Environment - 8 237
Fungi cannot prepare their own food due to the absence of chlorophyll. They get their
food from dead and decaying organic matter. So, they are called saprophytes. However,
some fungi are parasites of plants and animals.
Characteristics of fungi
1. Fungi contain eukaryotic cells which may contain more than one nucleus.
2. The plant body is a thallus, i.e. without root, stem and leaves.
3. Their cell wall is made of fungus cellulose.
4. They do not have chlorophyll.
5. Their body is made of hyphae, i.e. long filaments or mycelia.
6. They may be unicellular (yeast) or multicellular (mushroom).
7. They reproduce asexually by budding, fragmentation and sporulation.
Economic importance of fungi
Useful effects of fungi
1. Fungi act on dead bodies and decompose them. Thus, they help to increase the
fertility of soil and recycle materials in the environment.
2. Some fungi, i.e. edible mushrooms are valuable food sources for human beings.
3. Some fungi like yeast are used in wine industries and bakeries.
4. Some fungi like penicillin are used as antibiotics.
Harmful effects of fungi
1. Fungi attack tissue of plants and animals and cause diseases.
2. Fungi infect humans and cause diseases like skin infection, vaginal yeast
infection, etc.
3. Many fungi attack food grains and vegetables and spoil them.
Differences between Bacteria and Fungi
S.N. Bacteria S.N. Fungi
1. They have prokaryotic cell.
2. They are unicellular. They have eukaryotic cell.
They may be unicellular or
multicellular.
3. Nuclear membrane is absent. Nuclear membrane is present.
4. They have autotrophic and
They have heterotrophic
heterotrophic nutrition. nutrition.
238 Oasis School Science and Environment - 8 BIOLOGY
16.5 Modification of Different Parts of Plants
A variety of plants are found in our surroundings.
Among them, some are small, some are medium
sized and others are big. Some plants are found
in water and others are found on land. Flowering
plants have different organs like root, stem, leaf,
flower, fruit, seed, etc. Flowering plants have many
similarities in the structure of root, stem and leaves. (a) Tap root (b) Fibrous root
The root, stem and leaves of plants perform certain Fig. 16.9
functions. Besides these functions, the root, stem and
leaves of different plants are modified to perform
various functions and adapt in their habitat. This process is called modification of parts
of plants.
a. Modification of roots
Roots are the underground parts of plants. They are usually brown, white and light
yellow in coluor. Plants have two types of roots, viz. fibrous roots and tap roots.
Roots fix the plant body firmly to the soil. They absorb water and minerals from the
soil. They also help to hold the soil together. Besides these primary functions, roots
are modified to perform other functions as follows:
i. For storage of food
Plants like carrot, turnip, radish, sweet potato, etc. have modified roots to store
food and use whenever required. In carrot, upper portion of root is wide and lower
portion of the root is tapered. In turnip, upper portion of root is round and lower
portion is tapered. In radish, the middle proton of root is swollen and lower portion
is tapered.
Roots of carrot Roots of turnip Roots of radish Roots of sweet potato
ii. For additional support Fig 16.10
In plants like banyan, peepal, etc., roots grow vertically downwards from the
branches. Sometimes they become so long that they penetrate the soil. These roots
are called prop roots. They provide additional support to the branches.
In sugarcane, maize, bamboo, etc., which have tall stem, additional roots arise from
the lower nodes of the plant and fix the plant body firmly.
penetrate /ˈpenɪtreɪt/ - to go into or through sth
BIOLOGY Oasis School Science and Environment - 8 239
Roots of banyan Roots of maize Roots of sugarcane
Fig. 16.11
iii. For vital functions
In aquatic plants like water
hyacinth, hydrilla, roots have
air storage tissues that help
in floating. Some plants have
roots having chlorophyll that
help in photosynthesis. Some
plants of marshy places have Fig. 16.12 Modified roots of water hyacinth and orchid
modified roots that help in
breathing. Some epiphytes like orchids have modified roots for absorbing food.
b. Modification of stem
Stem is the part of plant that grows above the soil. Stem consists of branches, sub-
branches, nodes and internodes. Stem transports water and minerals from root to
the leaves. It also transports prepared food from leaves to different parts of plant.
It supports leaves, flowers and fruits. Besides these primary functions, stems of
some plants are modified to perform some additional functions like support, food
manufacturing and storage.
i. Underground modification to store food
In plants like onion, garlic, potato and ginger, the stem is modified to store food. The
stem of these plants have special buds that help in reproduction. Their stems are
thick and fleshy that store food in the form of starch.
Garlic Ginger Onion Potato
Fig. 16.13 Modified stems of some plants
240 Oasis School Science and Environment - 8 BIOLOGY
ii. Sub-aerial modification to reproduce
Some plants like grass, runner, fern, etc.
have modified sub-aerial stem that help
in reproduction. Some part of their stem
remains above the soil and some below the
soil. These stems have buds that grow into
new plants.
iii. Aerial modification to provide support
In plants like passion flower, cucumber, pumpkin, etc., stem has special wire like
structures called tendril. The tendril coils round any object near it and helps the
plant to climb up.
Stem tendril in passion flower Tendril in cucumber
Fig. 16.15
iv. Aerial modification to protect the plant
Some plants like cactus, orange, lemon, etc. have thorns that act as defensive organs.
Fig. 16.16 Modified stems to protect the plant
v. Aerial modification to store food and water
Certain desert plants like Cactus, Opuntia, Aloe, etc. have thick and fleshy stem
modified for storing food and water.
tendril /ˈtendrɪl/ - a thin curling stem that grows from a climbing plant
BIOLOGY Oasis School Science and Environment - 8 241
Fig. 16.17 Stems of Opuntia and Aloe vera store food and water
c. Modification of leaf
Leaf is the third major part of a plant. It is a lateral outgrowth of the stem or the
branch. Different plants have different types of leaves. The major functions of the
leaf is to prepare food by photosynthesis. It also helps in breathing and transpiration.
Besides these basic functions, leaves of plants are modified to perform some other
functions. Some of them are given below.
In pea plant, the leaf is modified into tendril that supports the plant as it climbs up.
In some plants like prickly pear (Opunita), leaves are modified into spines that help
to reduce the loss of water by reducing the rate of transpiration.
Some plants have thorns in their leaves that help in protection. Insectivorous plants
like pitcher plant, venus-fly-trap, bladderwort, etc. have modified leaves for catching
insects.
Thorns in leaf of Cactus Pitcher plant Venus fly trap Tendril of pea
Fig. 16.18 Modification of leaves
16. 6 Seed
A seed is a ripened ovule of a flowering plant. It is a small embryonic plant enclosed in
a covering called the seed coat. A seed is the product of a mature ovule of gymnosperms
and angiosperms.
Flowering plants produce different types of seed. Seeds of some plants are very small
whereas seeds of some plants are large. Seeds of mango, coconut, peach, pumpkin, bean,
etc. are examples of large seeds. The seeds of one plant differ from those of other plants.
transpiration /trænspɪˈreɪʃn/ - the process of water passing out from the surface of a plant or leaf
242 Oasis School Science and Environment - 8 BIOLOGY
Avocado seed Wheat seeds Maize seeds Pea seeds
Fig. 16.19 Some seeds
Activity 2
• Collect some seeds of pea, soyabean, gram, maize, wheat, etc. and keep them
in water in separate beakers for one day. Those seeds absorb water and become
larger.
• Observe their size and draw their diagrams.
• Gently remove their seed coat and observe their internal parts.
• Draw the neat figures showing their internal structure.
Structure of a Seed
Various plants have various types of seeds. Their shapes and sizes are different, however
their basic structure is similar. A typical seed has three main parts–embryo, a supply of
nutrients for the embryo and a seed coat.
The embryo is an Testa Testa
immature plant from Tegmen
which a new plant will Cotyledon Plumule
grow under favourable Plumule
conditions. A seed consists Epicotyl
of cotyledons, radicle Hypocotyl
and plumule. It has one Radicle
cotyledon or seed leaf in
monocots, two cotyledons (a) Structure of a maize seed (b) Structure of a gram seed
in dicots and one or
more in gymnosperms. Fig. 16.20
In monocots, the storage
tissue is called endosperm but the cotyledons themselves serve as storage tissue in dicots.
The radicle is the embryonic root. It is the first portion of the embryo to break through the
seed coat. It develops root hairs that absorb water and attach the embryo to particles of soil.
The plumule is the embryonic shoot. The embryonic stem above the part of attachment of
cotyledons is called epicotyl and below the point of attachment is called hypocotyl. After
germination, plumule develops into stem and leaves.
embryo /ˈembrɪəʊ/ - very early stage of an organism
BIOLOGY Oasis School Science and Environment - 8 243
Seed coat is a hard and tough structure derived from the integument (outer layer) of
ovule. This protective covering is called testa. In flowering plants, a thin and membranous
covering is found inside the testa which is known as tegmen. The seed coat in a mature
seed can be paper-thin layer (e.g. peanut) or thick and hard (e.g. coconut, walnut, peach,
etc.). The seed coat helps protect the embryo from mechanical injury and from drying out.
In addition to these three basic parts, some seeds have additional projections from the
seed coat. Those projections serve to aid the absorption of water when seed is about to
germinate. There is a small scar on the seed coat called the hilum. It is found where the
seed was attached to the ovary wall. There is a small opening near the hilum which is
called micropyle. Water enters inside the seed through this opening. So seeds are enlarged
when soaked in water or sown on the soil. It helps in the germination of seed.
Differences between Radicle and Plumule
Radicle Plumule
1. The radicle is the embryonic root 1. The plumule is the embryonic shoot.
2. It grows towards the soil. 2. It grows away from the soil.
Types of Seeds
There are two types of seeds on the basis of presence of food storage tissue, viz.
endospermic seeds and non-endospermic seeds.
1. Endospermic seeds
The seeds in which food is stored in endosperm are called endospermic seeds. All
monocot seeds are endospermic seeds. Examples: Seeds of maize, wheat, rice, bajra,
etc. Castor seed is a dicot seed which contains endosperm.
2. Non-endospermic seeds
The seeds in which food is stored in cotyledons are also called non-endospermic
seeds. All dicot seeds are non-endospermic seeds except castor seeds. Examples:
seeds of pea, soyabean, gram, orange, apple, etc.
On the basis of number of cotyledons, there are two types of seeds, viz. monocot
seeds and dicot seeds.
1. Monocotyledonous seeds [mono–one, cotyledon–seed leaf]
The seeds having only one cotyledon are called monocotyledonous seeds. In
monocot seeds, endosperm occupies the major portion of the seed. Maize seed is
an example of a monocot seed. The broader part of maize seed contains endosperm
which consists of yellow or white reserve food material. The narrower part of the
seed contains cotyledon. In the cotyledon, a small embryo is present. Plumule is
present on the broader part of the seed whereas radicle is present on the opposite
side of the plumule. Endosperm and embryo are separated by a thin membrane in
the seed of maize.
endosperm /ˈendə(ʊ)spɜːm/ - the part of the plant seed that provides food for the embryo
244 Oasis School Science and Environment - 8 BIOLOGY
The words you are searching are inside this book. To get more targeted content, please make full-text search by clicking here.
Science and Environment 8
Discover the best professional documents and content resources in AnyFlip Document Base.
Search