Living Science Chemistry Class 8 ICSE Solutions Chapter 6 Chemical Reactions

ICSE Living Science Chemistry for Class 8 Solutions Chapter 6 Chemical Reactions

Think And Answer (Page 72)

A green amorphous metal carbonate powder (P) on heating decomposes to evolve carbon dioxide, and a black residue is left behind, which is the oxide of the metal present in P. Identify P.
Answer:
The green amorphous metal carbonate powder (P) is copper carbonate (CuCO₃).

Check Your Progress (Page 74)

Answer these questions.
Question 1.
Name the attractive forces that hold atoms or ions together to form molecules or compounds.
Answer:
Chemical bonds.

Question 2.
Name the type of chemical reaction that occurs when magnesium ribbon is burned in air.
Answer:
Synthesis or combination reaction.

Question 3.
What happens to limestone on heating?
Answer:
When limestone (calcium carbonate) is heated, it breaks up into calcium oxide (quicklime) and carbon dioxide.

Question 4.
What type of chemical reaction is involved when silver chloride is exposed to light?
Answer:
Photo chemical decomposition reaction.

Question 5.
Name the type of chemical reaction that occurs between a solution of sodium chloride and a solution of silver nitrate.
Answer:
Precipitation reaction.

Question 6.
What type of reaction is the following?
acid + base → salt + water
Answer:
Neutralization reaction.

Think And Answer (Page 76)

When water is added to a chemical compound X, a lot of heat energy is evolved along with the formation of calcium hydroxide. What is X?
Answer:
Chemical compound X is quicklime (calcium oxide).

A. Tick The most appropriate answer

Question 1.
Attractive forces that hold atoms or Ions together to form molecules or compounds are called
a. Ionic bonds
b. covalent bonds.
c. chemical bonds
d. none of these.
Answer:
c. chemical bonds

Question 2.
An atom that loses electrons becomes a positively-charged ion called
a. cation.
b. anion
c. ion.
d. all of these
Answer:
a. cation.

Question 3.
When two atoms share electrons, they form a bond called
a. ionic bond.
b. covalent bond.
c. chemical bond.
d. electrovalent bond.
Answer:
b. covalent bond.

Question 4.
2Mg(s) + O₂(g) → 2MgO(s) is an example of a –
a. combination reaction.
b. double displacement reaction.
c. decomposition reaction.
d. displacement reaction
Answer:
a. combination reaction.

Question 5.
AB → A + B is the representation of a –
a. combination reaction.
b. double displacement reaction.
c. decomposition reaction.
d. displacement reaction
Answer:
c. decomposition reaction

Question 6.
When chlorine gas is passed through an aqueous solution of potassium bromide, the solution turns brown due to the liberation of –
a. chlorine.
b. oxygen.
c. bromine.
d. iodine.
Answer:
d. iodine.

Question 7.
Solutions of silver nitrate and sodium chloride react to form a white precipitate of –
a. sodium chloride.
b. silver nitrate.
c. sodium nitrate.
d. silver chloride.
Answer:
d. silver chloride

Question 8.
Our stomach produces –
a. hydrochloric acid.
b. sulphuric acid,
c. nitric acid.
d. none of these
Answer:
a. hydrochloric acid.

Question 9.
As you move down the activity series, the reactivity of metals –
a. increases
b. does not change,
c. decreases.
d. varies.
Answer:
c. decreases.

Question 10.
Chemical reactions in which heat energy is evolved are called –
a. endothermic reactions.
b. decomposition reactions.
c. displacement reactions.
d. exothermic reactions.
Answer:
d. exothermic reactions.

B. Fill in the blanks

Question 1.
Metals react with non-metals to form stable compounds called …………….. compounds.
Answer:
Metals react with non-metals to form stable compounds called ionic compounds or electrovalent compounds.

Question 2.
An atom that gains electrons becomes a negatively-charged ion called ……………..
Answer:
An atom that gains electrons becomes a negatively-charged ion called anion ……………..

Question 3.
The attractive force between oppositely charged ions that holds ions together is called a …………….. bond.
Answer:
The attractive force between oppositely charged ions that holds ions together is called a chemical bond.

Question 4.
An acid turns blue litmus paper or solution ……………..
Answer:
An acid turns blue litmus paper or solution red.

Question 5.
Salts are compounds formed by the combination of an acid and a ……………..
Answer:
Salts are compounds formed by the combination of an acid and a base.

Question 6.
The most reactive metal is at the ……………..of the activity series of metals.
Answer:
The most reactive metal is at the top of the activity series of metals.

