Physical and Chemical Changes Class 7 Worksheet Science Chapter 5

Q.1. True/False
(i) Formation of manure from leaves is a physical change.

False

(ii)  Iron pipes coated with zinc do not get rusted easily.

True

(iii)  Iron and rust are the same substances.

False

(iv)  The chemical name of baking soda is Sodium hydrogencarbonate.

True

(v) Changes in which new substances are formed are called chemical changes.

True

Q.2. Fill in the blanks.
(i) Cutting a log of wood into pieces is a ____ change.

physical

(ii) Condensation is a ____ change.

physical

(iii) When carbon dioxide is passed through lime water, it turns milky due to the formation of ____.

Calcium Carbonate

(iv) Two methods by which rusting of iron can be prevented are ____ and ____.

painting or greasing, galvanisation

(v)  Changes in which only ____ properties of a substance change are called physical changes.

physical

(vi)  A medicine is the end product of a chain of ____.

chemical reactions

Q.3. What is added to steel to make it stainless?

Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese.

Q.4. Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese.

The process of rusting can be represented by the following equation: Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → rust (iron oxide Fe₂O₃)

Q.5. What happens when magnesium oxide is dissolved in water?

On dissolving the magnesium oxide in water it forms a new substance. This change can be written in the form of the following equation: Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide [Mg(OH)₂]

Q.6. What happens when magnesium ribbon is burnt in air?

When the magnesium ribbon burnt in air, it reacts with oxygen found in the air to form Magnesium Oxide. After it burns, it forms a white powder of the magnesium oxide. The change can be represented by the following equation: Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)

Q.7. What happens when an iron nail is dipped in copper sulphate solution?

When an iron nail is dipped in the copper sulphate solution (CuSO₄) then iron displaces copper from the copper sulphate because iron is more reactive than copper. The change of colour of the solution from blue to green is due to the formation of iron sulphate (FeSO₄), a new substance. The brown deposit on the iron nail is copper, another new substance. We can write the reaction as: Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit

Q.8. What type of change is formation of manure?

Formation of manure is a chemical change.

Q.9. What do you understand by chemical change?

A change in which one or more new substances are formed is called a chemical change.

Q.10. What is galvanisation?

The process of depositing a layer of metal like zinc or chromium on iron is called galvanisation.

Q.11. Why formation of manure from leaves is a chemical change?

Formation of manure from leaves is a chemical change because manure formed has a different composition from leaves.

Q.12. Why a slice of an apple acquires a brown colour if it is not consumed immediately?

A slice of an apple acquires a brown colour if it is not consumed immediately due to the formation of new substances.

Q.13. Why tearing of paper into pieces is a physical change?

Tearing of a paper is a physical change because when the paper is torn only the shape and size of the paper is changed, no new substance is formed.

Q.14. Why is spoiling of food a chemical change?

Spoiling of food is an unwanted quality change in a foodstuff, such as staling, discoloration, the development of off-flavours and odours. It is a chemical change as it cannot be brought back to its original form.

Q.15. Ozone acts as a natural shield against ultraviolet radiation. Explain?

It protects us from the harmful ultraviolet radiations which come from the sun. Ozone absorbs this radiation and breaks down to oxygen. If ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface and cause harm to us and other life forms.

Q.16. What is rusting?

If a piece of iron is left in the open for some time, it acquires a film of brownish substance. This substance is called rust the process is called rusting.
The process of rusting can be represented by the following equation: Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → rust (iron oxide Fe₂O₃)

Q.17. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Burning of wood
Burning is a non-reversible chemical change because when we burn wood new substances are formed as the carbon in the wood reacts with oxygen in the air to create ash and smoke, and energy in the form of light and heat.
Cutting of wood it into small pieces
Cutting of wood into small pieces are physical change as no new substance is formed. Only shape and size changes when wood is cut into small pieces.

Q.18. What is crystallisation?

Large crystals of pure substances can be formed from their solutions. The process is called crystallisation.

Q.19. Why does rusting takes place faster during rainy season?

Rusting takes place faster during rainy season because the content of moisture in air is high in rainy season.

Q.20. What is a physical change?

A change in which a substance undergoes a change in its physical properties is called a physical change.