Changes Around Us: Physical and Chemical Changes Chapter 6

Physical & Chemical Changes – Quick Review

1.Which two statements best describe a physical change?

• (i) Only the shape or state may alter.
• (ii) A brand-new substance appears.
• (iii) Properties stay the same.
• (iv) Chemical bonds break forever.

(a) (i) and (iii)   (b) (ii) and (iv)   (c) (i)(ii)(iii)   (d) (ii)(iii)(iv)

Answer – (a): In a physical change, matter keeps its identity; only shape or state alters, giving no new substance.

2.Which of these changes can be reversed easily?

(a) Stitching cloth pieces into a shirt   (b) Burning dry leaves in a pit  
(c) Dissolving sugar crystals in warm water   (d) Baking wet clay into bricks

Answer – (c): Sugar can be recovered by evaporating water, so the change is reversible, unlike stitching, burning or baking.

3.Why is dissolving salt in water called a reversible physical change?

Heating the salty water lets water escape as vapour, leaving unchanged salt crystals; therefore dissolving salt is a reversible physical change.

4.Explain why curdling of milk is considered a chemical change.

Curdling forms curd and buttermilk because bacteria create lactic acid. Milk cannot be converted back, so new substances form, classifying it as a chemical change.

5.Which option shows a chemical change?

(a) Melting butter   (b) Burning paper   (c) Breaking chalk   (d) Freezing fruit juice

Answer – (b): Burning paper produces ash, smoke and gases with entirely new properties; this irreversible formation proves a chemical change occurred.

6.What is meant by a chemical change? Give one everyday example.

A chemical change makes one or more new substances with different properties.
Example: Rusting of iron or cooking food.

7.Which process below mainly involves physical weathering of rocks?

(a) Acid rain   (b) Rusting   (c) Wind abrasion   (d) Dissolving limestone in water

Answer – (c): Strong winds carrying sand grind rock surfaces into smaller chips without altering minerals; therefore wind abrasion is simple physical weathering.

8.What are some simple ways to prevent iron objects from rusting?

1. Painting – A coat of paint keeps air and water away from the iron.
2. Oiling/Greasing – Thin film prevents moisture reaching the metal.
3. Galvanizing – Coating with zinc shields iron from oxygen and water.
4. Using Stainless Steel – Chromium forms a passive layer that stops rust.

9.Is rusting of iron a physical or chemical change? Give a reason.

Rusting is a slow chemical change because iron reacts with oxygen and moisture, forming brown iron-oxide that cannot turn back into shiny iron.

10.Explain why melting of ice is a reversible physical change.

Ice becomes water on heating, and the water can freeze back into ice on cooling. No new substance forms, making it a reversible physical process.