Question 7.
Chemical reactions in which heat is absorbed are called ……………..
Answer:
Chemical reactions in which heat is absorbed are called endothermic.

Question 8.
…………….. are formed when metals or non-metals react with oxygen in the air.
Answer:
Oxides are formed when metals or non-metals react with oxygen in the air.

C. Match the columns

1. Electronegative ion	a. neutralization reaction
2. Reaction between sodium hydroxide and hydrochloric acid	b. basic oxides
3. Limestone on heating	c. show neither basic nor acidic properties
4. Dissolution of ammonium chloride in water	d. exothermic reaction
5. Oxides of metals	e. acidic oxides
6. Oxides of non-metals	f. show both basic and acidic properties
7. Amphoteric oxides	g. anion
8. Neutral oxides	h. decomposition reaction
	i. endothermic reaction

Answer:

1. Electronegative ion	g. anion
2. Reaction between sodium hydroxide and hydrochloric acid	a. neutralization reaction
3. Limestone on heating	h. decomposition reaction
4. Dissolution of ammonium chloride in water	i. endothermic reaction
5. Oxides of metals	b. basic oxides
6. Oxides of non-metals	e. acidic oxides
7. Amphoteric oxides	f. show both basic and acidic properties
8. Neutral oxides	c. show neither basic nor acidic properties

D. Write true or false. Correct the false statements

Question 1.
The attractive force between oppositely charged ions that holds the ions together is called an ionic bond.
Answer:
False; The attractive force between oppositely charged ions that hold the ions together is called an chemical bond.

Question 2.
When electricity is passed through acidified water, it decomposes to give hydrogen and carbon dioxide.
Answer:
False; When electricity is passed through acidified water, it decomposes to give hydrogen and oxygen.

Question 3.
Potassium chlorate on heating gives potassium chloride and oxygen.
Answer:
True

Question 4.
When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.
Answer:
True

Question 5.
Neutralization reactions between acids and bases are endothermic reactions.
Answer:
False; Neutralization reactions between acids and bases are exothermic reactions.

Question 6.
When metal carbonates are heated strongly, metal oxides are formed.
Answer:
True

Question 7.
Acidic oxides that are dissolved in water turn red litmus blue.
Answer:
False; Acidic oxides that are dissolved in water turn blue litmus red.

E. Name the following

Question 1.
Substances that take part in a chemical reaction
Answer:
Reactants

Question 2.
Substances produced because of a reaction
Answer:
Products

Question 3.
Stable compounds that are formed when metals react with non-metals
Answer:
Ionic or electrovalent

Question 4.
A reaction in which a substance breaks up into two or more simpler substances
Answer:
Decomposition reaction

Question 5.
A reaction in which an element displaces another element from its compound
Answer:
Displacement reaction

Question 6.
A substance that alters the speed of a chemical reaction but does not take part in the reaction
Answer:
Catalyst

Question 7.
The insoluble residue formed when solutions of two compounds react with each other
Answer:
Precipitate

Question 8.
Compounds that are formed when metals or non-metals react with oxygen in the air
Answer:
Oxides

F. Classify the following reactions as combination, displacement, decomposition or neutralization reactions.

Question 1.
CaCO₃ → CaO + CO₂
Answer:
decomposition reaction

Question 2.
Cu + 2AgNO₃ → CU(NO₃)₂+ 2Ag
Answer:
displacement reaction

Question 3.
NaOH + HCl → NaCl + H₂O
Answer:
neutralization reaction.

Question 4.
Fe + S → FeS
Answer:
combination, or synthesis reaction

G Complete the following chemical equations and balance them.

Question 1.

Answer:

Question 2.
Cl₂(g) + KBr(aq) → + Br₂(g)
Answer:
Cl₂(g) + 2KBr(aq) → 2KCl(aq) + Br₂(g)

Question 3.
BaCl₂ + Na₂SO₄(aq) → BaSO₄ + NaCl
Answer:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

Question 4.
CuO → CuSO₄+ H₂O
Answer:
CuO + H₂SO₄ → CuSO₄ + H₂O

Question 5.
Pb(NO₃)₂ + NO₂ + O₂
Answer:
2Pb(NO₃)₂→ 2PbO + 4NO₂ + O₂

H. Answer the following in short

Question 1.
What is a chemical reaction?
Answer:
A chemical reaction is defined as a process in which one or more substances are converted into one or more different substances.

Question 2.
Differentiate between an electrovalent bond and a covalent bond.
Answer:
The attractive force between oppositely charged ions that holds the ions together is called an ionic bond or electrovalent bond. When two atoms share electrons, they form a bond called a covalent bond.

Question 3.
What is a double displacement reaction?
Answer:
A chemical reaction in which two compounds in their aqueous solution interchange their ions to form new compounds is called a double displacement reaction or double decomposition reaction.

Question 4.
What is a neutralization reaction?
Answer:
A neutralization reaction is a type of double displacement reaction in which an acid and a base react together to form a salt and water.

Question 5.
Differentiate between an acid and a base.
Answer:
An acid yields hydronium ions (H₃O⁺) when dissolved in water.
A base is either a metallic hydroxide or a metallic oxide that reacts with an acid to form salt and water.

Question 6.
What type of reaction is governed by the position of a metal in the metal activity series?
Answer:
Displacement reactions

Question 7.
What is the difference between an exothermic reaction and an endothermic reaction?
Answer:
Chemical reactions in which heat energy is evolved are called exothermic reactions. Chemical reactions in which heat is absorbed are called endothermic reactions.

Question 8.
Name the types of oxides.
Answer:
Based on their acid-base characteristics, oxides are classified into acidic oxides, basic oxides, amphoteric oxides and neutral oxides.

I. Answer the following in detail

Question 1.
Name all the types of chemical reactions and write an example of each.
Answer:
The chemical reactions are classified into the following types.

• Combination reactions
• Decomposition reactions
• Displacement reactions
• Double displacement reactions

Combination reactions :
A reaction in which two or more elements or substances combine to form a single product is called a combination reaction. This type of reaction is also known as a synthesis reaction. The general format of a combination reaction is as follows.
A + B → AB

Example : Burning of magnesium: When a piece of magnesium ribbon is burned or ignited in air (oxygen), it bums with a white dazzling light to form a white powder of magnesium oxide

A reaction in which a single compound breaks down into two or more simpler substances is known as a decomposition reaction. Generally, a decomposition reaction is represented as :
AB → A + B
When decomposition is carried out by heating, it is called thermal decomposition.

Example : Decomposition of limestone on heating : When limestone (calcium carbonate) is heated, it breaks up into calcium oxide (quicklime) and carbon dioxide.

Decomposition reactions that take place when an electric current is passed through the compound in its molten state or in its aqueous solution are called electrolytic decomposition reactions or simply electrolysis.

Example : When electricity is passed through acidified water, it decomposes to give hydrogen and oxygen.

Decomposition reactions that take place on the absorption of light are called photochemical decomposition reactions. Example : When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.

Displacement reactions :
A reaction in which a more active (reactive), element displaces a less active (reactive) element from its compound when reacted together is called a displacement reaction.
Generally, a displacement reaction is represented as :
A + BC → AC + B

Dpuble displacement reactions :
A chemical reaction in which two compounds in their aqueous solution interchange their ions to form new compounds is called a double displacement reaction or double decomposition reaction. A double displacement reaction is represented as :
AB + CD → AD + CB
These reactions are of two types :

• precipitation reactions and
• neutralization reactions.

Precipitation reactions :
A double displacement reaction usually occurs in solution. When one of the products formed is an insoluble or nearly insoluble solid, the type of double displacement reaction is known as a precipitation reaction.
Reaction between a solution of sodium chloride and a solution of silver nitrate : When solutions of silver nitrate and sodium chloride react, a white precipitate of silver chloride is produced along with sodium nitrate.

Neutralization reactions :
A neutralization reaction is a type of double displacement reaction in which an acid and a base react together to form a salt and water.

Example : Reaction between sodium hydroxide and hydrochloric acid: When a solution of sodium hydroxide reacts with dilute hydrochloric acid, sodium chloride (salt) and water are formed.

Question 2.
Explain how the principle of a displacement reaction is based on the reactivity of metals and non-metals.
Answer:
The principle of displacement reactions is based on the reactivity of metals and non-metals. The activity series of metals helps us to predict which metals will replace other metals. It has been observed that a more reactive metal can replace the ion of any metal lower than it on the list. For example, copper can react with a solution of silver nitrate and displace the silver to form , topper nitrate.

A highly reactive non-metal can also displace a less reactive non-metal in a reaction. For example, more active halogens can displace ions of less active halogens from their compounds.

Question 3.
Describe an experiment to study a neutralization reaction.
Answer:
Aim : To study the reaction between acids and bases, that is, neutralization reaction.
Materials required: Sodium hydroxide solution, phenolphthalein solution, dilute hydrochloric acid, red and blue litmus papers, a burette, a conical flask, a pipette, a clamping stand.

Procedure :
1. Take about 5mL of dilute hydrochloric acid solution in a conical flask. Add a drop or two of phenolphthalein to it.
2. Take sodium hydroxide solution in a burette. Add this solution to the hydrochloric acid in the conical flask drop by drop with constant stirring.

Observation :
A stage is reached when the solution in the conical flask just changes to pink.
(i) Divide the solution in the conical flask into two parts. To one add a blue litmus paper and to the other a red litmus paper.
Observation : There is no change in either of the litmus papers.
Conclusion : A neutralization reaction has taken place between the two reactants since the solution obtained is neither acidic nor basic in nature.

Question 4.
Explain exothermic reactions with the help of examples.
Answer:
Exothermic reactions : Chemical reactions in which heat energy is evolved are called exothermic reactions.
Examples :
(i) Dissolution of quicklime in water: When water is added to quicklime (calcium oxide), a lot of heat energy is produced along with calcium hydroxide.

(ii) Neutralization reactions between acids and bases: Neutralisation reactions between acids and bases are exothermic reactions. When a solution of sodium hydroxide reacts with dilute hydrochloric acid, heat is produced along with sodium chloride (salt) and water.

Question 5.
Describe the methods used to prepare oxides.
Answer:
Preparation of oxides :
1. By the direct combination of metals and non-metals with oxygen: When they are heated, many metals and non-metals react with oxygen in the air to produce the respective metal oxides and non-metal oxides.

2. By the thermal decomposition of metal carbonates :
When metal carbonates are heated strongly, metal oxides are formed.

3. By the thermal decomposition of metal nitrates: Metal nitrates decompose on heating to give metal oxide, brown fumes of nitrogen dioxide and oxygen.

4. By the thermal decomposition of metal sulphates: Metal decomposition of metal sulphates decompose to produce metal oxides and gaseous products such as sulphur dioxide and sulphur trioxide.

Question 6.
Differentiate among acidic oxides, basic oxides, amphoteric oxides and neutral oxides.
Answer:
Classification of oxides : Based on their acid-base characteristics, oxides are classified into acidic oxides, basic oxides, amphoteric oxides and neutral oxides.
Acidic oxides : Acidic oxides are the oxides of non-metals. They dissolve in water to produce acids. For example,

Basic oxides : Basic oxides are the oxides of metals. They are usually insoluble in water. Basic oxides that are soluble in water form alkalis. For example,

Amphoteric oxides :
Amphoteric oxides are metallic oxides that show both basic and acidic properties. ZnO, Al₂O₃ and PbO are examples of amphoteric oxides.

Neutral oxides : Neutral oxides show neither basic nor acidic properties. This means that they do not form salt while reacting with acids or bases. Neutral oxides do not affect litmus paper or solution. Some examples of neutral oxides are carbon monoxide (CO), nitrous oxide (N₂O) and nitric oxide (NO).

Think And Answer

Question 1.
Sodium chloride is an ionic compound. Why?
Answer:
Sodium chloride is an ionic compound because when sodium metal reacts with chlorine gas, a sodium atom loses one electron, which is gained by a chlorine atom. As a result, a soidum ion and a chloride ion are formed. The oppositely charged ions attract each other. The attractive force between oppositely charged ions that holds the ions together is called an ionic bond or electrovalent bond.

Question 2.
The reaction between solutions of silver nitrate and sodium chloride is a precipitation reaction. Explain.
Answer:
When solutions of silver nitrate and sodium chloride react, a white precipitate of silver chloride is produced along with sodium nitrate.

Question 3.
The dissolution of quicklime in water is an exothermic reaction. Why?
Answer:
When water is added to quicklime (calcium oxide), a lot of heat energy is produced along with calcium hydroxide.

Question 4.
Acidic oxides that are dissolved in water turn blue litmus red. Why?
Answer:
Acidic oxides are the oxides of non-metals. They dissolve in water to produce acids. Acids turn blue litmus solution red. Hence, acidic oxides that are dissolved in water turn blue litmus red.

Question 5.
Nitrous oxide is a neutral oxide. Explain.
Answer:
Nitrous oxide is a neutral oxide because it show neither basic nor acidic properties. This means that it does not form salt while reacting with acids or bases. It does not affect litmus paper or solution.

Living Science Chemistry Class 8 ICSE Solutions