Question

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Answer 1

1) Classify the following changes in as many ways as possible

(a)Growth of a tree

(b) Burning of paper

(c) Weathering of a rock

(d) Making ice-cream from milk

Ans)

(a)Growth of a tree

• Irreversible Change and Physical Change.

(b) Burning of paper

• Irreversible Change and Physical Change.

(c) Weathering of a rock

• Irreversible Change and Physical Change.

(d) Making ice-cream from milk

• Chemical Change and Reversible Change.

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2)State two changes which are reversible

• Melting
• Boiling
• Freezing

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3. Mention one application of expansion on heating and contraction on cooling.

→Liquid Thermometer.

• Liquid thermometers are the most commonly used. They are simple, inexpensive, long-lasting and accurate. The working of a liquid thermometer is based on the principle that liquids expand on heating and contract on cooling.

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4. Classify the following into physical and chemical changes.

(a) Melting of wax

(b) Burning of agarbatti

(c) Tearing of cloth

(d)Cooking of food

Ans

(a) Melting of wax

• Physical Change.

(b) Burning of agarbatti

• Physical Change.

(c) Tearing of cloth

• Physical Change.

(d)Cooking of food

• Chemical Change.

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5. Write two points of difference between a chemical change and a physical change

→Physical Change

• A physical change affects only the physical properties of a substance, such as size, shape and state.

• No new substance is formed in a physical change.

• A physical change can be either reversible or irreversible.

→Chemical Change

• There may be a change in colour.
• Evolution of a gas or effervescence may be observed.

• A chemical change are generally irreversible.

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$\red{\boxed{ \it \: Know \: More}}$

$\green{ \it\boxed{PHYSICAL \: CHANGES}}$

Changes that affect only the physical properties of a substance and where no new substances are formed are called physical changes.

$Few \: Examples$

• Melting of butter

• Tearing of paper

• Breaking of glass

• Dissolution of sugar in water

• Evaporation of seawater

• Condensation of water vapour

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$\green{ \boxed{ \it \: CHEMICAL \: CHANGES}}$

Changes that involve the formation of new substances are called chemical changes.

$Few \: Examples$

• burning of paper

• Curdling of milk

• Cooking of food

• Digestion of food in the body

• photosynthesis in plants

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$\green {\boxed{ \it \: REVERSIBLE \: AND \: IRREVERSIBLE \: CHANGES}}$

• A reversible change is one that can be reversed by removing the cause of the change.

• An irreversible change is one that cannot be easily reversed.

Answer 2

Answers:-

1. (a) Growth of a tree:

• Irreversible Change

• Slow Change

• Non-Periodic Change

• Chemical Change

(b) Burning of paper:

• Irreversible Change

• Fast Change

• Non-Periodic Change

• Physical Change

(c) Weathering of a rock:

• Irreversible Change

• Slow Change

• Non-Periodic Change

• Physical and Chemical Change

(d) Making ice cream from milk

• Irreversible Change

• Slow Change

• Non-Periodic Change

• Physical Change

• Man-Made Change

2. Change of ice to water, and Change of water to steam

3. This expansion and contraction phenomenon is used in thermometers. When the temperature of the person is high, the mercury in the thermometer rises. Similarly, when the temperature is normal, nothing happens to the mercury since it doesn't expand in absence of heat.

4. (a) Physical Change

(b) Chemical Change

(c) Physical Change

(d) Chemical Change

5. Physical Change:

• Mostly reversible. Exceptions: burning of paper, etc.

• There is no change in the internal arrangement of the molecules, only the shape or size changes.

Chemical Change:

• Mostly irreversible.

• There is a change in the internal arrangement of the molecules.