Selina Textbook Solutions: Chemical Reactions | Chemistry Class 8 ICSE PDF Download

 Page 1


Chemical Reactions 
Question 1. 
(a) Define a chemical reaction. 
(b) What happens during a chemical reaction ? 
(c) What do you understand by a chemical bond ? 
Answer: 
(a) Any chemical change in matter which involves transformation into one or more 
substances with entirely different properties is called a chemical reaction. 
(b) A chemical reaction involves breaking of chemical bonds between the atoms or 
groups of atoms of reacting substances and rearrangement of atoms making new bonds 
to form new substances. 
 
(c) A chemical bond is the attractive force that holds the atoms of a molecule together, 
in a compound. 
Question 2. 
Give one example each of which illustrates the following characteristics of a chemical 
reaction: 
(a) evolution of a gas 
(b) change of colour 
(c) change in state 
Answer: 
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an 
effervescence 
 
Page 2


Chemical Reactions 
Question 1. 
(a) Define a chemical reaction. 
(b) What happens during a chemical reaction ? 
(c) What do you understand by a chemical bond ? 
Answer: 
(a) Any chemical change in matter which involves transformation into one or more 
substances with entirely different properties is called a chemical reaction. 
(b) A chemical reaction involves breaking of chemical bonds between the atoms or 
groups of atoms of reacting substances and rearrangement of atoms making new bonds 
to form new substances. 
 
(c) A chemical bond is the attractive force that holds the atoms of a molecule together, 
in a compound. 
Question 2. 
Give one example each of which illustrates the following characteristics of a chemical 
reaction: 
(a) evolution of a gas 
(b) change of colour 
(c) change in state 
Answer: 
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an 
effervescence 
 
(b) When blue coloured copper sulphate reacts with hydrogen sulphide gas, a black 
coloured substance copper sulphide is formed. 
 
(c) The reaction between hydrogen sulphide and chlorine (both gases) produces 
sulphur (solid) and hydrogen chloride (gas). 
 
Question 3. 
How do the following help in bringing about a chemical change? 
(a) pressure (b) light 
(c) catalyst (d) heat. 
Answer: 
(a) Some chemical reactions take place when reactants are subjected to high pressure. 
e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas. 
 
(b) Some chemical reactions can take place in the presence of light. Ex. 
Photosynthesis. 
 
(c) A catalyst can either increases or decreases the rate of chemical reaction and some 
chemical reactions need a catalyst to change the rate of the reaction, in case it is too 
slow or too fast. 
1. Positive catalyst: When a catalyst increase the rate of reaction finely divided iron 
is used as a positive catalyst in the manufacturing of ammonia from hydrogen and 
nitrogen. 
 
Page 3


Chemical Reactions 
Question 1. 
(a) Define a chemical reaction. 
(b) What happens during a chemical reaction ? 
(c) What do you understand by a chemical bond ? 
Answer: 
(a) Any chemical change in matter which involves transformation into one or more 
substances with entirely different properties is called a chemical reaction. 
(b) A chemical reaction involves breaking of chemical bonds between the atoms or 
groups of atoms of reacting substances and rearrangement of atoms making new bonds 
to form new substances. 
 
(c) A chemical bond is the attractive force that holds the atoms of a molecule together, 
in a compound. 
Question 2. 
Give one example each of which illustrates the following characteristics of a chemical 
reaction: 
(a) evolution of a gas 
(b) change of colour 
(c) change in state 
Answer: 
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an 
effervescence 
 
(b) When blue coloured copper sulphate reacts with hydrogen sulphide gas, a black 
coloured substance copper sulphide is formed. 
 
(c) The reaction between hydrogen sulphide and chlorine (both gases) produces 
sulphur (solid) and hydrogen chloride (gas). 
 
Question 3. 
How do the following help in bringing about a chemical change? 
(a) pressure (b) light 
(c) catalyst (d) heat. 
Answer: 
(a) Some chemical reactions take place when reactants are subjected to high pressure. 
e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas. 
 
(b) Some chemical reactions can take place in the presence of light. Ex. 
Photosynthesis. 
 
(c) A catalyst can either increases or decreases the rate of chemical reaction and some 
chemical reactions need a catalyst to change the rate of the reaction, in case it is too 
slow or too fast. 
1. Positive catalyst: When a catalyst increase the rate of reaction finely divided iron 
is used as a positive catalyst in the manufacturing of ammonia from hydrogen and 
nitrogen. 
 
2. Negative Catalyst: When a catalyst decreases the rate of reaction. 
Ex. Phosphoric acid act as a negative catalyst to decrease the rate of the 
decomposition of hydrogen peroxide. 
(d) Some chemical reactions take place only in the presence of heat. 
e.g. When lead nitrate is heated, it breaks into lead monoxide, nitrogen dioxide and 
oxygen. 
 
Question 4. 
(a) Define catalyst. 
(b) What are (i) positive catalysts and (ii) negative catalysts? Support your answer with 
one example for each of them. 
(c) Name three biochemical catalysts found in the human body. 
Answer: 
(a) Catalyst: A catalyst is a substance that either increases or decreases the rate of a 
chemical reaction without itself undergoing any chemical change during the reaction. 
(b) (i) Positive catalyst: When a catalyst increases the rate of chemical reaction, it is 
called positive catalyst. 
e.g. when potassium chlorate heated to 700°C decomposes to evolve oxygen gas, 
when MnO2 is added the decomposition takes place at 300°C 
 
(ii) Negative catalyst: When a catalyst decreases the rate of chemical reaction it is 
called negative catalyst. 
Example. Phosphoric acid acts as a negative catalyst to decrease the rate of the 
decomposition of hydrogen peroxide. Alcohol too acts as a negative catalyst in certain 
chemical reactions. 
(c) Biochemical catalysts found in human body: 
1. Pepsin 
2. Tryspin 
3. lipase. 
Question 5. 
What do you observe when 
(a) dilute sulphuric acid is added to granulated zinc? 
(b) a few pieces of iron are dropped in a blue solution of copper sulphate? 
(c) silver nitrate is added to a solution of sodium chloride? 
(d) ferrous sulphate solution is added to an aqueous solution of sodium hydroxide. 
Page 4


Chemical Reactions 
Question 1. 
(a) Define a chemical reaction. 
(b) What happens during a chemical reaction ? 
(c) What do you understand by a chemical bond ? 
Answer: 
(a) Any chemical change in matter which involves transformation into one or more 
substances with entirely different properties is called a chemical reaction. 
(b) A chemical reaction involves breaking of chemical bonds between the atoms or 
groups of atoms of reacting substances and rearrangement of atoms making new bonds 
to form new substances. 
 
(c) A chemical bond is the attractive force that holds the atoms of a molecule together, 
in a compound. 
Question 2. 
Give one example each of which illustrates the following characteristics of a chemical 
reaction: 
(a) evolution of a gas 
(b) change of colour 
(c) change in state 
Answer: 
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an 
effervescence 
 
(b) When blue coloured copper sulphate reacts with hydrogen sulphide gas, a black 
coloured substance copper sulphide is formed. 
 
(c) The reaction between hydrogen sulphide and chlorine (both gases) produces 
sulphur (solid) and hydrogen chloride (gas). 
 
Question 3. 
How do the following help in bringing about a chemical change? 
(a) pressure (b) light 
(c) catalyst (d) heat. 
Answer: 
(a) Some chemical reactions take place when reactants are subjected to high pressure. 
e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas. 
 
(b) Some chemical reactions can take place in the presence of light. Ex. 
Photosynthesis. 
 
(c) A catalyst can either increases or decreases the rate of chemical reaction and some 
chemical reactions need a catalyst to change the rate of the reaction, in case it is too 
slow or too fast. 
1. Positive catalyst: When a catalyst increase the rate of reaction finely divided iron 
is used as a positive catalyst in the manufacturing of ammonia from hydrogen and 
nitrogen. 
 
2. Negative Catalyst: When a catalyst decreases the rate of reaction. 
Ex. Phosphoric acid act as a negative catalyst to decrease the rate of the 
decomposition of hydrogen peroxide. 
(d) Some chemical reactions take place only in the presence of heat. 
e.g. When lead nitrate is heated, it breaks into lead monoxide, nitrogen dioxide and 
oxygen. 
 
Question 4. 
(a) Define catalyst. 
(b) What are (i) positive catalysts and (ii) negative catalysts? Support your answer with 
one example for each of them. 
(c) Name three biochemical catalysts found in the human body. 
Answer: 
(a) Catalyst: A catalyst is a substance that either increases or decreases the rate of a 
chemical reaction without itself undergoing any chemical change during the reaction. 
(b) (i) Positive catalyst: When a catalyst increases the rate of chemical reaction, it is 
called positive catalyst. 
e.g. when potassium chlorate heated to 700°C decomposes to evolve oxygen gas, 
when MnO2 is added the decomposition takes place at 300°C 
 
(ii) Negative catalyst: When a catalyst decreases the rate of chemical reaction it is 
called negative catalyst. 
Example. Phosphoric acid acts as a negative catalyst to decrease the rate of the 
decomposition of hydrogen peroxide. Alcohol too acts as a negative catalyst in certain 
chemical reactions. 
(c) Biochemical catalysts found in human body: 
1. Pepsin 
2. Tryspin 
3. lipase. 
Question 5. 
What do you observe when 
(a) dilute sulphuric acid is added to granulated zinc? 
(b) a few pieces of iron are dropped in a blue solution of copper sulphate? 
(c) silver nitrate is added to a solution of sodium chloride? 
(d) ferrous sulphate solution is added to an aqueous solution of sodium hydroxide. 
(e) solid lead nitrate is heated? 
(f) when dilute sulphuric acid is added to barium chloride solution ? 
Answer: 
(a) When Zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with an 
effervesence. 
Zn + dil. H2SO4 ? Zn SO4 + H2. 
(b) When a few pieces of iron are dropped into a blue coloured copper sulphate 
solution, the blue colour of the solution fades and eventually turns into green. 
(c) When a solution of silver nitrate is added to a solution of sodium chloride, white 
insoluble ppt. of silver chloride is formed. 
AgNO3 (aq) + NaCl (aq) ? AgCl (ppt) + NaNO3 (aq) 
(d) When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green 
ppt. of ferrous hydroxide is formed. 
FeSO4 (aq) + 2NaOH (aq) ? Fe(OH)2 ? + Na2SO4(aq) 
(e) When solid lead nitrate is heated, it decomposes to produce light yellow solid lead 
monoxide, reddish brown nitrogen dioxide gas and colourless oxygen gas. 
 
(f) When few drops of dilute sulphuric acid is added to barium chloride solution, a white 
precipitate of barium sulphate is formed. 
 
Question 6. 
Complete and balance the following chemical equations: 
Page 5


Chemical Reactions 
Question 1. 
(a) Define a chemical reaction. 
(b) What happens during a chemical reaction ? 
(c) What do you understand by a chemical bond ? 
Answer: 
(a) Any chemical change in matter which involves transformation into one or more 
substances with entirely different properties is called a chemical reaction. 
(b) A chemical reaction involves breaking of chemical bonds between the atoms or 
groups of atoms of reacting substances and rearrangement of atoms making new bonds 
to form new substances. 
 
(c) A chemical bond is the attractive force that holds the atoms of a molecule together, 
in a compound. 
Question 2. 
Give one example each of which illustrates the following characteristics of a chemical 
reaction: 
(a) evolution of a gas 
(b) change of colour 
(c) change in state 
Answer: 
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an 
effervescence 
 
(b) When blue coloured copper sulphate reacts with hydrogen sulphide gas, a black 
coloured substance copper sulphide is formed. 
 
(c) The reaction between hydrogen sulphide and chlorine (both gases) produces 
sulphur (solid) and hydrogen chloride (gas). 
 
Question 3. 
How do the following help in bringing about a chemical change? 
(a) pressure (b) light 
(c) catalyst (d) heat. 
Answer: 
(a) Some chemical reactions take place when reactants are subjected to high pressure. 
e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas. 
 
(b) Some chemical reactions can take place in the presence of light. Ex. 
Photosynthesis. 
 
(c) A catalyst can either increases or decreases the rate of chemical reaction and some 
chemical reactions need a catalyst to change the rate of the reaction, in case it is too 
slow or too fast. 
1. Positive catalyst: When a catalyst increase the rate of reaction finely divided iron 
is used as a positive catalyst in the manufacturing of ammonia from hydrogen and 
nitrogen. 
 
2. Negative Catalyst: When a catalyst decreases the rate of reaction. 
Ex. Phosphoric acid act as a negative catalyst to decrease the rate of the 
decomposition of hydrogen peroxide. 
(d) Some chemical reactions take place only in the presence of heat. 
e.g. When lead nitrate is heated, it breaks into lead monoxide, nitrogen dioxide and 
oxygen. 
 
Question 4. 
(a) Define catalyst. 
(b) What are (i) positive catalysts and (ii) negative catalysts? Support your answer with 
one example for each of them. 
(c) Name three biochemical catalysts found in the human body. 
Answer: 
(a) Catalyst: A catalyst is a substance that either increases or decreases the rate of a 
chemical reaction without itself undergoing any chemical change during the reaction. 
(b) (i) Positive catalyst: When a catalyst increases the rate of chemical reaction, it is 
called positive catalyst. 
e.g. when potassium chlorate heated to 700°C decomposes to evolve oxygen gas, 
when MnO2 is added the decomposition takes place at 300°C 
 
(ii) Negative catalyst: When a catalyst decreases the rate of chemical reaction it is 
called negative catalyst. 
Example. Phosphoric acid acts as a negative catalyst to decrease the rate of the 
decomposition of hydrogen peroxide. Alcohol too acts as a negative catalyst in certain 
chemical reactions. 
(c) Biochemical catalysts found in human body: 
1. Pepsin 
2. Tryspin 
3. lipase. 
Question 5. 
What do you observe when 
(a) dilute sulphuric acid is added to granulated zinc? 
(b) a few pieces of iron are dropped in a blue solution of copper sulphate? 
(c) silver nitrate is added to a solution of sodium chloride? 
(d) ferrous sulphate solution is added to an aqueous solution of sodium hydroxide. 
(e) solid lead nitrate is heated? 
(f) when dilute sulphuric acid is added to barium chloride solution ? 
Answer: 
(a) When Zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with an 
effervesence. 
Zn + dil. H2SO4 ? Zn SO4 + H2. 
(b) When a few pieces of iron are dropped into a blue coloured copper sulphate 
solution, the blue colour of the solution fades and eventually turns into green. 
(c) When a solution of silver nitrate is added to a solution of sodium chloride, white 
insoluble ppt. of silver chloride is formed. 
AgNO3 (aq) + NaCl (aq) ? AgCl (ppt) + NaNO3 (aq) 
(d) When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green 
ppt. of ferrous hydroxide is formed. 
FeSO4 (aq) + 2NaOH (aq) ? Fe(OH)2 ? + Na2SO4(aq) 
(e) When solid lead nitrate is heated, it decomposes to produce light yellow solid lead 
monoxide, reddish brown nitrogen dioxide gas and colourless oxygen gas. 
 
(f) When few drops of dilute sulphuric acid is added to barium chloride solution, a white 
precipitate of barium sulphate is formed. 
 
Question 6. 
Complete and balance the following chemical equations: 
 
Answer: