Selina Textbook Solutions: Elements, Compounds and Mixtures | Chemistry Class 8 ICSE PDF Download

 Page 1


Elements, Compounds and Mixtures 
Points to Remember: 
• Every substance is made up of very tiny particles, called molecules. Molecules are 
formed from even smaller particles called atoms. 
• Element— 
(a) Element is the simplest pure substance. It cannot be divided further into 
simpler substances by any chemical method, e.g. oxygen, hydrogen, sulphur, etc. 
(b) At present 116 elements are known, of which 92 are natural elements. 
• Based on their properties, elements are classified into: metals, non-metals, 
metalloids, noble gases. 
• Metals are ductile, malleable, good conductors of heat and electricity, high melting 
and boiling points. Metals are sonorous, e.g., Iron, Gold, Silver, etc. 
• Non-metals are solids and brittle in nature, bad conductor of heat and electricity 
(exception Graphite) low melting and boiling points, e.g. sulphur, carbon, 
hydrogen, etc. 
• Metalloids— These elements show properties of both metals and non-metals. 
They are hard solids, e.g. Boron, Silicon, Arsenic. 
• Inert or noble gases— These elements do not react chemically with other 
elements or compounds are called noble (Inert) gases, e.g., helium, neon, argon, 
etc. 
• Symbols of Elements— Each element is denoted by a symbol usually to first 
letter. 
Examples: Oxygen by O, Hydrogen by H. 
• Atom— “An Atom is the smallest particle of an element that can take part in a 
chemical reaction but may or may not have independent existence.” 
The atom of an element exhibits all the properties of that element. 
• Molecule— A molecule is the smallest particle of a pure substance of element or 
compound which has independent existence. It exhibits all the properties of pure 
substance. 
• Atomicity— The number of atoms of an element that join together to form a 
molecule of that element is known as the atomicity. 
• Molecular Formula— of an element is the symbolic representation of its 
molecule. It indicates the number of atoms present in it. e.g. Magnesium oxide 
– MgO. 
 
 
 
 
 
Page 2


Elements, Compounds and Mixtures 
Points to Remember: 
• Every substance is made up of very tiny particles, called molecules. Molecules are 
formed from even smaller particles called atoms. 
• Element— 
(a) Element is the simplest pure substance. It cannot be divided further into 
simpler substances by any chemical method, e.g. oxygen, hydrogen, sulphur, etc. 
(b) At present 116 elements are known, of which 92 are natural elements. 
• Based on their properties, elements are classified into: metals, non-metals, 
metalloids, noble gases. 
• Metals are ductile, malleable, good conductors of heat and electricity, high melting 
and boiling points. Metals are sonorous, e.g., Iron, Gold, Silver, etc. 
• Non-metals are solids and brittle in nature, bad conductor of heat and electricity 
(exception Graphite) low melting and boiling points, e.g. sulphur, carbon, 
hydrogen, etc. 
• Metalloids— These elements show properties of both metals and non-metals. 
They are hard solids, e.g. Boron, Silicon, Arsenic. 
• Inert or noble gases— These elements do not react chemically with other 
elements or compounds are called noble (Inert) gases, e.g., helium, neon, argon, 
etc. 
• Symbols of Elements— Each element is denoted by a symbol usually to first 
letter. 
Examples: Oxygen by O, Hydrogen by H. 
• Atom— “An Atom is the smallest particle of an element that can take part in a 
chemical reaction but may or may not have independent existence.” 
The atom of an element exhibits all the properties of that element. 
• Molecule— A molecule is the smallest particle of a pure substance of element or 
compound which has independent existence. It exhibits all the properties of pure 
substance. 
• Atomicity— The number of atoms of an element that join together to form a 
molecule of that element is known as the atomicity. 
• Molecular Formula— of an element is the symbolic representation of its 
molecule. It indicates the number of atoms present in it. e.g. Magnesium oxide 
– MgO. 
 
 
 
 
 
Exercise 3(A) 
Question 1. 
Define: (a) Elements (b) Compounds 
Answer: 
(a) Elements: Element is a substance which cannot be broken further into simpler 
substances and has a definite set of properties. Elements are made up of only one kind 
of atoms. 
(b) Compounds: Compounds are pure substances composed of two or more elements 
in definite proportion by mass and has properties, entirely different from those of its 
constituents elements. 
Compound, are made up of different types of atoms combined chemically. 
Question 2. 
Give two examples for each of the following: 
(a) Metals (b) Non-metals 
(c) Metalloids (d) Inert gases 
Answer: 
(a) Metals: Iron, silver, gold. 
(b) Non-metals: Carbon, sulphur, oxygen. 
(c) Metalloids: Antimony, silicon, boron. 
(d) Inert gases: Helium, argon, neon. 
Question 3. 
Differentiate between: 
(a) Pure and impure substances 
(b) Homogenous and heterogenous substances 
Answer: 
(a) Pure substances — 
1. Pure substances have definite composition and definite physical and chemical 
properties. 
2. They are all homogeneous i.e. their composition is uniform throughout the bulk. 
3. Examples: Elements and compounds. 
Impure substances — 
1. Impure substances are made up of two or more pure substances mixed together 
in any proportion. 
2. They may be homogeneous or hetergeneous i.e. their composition is not uniform 
throughout the bulk. 
3. They are all mixtures. 
Examples: air, sea water, petroleum, a solution of sugar in water are all impure 
substances. 
Page 3


Elements, Compounds and Mixtures 
Points to Remember: 
• Every substance is made up of very tiny particles, called molecules. Molecules are 
formed from even smaller particles called atoms. 
• Element— 
(a) Element is the simplest pure substance. It cannot be divided further into 
simpler substances by any chemical method, e.g. oxygen, hydrogen, sulphur, etc. 
(b) At present 116 elements are known, of which 92 are natural elements. 
• Based on their properties, elements are classified into: metals, non-metals, 
metalloids, noble gases. 
• Metals are ductile, malleable, good conductors of heat and electricity, high melting 
and boiling points. Metals are sonorous, e.g., Iron, Gold, Silver, etc. 
• Non-metals are solids and brittle in nature, bad conductor of heat and electricity 
(exception Graphite) low melting and boiling points, e.g. sulphur, carbon, 
hydrogen, etc. 
• Metalloids— These elements show properties of both metals and non-metals. 
They are hard solids, e.g. Boron, Silicon, Arsenic. 
• Inert or noble gases— These elements do not react chemically with other 
elements or compounds are called noble (Inert) gases, e.g., helium, neon, argon, 
etc. 
• Symbols of Elements— Each element is denoted by a symbol usually to first 
letter. 
Examples: Oxygen by O, Hydrogen by H. 
• Atom— “An Atom is the smallest particle of an element that can take part in a 
chemical reaction but may or may not have independent existence.” 
The atom of an element exhibits all the properties of that element. 
• Molecule— A molecule is the smallest particle of a pure substance of element or 
compound which has independent existence. It exhibits all the properties of pure 
substance. 
• Atomicity— The number of atoms of an element that join together to form a 
molecule of that element is known as the atomicity. 
• Molecular Formula— of an element is the symbolic representation of its 
molecule. It indicates the number of atoms present in it. e.g. Magnesium oxide 
– MgO. 
 
 
 
 
 
Exercise 3(A) 
Question 1. 
Define: (a) Elements (b) Compounds 
Answer: 
(a) Elements: Element is a substance which cannot be broken further into simpler 
substances and has a definite set of properties. Elements are made up of only one kind 
of atoms. 
(b) Compounds: Compounds are pure substances composed of two or more elements 
in definite proportion by mass and has properties, entirely different from those of its 
constituents elements. 
Compound, are made up of different types of atoms combined chemically. 
Question 2. 
Give two examples for each of the following: 
(a) Metals (b) Non-metals 
(c) Metalloids (d) Inert gases 
Answer: 
(a) Metals: Iron, silver, gold. 
(b) Non-metals: Carbon, sulphur, oxygen. 
(c) Metalloids: Antimony, silicon, boron. 
(d) Inert gases: Helium, argon, neon. 
Question 3. 
Differentiate between: 
(a) Pure and impure substances 
(b) Homogenous and heterogenous substances 
Answer: 
(a) Pure substances — 
1. Pure substances have definite composition and definite physical and chemical 
properties. 
2. They are all homogeneous i.e. their composition is uniform throughout the bulk. 
3. Examples: Elements and compounds. 
Impure substances — 
1. Impure substances are made up of two or more pure substances mixed together 
in any proportion. 
2. They may be homogeneous or hetergeneous i.e. their composition is not uniform 
throughout the bulk. 
3. They are all mixtures. 
Examples: air, sea water, petroleum, a solution of sugar in water are all impure 
substances. 
(b) Homogeneous mixture — is a mixture where the components that make up the 
mixture are uniformly distributed throughout the mixture. 
Example — air, sugar water, rain water. 
Heterogeneous mixture — is a mixture, where the components of the mixture are not 
uniform or have localized regios with different properties. 
Example—Cereal in milk, vegetable soup. 
Question 4. 
Write the chemical name of the following and also give their molecular formulae: 
(a) Baking soda (b) Vinegar 
(c) Marble (d) Sand 
Answer: 
(a) Sodium bicarbonate (Baking soda) — NaHCO3 
(b) Acetic acid (Vinegar) — CH3COOH 
(c) Calcium carbonate (Marble) — CaCO3 
(d) Silicon dioxide (Sand) — SiO2 
Question 5. 
Name: 
(a) a soft metal 
(b) a metal which is brittle 
(c) a non-metal which is lustrous 
(d) a liquid metal 
(e) a metal which is a poor conductor of electricity. 
(f) a non-metal which is a good conductor of electricity. 
(g) a liquid non-metal 
(h) the hardest naturally occurring substance 
(i) an inert gas 
Answer: 
(a) Gold 
(b) Zinc 
(c) Iodine 
(d) Mercury 
(e) Tungsten 
(f) Graphite 
(g) Bromine 
(h) Diamond 
(i) Neon, helium 
Question 6. 
How is sodium chloride different from its constituent elements ? 
Answer: 
The properties of sodium chloride are completely different from those of sodium and 
chlorine. Sodium is a soft, highly reactive metal. Chlorine is a poisonous non-metallic 
Page 4


Elements, Compounds and Mixtures 
Points to Remember: 
• Every substance is made up of very tiny particles, called molecules. Molecules are 
formed from even smaller particles called atoms. 
• Element— 
(a) Element is the simplest pure substance. It cannot be divided further into 
simpler substances by any chemical method, e.g. oxygen, hydrogen, sulphur, etc. 
(b) At present 116 elements are known, of which 92 are natural elements. 
• Based on their properties, elements are classified into: metals, non-metals, 
metalloids, noble gases. 
• Metals are ductile, malleable, good conductors of heat and electricity, high melting 
and boiling points. Metals are sonorous, e.g., Iron, Gold, Silver, etc. 
• Non-metals are solids and brittle in nature, bad conductor of heat and electricity 
(exception Graphite) low melting and boiling points, e.g. sulphur, carbon, 
hydrogen, etc. 
• Metalloids— These elements show properties of both metals and non-metals. 
They are hard solids, e.g. Boron, Silicon, Arsenic. 
• Inert or noble gases— These elements do not react chemically with other 
elements or compounds are called noble (Inert) gases, e.g., helium, neon, argon, 
etc. 
• Symbols of Elements— Each element is denoted by a symbol usually to first 
letter. 
Examples: Oxygen by O, Hydrogen by H. 
• Atom— “An Atom is the smallest particle of an element that can take part in a 
chemical reaction but may or may not have independent existence.” 
The atom of an element exhibits all the properties of that element. 
• Molecule— A molecule is the smallest particle of a pure substance of element or 
compound which has independent existence. It exhibits all the properties of pure 
substance. 
• Atomicity— The number of atoms of an element that join together to form a 
molecule of that element is known as the atomicity. 
• Molecular Formula— of an element is the symbolic representation of its 
molecule. It indicates the number of atoms present in it. e.g. Magnesium oxide 
– MgO. 
 
 
 
 
 
Exercise 3(A) 
Question 1. 
Define: (a) Elements (b) Compounds 
Answer: 
(a) Elements: Element is a substance which cannot be broken further into simpler 
substances and has a definite set of properties. Elements are made up of only one kind 
of atoms. 
(b) Compounds: Compounds are pure substances composed of two or more elements 
in definite proportion by mass and has properties, entirely different from those of its 
constituents elements. 
Compound, are made up of different types of atoms combined chemically. 
Question 2. 
Give two examples for each of the following: 
(a) Metals (b) Non-metals 
(c) Metalloids (d) Inert gases 
Answer: 
(a) Metals: Iron, silver, gold. 
(b) Non-metals: Carbon, sulphur, oxygen. 
(c) Metalloids: Antimony, silicon, boron. 
(d) Inert gases: Helium, argon, neon. 
Question 3. 
Differentiate between: 
(a) Pure and impure substances 
(b) Homogenous and heterogenous substances 
Answer: 
(a) Pure substances — 
1. Pure substances have definite composition and definite physical and chemical 
properties. 
2. They are all homogeneous i.e. their composition is uniform throughout the bulk. 
3. Examples: Elements and compounds. 
Impure substances — 
1. Impure substances are made up of two or more pure substances mixed together 
in any proportion. 
2. They may be homogeneous or hetergeneous i.e. their composition is not uniform 
throughout the bulk. 
3. They are all mixtures. 
Examples: air, sea water, petroleum, a solution of sugar in water are all impure 
substances. 
(b) Homogeneous mixture — is a mixture where the components that make up the 
mixture are uniformly distributed throughout the mixture. 
Example — air, sugar water, rain water. 
Heterogeneous mixture — is a mixture, where the components of the mixture are not 
uniform or have localized regios with different properties. 
Example—Cereal in milk, vegetable soup. 
Question 4. 
Write the chemical name of the following and also give their molecular formulae: 
(a) Baking soda (b) Vinegar 
(c) Marble (d) Sand 
Answer: 
(a) Sodium bicarbonate (Baking soda) — NaHCO3 
(b) Acetic acid (Vinegar) — CH3COOH 
(c) Calcium carbonate (Marble) — CaCO3 
(d) Silicon dioxide (Sand) — SiO2 
Question 5. 
Name: 
(a) a soft metal 
(b) a metal which is brittle 
(c) a non-metal which is lustrous 
(d) a liquid metal 
(e) a metal which is a poor conductor of electricity. 
(f) a non-metal which is a good conductor of electricity. 
(g) a liquid non-metal 
(h) the hardest naturally occurring substance 
(i) an inert gas 
Answer: 
(a) Gold 
(b) Zinc 
(c) Iodine 
(d) Mercury 
(e) Tungsten 
(f) Graphite 
(g) Bromine 
(h) Diamond 
(i) Neon, helium 
Question 6. 
How is sodium chloride different from its constituent elements ? 
Answer: 
The properties of sodium chloride are completely different from those of sodium and 
chlorine. Sodium is a soft, highly reactive metal. Chlorine is a poisonous non-metallic 
gas while sodium chloride is a very useful non poisonous compound which is added to 
our food to get minerals and also to add taste to it. 
Question 7. 
Why is iron sulphide a compound ? 
Answer: 
Iron sulphide is a compound which can be broken into the elements iron and sulphur they both 
have different properties. The properties of compound are entirely different from there of its 
constituents elements. 
Exercise 3(B) 
Question 1. 
Classify the following substances into compounds and mixtures: 
Answer: 
Carbon dioxide, air, water, milk, common, salt, blood, fruit juice, iron sulphide. 
Carbon dioxide — (Compound) 
air — (Mixture) 
water — (Compound) 
milk — (Mixture) 
common salt — (Compound) 
blood — (Mixture) 
fruit juice — (Mixture) 
iron sulphide — (Compound) 
Question 2. 
Give one example for each of the following types of mixtures 
(a) solid-solid homogenous mixture 
(b) solid-liquid heterogenous mixture 
(c) misicible liquids 
(d) liquid-gas homogenous mixture 
Answer: 
(a) Solid-solid homogenous mixture — Alloys of metals e.g. brass, bronze stainless 
steel etc. 
(b) Solid-liquid heterogenous mixture — Sand and water, mud and water, sugar and oil. 
(c) Misicible liquids — water and ethanol. 
(d) Liquid-gas homogenous mixture — Air 
Question 3. 
Suggest a suitable technique to separate the constituents of the following mixtures. Also 
give the reason for selecting the particular method. 
(a) Salt from sea water 
(b) Ammonium chloride from sand 
(c) Chalk powder from water 
(d) Iron from sulphur 
(e) Water and alcohol 
Page 5


