Living Science Chemistry Class 7 ICSE Solutions Chapter 2 Physical and Chemical Changes

ICSE Living Science Chemistry for Class 7 Solutions Chapter 2 Physical and Chemical Changes

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Answer these questions
Question 1.
Write skeletal equations for the following word equations.
a. magnesium + oxygen → magnesium oxide
Answer:
Mg + O₂ →MgO

b. hydrogen + chlorine → hydrogen chloride
Answer:
H₂ + Cl₂ →HCl

c. calcium carbonate → calcium oxide + carbon dioxide
Answer:
CaCO₃ → CaO + CO₂

d. zinc carbonate → zinc oxide + carbon dioxide
Answer:
ZnCO₃ → ZnO + CO₂

Question 2.
Balance the following equations.
a. Ca + O₂ → CaO
Answer:
2Ca + O₂ → 2CaO

b. HgO → Hg + O₂
Answer:
2HgO → 2Hg + O₂

c. Fe + O₂ →Fe₂O₃
Answer:
4Fe + 3O₂ → 2Fe₂O₃

d. N₂ + H₂ → NH₃
Answer:
N₂ + 3H₂ → 2NH₃

A. Tick the most appropriate answer.

Question 1.
Which of the following is / are characteristic (s) of a physical change ?
a. Temporary change
b. Reversible
c. Formation of no new substance AH of the above
d. All of the above
Answer:
d. All of the above

Question 2.
Which of the following is not a chemical change ?
a. Rusting of iron
b. Cooking of food
c. Digestion of food
d. Freezing of water
Answer:
d. Freezing of water

Question 3.
Chemical equation are balanced to satisfy the
a. Law of conservation of mass
b. Law of constant proportions
c. Law of reciprocal proportions
d. Law of multiple proportions.
Answer:
a. Law of conservation of mass

Question 4.
The following reaction is used for the preparation of oxygen gas in the laboratory

Which of the following statements is / are correct about the reaction ?
a. It is a decomposition reaction and takes place when heat is supplied.
b. It is a combination reaction.
c. It is a decomposition reaction and accompanied by the release of heat.
d. It is a decomposition reaction and takes place in the presence of light.
Answer:
a. It is a decomposition reaction and takes place when heat is supplied.

Question 5.
A decomposition reaction is represented as
a. AB → A + B
b. A + B → AB
c. A → B
d. A+B → B + A
Answer:
a. AB → A + B

B. Fill in the blanks.

Question 1.
A ………….. change is temporary and reversible.
Answer:
A physical change is temporary and reversible.

Question 2.
In a chemical change, the original substance loses its own ………….. and properties.
Answer:
In a chemical change, the original substance loses its own composition and properties.

Question 3.
A …………. change is generally associated with the evolution or absorption of energy.
Answer:
A chemical change is generally associated with the evolution or absorption of energy.

Question 4.
A …………. represents a chemical change in terms of symbols and formulae.
Answer:
A chemical equation represents a chemical change in terms of symbols and formulae.

Question 5.
In a ………….. equation, the total number of atoms on the left-hand side is equal to the total-number of atoms on the right-hand side.
Answer:
In a balanced chemical equation, the total number of atoms on the left-hand side is equal to the total-number of atoms on the right-hand side.

C. State if the following statements are true or false. Correct the statement if it is false.

Question 1.
The law of conservation of mass states that mass is neither created nor destroyed.
Answer:
True

Question 2.
New substances are formed during a chemical reaction due to the rearrangement of the atoms of the original substances.
Answer:
True

Question 3.
In a skeletal equation, the mass of the reactants is equal to the mass of the products.
Answer:
False. In a skeletal equation, the mass of the reactants is not equal to the mass of the products.

Question 4.
In a combination reaction, a new compound is formed.
Answer:
True

Question 5.
Decomposition reactions generally take place in the presence of light.
Answer:
Tme

D. Name the following :

Question 1.
The substances which react with one another in a chemical reaction.
Answer:
Reactants

Question 2.
The substances which are produced by a chemical reaction
Answer:
Products

Question 3.
A chemical reaction in which two substances combine to form a new compound.
Answer:
Combination reaction

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

E. Answer the following in short.

Question 1.
What is a physical change ?
Answer:
A physical change is a temporary change in which no new substance is formed, and the chemical composition of the original substance is not altered. Examles : melting of wax, freezing of water, breaking a chalk piece, etc.

Question 2.
What is a chemical change ?
Answer:
A chemical change is a permanent change in which the original substance loses its own composition and properties and give rise to one or more new substances.
Examples : Rusting of iron, digestion of food, ripening of fruits, etc.

Question 3.
Why is the burning of paper a chemical change ?
Answer:
If we bum a piece of paper, new substances like carbon dioxide, water vapour, smoke and ash are produced. The molecules of paper and oxygen combine and undergo changes to form molecules of these new substances. The change is permanent and cannot be reversed. Thus, the burning of paper is a chemical change. –

Question 4.
What is a chemical equation ?
Answer:
The representation of a chemical reaction by using the symbols and formulae of the reactants and the products involved in the reaction is called its chemical equation.
Example:

Question 5.
State the law of conservation of mass.
Answer:
Law of conservation of mass states that mass is neither created nor destroyed during chemical reaction.

Question 6.
State the difference between a balanced equation and an unbalanced equation.
Answer:
In a balanced chemical equation, the number of atoms of each element on both sides of the chemical equation are same, (i.e. number of atoms of each element on the reactant side is equal to the number of atoms of the same element on the product side.)
Example:
2Mg + O₂ → 2MgO
In an unbalanced chemical equation the number of atoms of each element on both sides of the chemical equation are not  equal. It is also called as a skeletal equation.
Example: Mg + O₂ → MgO

F. Write chemical equations in symbols and formulae for the following word equations.

Question 1.
iron + sulphur → iron (II) sulphide
Answer:

Question 2.
water + carbon dioxide -» carbonic acid

Question 3.
calcium carbonate → calcium oxide + carbon dioxide
Answer:

Question 4.
silver chloride → silver + chlorine
Answer:

G Answer the following in details.

Question 1.
Write the characteristics of physical changes.
Answer:
Characteristics of physical changes :

1. Generally, the change is temporary and can be reversed by changing the conditions.
2. No new substance is formed. So there is no change in the mass of the substance undergoing a physical change.
3. There is no change in the chemical composition of the original substance.
4. There may or may not be a change in the physical properties like state, shape, size or colour of the original substance.
5. No significant change in energy is involved.

Question 2.
Write the characteristics of chemical changes.
Answer:
Characteristics of a chemical change :

1. One or more new substances are formed with composition and properties different from the original substances.
2. The change is permanent and irreversible.
3. An energy change is generally involved in a chemical change. Energy is either absorbed or evolved during a chemical change. Absorption of heat energy in a chemical reaction is called endothermic while evolution of heat energy is called exothermic reaction.
4. The mass of the substances undergoing a chemical change is generally altered. However, the total mass of the substances involved in a chemical change remains the same. Mass is neither created nor destroyed during a chemical change.

Question 3.
Give the main difference between physical changes and chemicals changes.
Answer:

Question 4.
What information does a balanced equation give us ?
Answer:
A balanced equation gives the following information :

1. Number of molecules of each of the reactants and products taking part in the reaction.
2. Number of atoms of each element of the reactants and the products taking part in the reaction.
3. It enables us to calculate the mass of the reactants and the products involved in the reaction.
4. It enables us to find that the mass of the reactants is equal to the mass of the products.
   For example, the equation
   

gives the following information :
(a) One molecule of calcium carbonate on heating gives one molecule of calcium oxide and one molecule of carbon dioxide.
(b) Mass of CaCO₃ = Mass of CaO + Mass of CO₂

Question 5.
What is a combination reaction ? Give one example of it.
Answer:
A type of chemical reaction in which two or more substances (element or compounds) combine to form a new substance (a compound) is called as a combination reaction. Generally, it is represented as :
A + B → AB

Example:
(i) Formation of hydrogen chloride

(ii) Burning of coal (carbon)

Question 6.
What is a decomposition reaction ? Give one example of it in which a catalyst is used.
Answer:
A type of chemical reaction in which a substance (usually a compound) breaks up into two or more simple substances is known as a decomposition reaction. A decomposition reaction takes place when the energy is supplied in the form of heat, electricity or light. Generally, it is represented as :
AB → A + B
Examples:
(i) Decomposition of potassium chlorate on heating in the presence of manganese dioxide (MnO₂) as a catalyst.

(ii) Decomposition of acidified water by passing electric current

H. Write the balanced chemical equation using the following information.

Question 1.
An aqueous calcium hydroxide solution (lime water) reacts with carbon dioxide gas to produce calcium carbonate and water.
Answer:

I. Complete the following chemical equations and balance them.

Question 1.
N₂ + ………..→ NO
Answer:
N₂ + O₂ → NO

Multiply NO by 2, we get
N₂ + O₂→ NO

Since the number of atoms of each element is equal on both sides, it is balanced now.

Question 2.
AgCl → ………….. + Cl₂
Answer:
AgCl → Ag + Cl₂

Multply AgCl on LHS by 2 to balance the Cl atoms.
2AgCl →Ag + Cl₂

Multply Ag on RHS by 2 to balance the Ag atoms.
2AgCl →2Ag + Cl₂

Question 3.
Mg + O₂ → …………..
Answer:
Mg + O₂ → MgO

Multply MgO on RHS by 2 to balance the oxygen atoms.
Mg + O₂ → MgO

Now, multply Mg on LHS to balance Mg atoms.
2Mg + O₂ + 2MgO

Question 4.
H₂ + ……….. → HCl
Answer:
H₂ + Cl₂ →HCl

Multply HC1 in RHS by 2 to balance both H and Cl atoms.
H₂ + Cl₂ → 2HCl

Now the equation is balanced.

Living Science Chemistry Class 7 ICSE Solutions