Chapter 2 – Physical and Chemical Change

Selina Concise Chemistry 2023 solutions for class 7 Chemistry. Chapter 2 – Physical and Chemical Change is provided here with simple step-by-step explanations. These solutions for Physical and Chemical Change (2nd Chapter) are extremely popular among class 10 students. The Physical and Chemical Change solution is handy for quickly completing your homework and preparing for exams.

Question 1.

(a) Define:

1. a physical change
2. a chemical change

(b) Give two examples for each of the above two changes.

Answer

(a) (i) Physical change: A temporary change in which no new substance is formed, the composition of the substance remains the same, though its state, shape and size may change.

(ii) Chemical change: A permanent change in which new substances are formed whose composition and properties are completely different from those of the original substances.

(b) Physical change:

1. Change of water into its vapours.
2. Heating of iron rod.

Chemical change:

1. Burning of wood.
2. Breathing.

Question 2.

What are reversible and irreversible changes? Give one example for each.

Answer

(a) Reversible change: When a change in a substance can be reversed by changing the conditions, it is said to be a reversible change.

Example: Melting of ghee or wax.

(b) Irreversible change: When a substance can not be brought back to its original state after a change, it is said to be an irreversible change.

Example: Souring of milk.

Question 3.

Mention a change which is always

1. desirable
2. undesirable
3. periodic

Answer

1. Desirable change: Cleaning your house regularly to maintain hygiene.
2. Undesirable change: Rotting of fruits and vegetables.
3. Periodic change: Seasonal changes in weather, such as the transition from summer to winter.

Question 4.

Is burning a physical change or a chemical change? Why?

Answer

Burning is a chemical change as the new substance is formed with new properties and it cannot be reversed.

Question 5.

A burning candle shows both physical and chemical changes. Explain ?

Answer

A burning candle produces wax vapours which solidify and form wax again is a physical change. At the same time, wax on burning produces water vapours and carbon dioxide which escape into the atmosphere which are new substances with new properties. It is a chemical change.

Hence burning of candles shows both physical and chemical changes.

Question 6.

State three differences between evaporation and boiling.

Answer

Question 7.

State four differences between physical and chemical changes.

Answer

Question 8.

What do you observe when :

1. water is boiled,
2. a piece of paper is burnt,
3. some ice cubes are kept in a glass tumbler,
4. solid ammonium chloride is heated,
5. an iron nail is kept in tap water for a few days,
6. a spoon of sugar is heated in a pan,
7. A lighted match stick is brought near the mouth of the test tube containing hydrogen gas,
8. quick lime is dissolved in water,
9. little amount of curd is added to a bowl containing warm milk and kept for five hours

Answer

1. Formation of bubbles, steam, and a change in state from liquid to gas.
2. Smoke, flames, and ash are produced, and a chemical change occurs, converting the paper into new substances.
3. The ice melts, and water droplets form on the surface of the tumbler due to condensation.
4. The solid ammonium chloride sublimes to form ammonium gas and hydrogen chloride gas.
5. The iron nail rusts, leading to a chemical change and the formation of reddish-brown rust.
6. The sugar caramelizes and turns brown, leading to a chemical change.
7. The hydrogen gas burns explosively with a pop sound, and water vapour is formed.
8. Quicklime reacts with water, producing heat and forming slaked lime.
9. The milk undergoes fermentation, and the proteins coagulate, leading to the formation of curd.

Question 9.

Name a chemical change which takes place in presence of:

1. Heat
2. Light
3. Electricity

Answer

(a) Heat:

• Burning of paper.
• Cooking food needs heat.

(b) Light :

• The formation of food by plants i.e. photosynthesis needs light.
• Hydrogen and chlorine react in presence of light

(c) Electricity :

• Water breaks into hydrogen and oxygen on passing electricity.
• Sodium chloride solution breaks up into its components sodium and Chlorine passing an electric current through sodium chloride solution.

Objective Type Questions

Question 1.

Fill in the blanks.

1. A new substance is always formed in a ____ change.
2. Melting of ice is a ____ change.
3. When a candle burns, wax melts. The melting of wax is a ____ change.
4. A chemical change occurs as a result of the rearrangement of ____ between two substances.
5. The burning of fuel is a ____ change.
6. The revolution of the earth around the sun is a natural ____ change.
7. Growing a seedling into a plant is a ____ change.

Answer

1. chemical
2. physical
3. physical
4. rearrangement of atoms
5. chemical
6. natural periodic
7. chemical

Question 2.

State whether the following are physical or chemical changes.

1. glowing of a bulb
2. burning of sugar
3. heating of water
4. growing of a piglet into a pig
5. burning of wood
6. passing electric current through a heater rod
7. water cycle in nature
8. respiration in living beings
9. shaping a piece of glass
10. lightning
11. dissolving sugar in water
12. heating a mixture of iron filings and sulphur
13. mixing oil with water
14. cutting wood into small pieces
15. photosynthesis
16. Digestion of food
17. Melting of wax
18. Boiling of an egg
19. Slaking of lime

Answer

Question 3.

Match the following:

Answer

Question 4.

Write true or false against the following statements :

1. Cutting paper into pieces is a chemical change.
2. Rusting of iron is a chemical change.
3. Earthquake is a desirable change.
4. The melting of ice is a physical change.
5. The burning of sugar is a temporary change.

Answer

1. False
2. True
3. False
4. True
5. False

Multiple Choice Questions

Tick (√) the correct alternative from the choice given for the following statements :

Question 1.

A substance which can not sublime is

1. iodine
2. camphor
3. sugar
4. dry ice

Answer

sugar

Explanation:

Sugar is a substance that cannot sublime. Sublimation is the process in which a solid directly changes into a gas without going through the liquid state.

Question 2.

When you put some ice cubes in a glass, droplets of water are formed on the outer wall of the glass. This explains the phenomenon of:

1. melting
2. freezing
3. condensation
4. evaporation

Answer

condensation.

Explanation:

When you put some ice cubes in a glass, the surrounding air cools down, and the water vapour in the air loses its heat energy. This results in the condensation of the water vapour into liquid droplets on the outer surface of the glass. This phenomenon is known as condensation, which is the process of water vapour changing into liquid water due to a decrease in temperature or an increase in pressure.

Question 3.

Burning is a

1. slow process
2. fast process
3. natural process
4. none of the above

Answer

fast process

Explanation:

Burning is a process in which a fuel reacts with an oxidizing agent to produce heat and light.

Question 4.

Which one of the following is volatile in nature ?

1. common salt
2. petrol
3. water
4. milk

Answer

petrol.

Explanation:

Volatile substances are those that have a tendency to vaporize or evaporate quickly at room temperature. Among the given options, petrol is a volatile substance because it is a mixture of hydrocarbons that are highly volatile and readily evaporate into the air. Common salt, water, and milk, on the other hand, are not volatile substances as they do not easily vaporize or evaporate at room temperature.

Question 5.

An example of both physical and chemical change is

1. burning of candle
2. melting of ice
3. cooking of food
4. blowing of bulb

Answer

Burning of a candle

Explanation:

The burning of a candle is an example of both physical and chemical change. The melting of wax and the solidification of the melted wax are physical changes, while the burning of the wick and wax to produce heat, light, water vapour, and carbon dioxide involves chemical changes, specifically combustion.

Question 6.

The compound rust is a hydrated oxide of

1. copper
2. aluminium
3. iron
4. gold

Answer

iron.

Explanation:

Rust is a reddish-brown compound that forms on iron or iron alloys when they are exposed to oxygen and moisture for an extended period. It is a hydrated form of iron oxide, which means it contains water molecules within its crystal structure. Therefore, rust is a hydrated oxide of iron, not copper, aluminium, or gold.

Question 7.

When sugar is heated, its colour changes into

1. red
2. brown
3. black
4. grey

Answer

brown.

Explanation:

When sugar is heated, it undergoes a chemical reaction known as caramelization. During this process, the sugar molecules break down and re-form into a new complex mixture of compounds, resulting in a brown colour and a characteristic flavour. The extent of browning depends on the temperature and duration of heating. Therefore, the correct option is brown.

Question 8.

A pop sound is heard when a lighted match stick is brought near the mouth of a jar. This indicates the release of gas.

1. oxygen
2. hydrogen
3. nitrogen
4. water-vapour

Answer

hydrogen.

Explanation:

When a lighted match stick is brought near the mouth of a jar, a pop sound is heard if the jar contains hydrogen gas. This is because hydrogen gas is highly flammable and reacts explosively with oxygen in the air in the presence of a spark or flame. The reaction produces water vapor, and a loud pop sound is heard due to the rapid expansion of gases. Therefore, the correct option is hydrogen.

Question 9.

When we add water to the following substances, which one will show a chemical change?

1. salt
2. sugar
3. oil
4. quick lime

Answer

quick lime

Explanation:

When water is added to quick lime (calcium oxide), a chemical reaction takes place, known as slaking of lime. In this reaction, quick lime reacts with water to produce slaked lime (calcium hydroxide), releasing heat in the process.