Elements, Compounds and Mixtures 
Points to Remember: 
• Every substance is made up of very tiny particles, called molecules. Molecules are 
formed from even smaller particles called atoms. 
• Element— 
(a) Element is the simplest pure substance. It cannot be divided further into 
simpler substances by any chemical method, e.g. oxygen, hydrogen, sulphur, etc. 
(b) At present 116 elements are known, of which 92 are natural elements. 
• Based on their properties, elements are classified into: metals, non-metals, 
metalloids, noble gases. 
• Metals are ductile, malleable, good conductors of heat and electricity, high melting 
and boiling points. Metals are sonorous, e.g., Iron, Gold, Silver, etc. 
• Non-metals are solids and brittle in nature, bad conductor of heat and electricity 
(exception Graphite) low melting and boiling points, e.g. sulphur, carbon, 
hydrogen, etc. 
• Metalloids— These elements show properties of both metals and non-metals. 
They are hard solids, e.g. Boron, Silicon, Arsenic. 
• Inert or noble gases— These elements do not react chemically with other 
elements or compounds are called noble (Inert) gases, e.g., helium, neon, argon, 
etc. 
• Symbols of Elements— Each element is denoted by a symbol usually to first 
letter. 
Examples: Oxygen by O, Hydrogen by H. 
• Atom— “An Atom is the smallest particle of an element that can take part in a 
chemical reaction but may or may not have independent existence.” 
The atom of an element exhibits all the properties of that element. 
• Molecule— A molecule is the smallest particle of a pure substance of element or 
compound which has independent existence. It exhibits all the properties of pure 
substance. 
• Atomicity— The number of atoms of an element that join together to form a 
molecule of that element is known as the atomicity. 
• Molecular Formula— of an element is the symbolic representation of its 
molecule. It indicates the number of atoms present in it. e.g. Magnesium oxide 
– MgO. 
 
 
 
 
 
Exercise 3(A) 
Question 1. 
Define: (a) Elements (b) Compounds 
Answer: 
(a) Elements: Element is a substance which cannot be broken further into simpler 
substances and has a definite set of properties. Elements are made up of only one kind 
of atoms. 
(b) Compounds: Compounds are pure substances composed of two or more elements 
in definite proportion by mass and has properties, entirely different from those of its 
constituents elements. 
Compound, are made up of different types of atoms combined chemically. 
Question 2. 
Give two examples for each of the following: 
(a) Metals (b) Non-metals 
(c) Metalloids (d) Inert gases 
Answer: 
(a) Metals: Iron, silver, gold. 
(b) Non-metals: Carbon, sulphur, oxygen. 
(c) Metalloids: Antimony, silicon, boron. 
(d) Inert gases: Helium, argon, neon. 
Question 3. 
Differentiate between: 
(a) Pure and impure substances 
(b) Homogenous and heterogenous substances 
Answer: 
(a) Pure substances — 
1. Pure substances have definite composition and definite physical and chemical 
properties. 
2. They are all homogeneous i.e. their composition is uniform throughout the bulk. 
3. Examples: Elements and compounds. 
Impure substances — 
1. Impure substances are made up of two or more pure substances mixed together 
in any proportion. 
2. They may be homogeneous or hetergeneous i.e. their composition is not uniform 
throughout the bulk. 
3. They are all mixtures. 
Examples: air, sea water, petroleum, a solution of sugar in water are all impure 
substances. 
(b) Homogeneous mixture — is a mixture where the components that make up the 
mixture are uniformly distributed throughout the mixture. 
Example — air, sugar water, rain water. 
Heterogeneous mixture — is a mixture, where the components of the mixture are not 
uniform or have localized regios with different properties. 
Example—Cereal in milk, vegetable soup. 
Question 4. 
Write the chemical name of the following and also give their molecular formulae: 
(a) Baking soda (b) Vinegar 
(c) Marble (d) Sand 
Answer: 
(a) Sodium bicarbonate (Baking soda) — NaHCO3 
(b) Acetic acid (Vinegar) — CH3COOH 
(c) Calcium carbonate (Marble) — CaCO3 
(d) Silicon dioxide (Sand) — SiO2 
Question 5. 
Name: 
(a) a soft metal 
(b) a metal which is brittle 
(c) a non-metal which is lustrous 
(d) a liquid metal 
(e) a metal which is a poor conductor of electricity. 
(f) a non-metal which is a good conductor of electricity. 
(g) a liquid non-metal 
(h) the hardest naturally occurring substance 
(i) an inert gas 
Answer: 
(a) Gold 
(b) Zinc 
(c) Iodine 
(d) Mercury 
(e) Tungsten 
(f) Graphite 
(g) Bromine 
(h) Diamond 
(i) Neon, helium 
Question 6. 
How is sodium chloride different from its constituent elements ? 
Answer: 
The properties of sodium chloride are completely different from those of sodium and 
chlorine. Sodium is a soft, highly reactive metal. Chlorine is a poisonous non-metallic 
gas while sodium chloride is a very useful non poisonous compound which is added to 
our food to get minerals and also to add taste to it. 
Question 7. 
Why is iron sulphide a compound ? 
Answer: 
Iron sulphide is a compound which can be broken into the elements iron and sulphur they both 
have different properties. The properties of compound are entirely different from there of its 
constituents elements. 
Exercise 3(B) 
Question 1. 
Classify the following substances into compounds and mixtures: 
Answer: 
Carbon dioxide, air, water, milk, common, salt, blood, fruit juice, iron sulphide. 
Carbon dioxide — (Compound) 
air — (Mixture) 
water — (Compound) 
milk — (Mixture) 
common salt — (Compound) 
blood — (Mixture) 
fruit juice — (Mixture) 
iron sulphide — (Compound) 
Question 2. 
Give one example for each of the following types of mixtures 
(a) solid-solid homogenous mixture 
(b) solid-liquid heterogenous mixture 
(c) misicible liquids 
(d) liquid-gas homogenous mixture 
Answer: 
(a) Solid-solid homogenous mixture — Alloys of metals e.g. brass, bronze stainless 
steel etc. 
(b) Solid-liquid heterogenous mixture — Sand and water, mud and water, sugar and oil. 
(c) Misicible liquids — water and ethanol. 
(d) Liquid-gas homogenous mixture — Air 
Question 3. 
Suggest a suitable technique to separate the constituents of the following mixtures. Also 
give the reason for selecting the particular method. 
(a) Salt from sea water 
(b) Ammonium chloride from sand 
(c) Chalk powder from water 
(d) Iron from sulphur 
(e) Water and alcohol 
(f) Sodium chloride and potassium nitrate 
(g) Calcium carbonate and sodium chloride 
Answer: 
(a) The technique used to separate the salt from seawater is Evaporation. 
Reason – Because this method is used to separate the components of the 
homogeneous solid-liquid mixture. In this method, sea water is collected in a shallow 
bed and allowed to evaporate in the sun. When all the water is evaporated, salt is left 
behind. By this method, we only get solid and liquid is evaporated in its vapour form. 
(b) Technique used to separate Ammonium chloride from sand is sublimation. 
Because this method is used for solid mixtures in which one of the components can 
sublime on heating. In this method, Ammonium chloride changes into vapours on 
heating and salt is left behind. 
(c) Technique used to separate chalk powder from water is filtration. 
Reason – Because this process is used to separate the components of a 
heterogeneous solid-liquid mixture in which solids are lights and insoluble in liquids. 
Substances used as filters are sand filter paper at C. These filters allows the liquid to 
pass through them, but not solids. 
(d) Technique to separate iron from sulpher is magnetic separation. 
Because, this method is used when one of the component of mixture is Iron. Iron gets 
attracted towards the magnet and hence get separated. 
(e) Technique used to separate water and Alcohol is Fractional Distillation. 
Because in this method, the vapours of water is left behind in the original vessel as the 
alcohol boils at lower temperature than water. Thus these two liquids can be separated. 
(f) Technique used is Fractional-crystallisation. 
Because: This method is used when solubility of solid components of mixture and 
different in the same solvent. Here, sodium chloride and potassium nitrate. Both are 
soluble in water but solubility of potassium nitrate is more. 
(g) Technique used is Solvent Extraction Method: Because, by this method, salts get 
dissolve in water while calcium carbonate being insoluble in water settles down in the 
container. And hence get separated about. 
Question 4. 
(a) Define mixture. 
(b) Why is it necessary to separate the constituents of a mixture. 
(c) State four differences between compounds and mixtures. 
Answer: