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

The formation of manure from leaves is a chemical change because it involves:

• Decomposition of organic material.
• Creation of new substances through the breakdown of leaves.
• Changes in chemical composition, not just physical appearance.

Thus, it is incorrect to classify this process as a physical change.

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

True

• Zinc provides a protective layer on iron pipes.
• This layer prevents moisture and oxygen from causing rust.
• As a result, galvanised iron pipes are less likely to rust.

(iii)  Iron and rust are the same substances.

False

Iron and rust are not the same. Here are the key differences:

• Iron is a pure metal.
• Rust is a compound formed when iron reacts with oxygen and moisture.
• Rust appears as a brownish substance on iron surfaces.
• They have different properties and compositions.

Therefore, the statement is False.

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

True

The chemical name of baking soda is sodium bicarbonate. It is widely used in:

• Cooking - as a leavening agent.
• Baking - to help dough rise.

In summary, the statement is true.

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

True

A chemical change occurs when substances transform into new substances. This involves:

• A change in chemical composition.
• Formation of new substances.

Examples include:

• Digestion of food.
• Rusting of iron.

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

physical

This is a physical change because it alters the form of the wood without changing its chemical composition.

(ii) Condensation is a ____ change.

physical
Condensation is a physical change as it involves a change in the state of matter from gas to liquid without altering the substance's chemical identity.

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

Calcium Carbonate
The milky appearance is due to the formation of calcium carbonate, which occurs when carbon dioxide reacts with lime water.

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

painting or greasing, galvanisation
Painting or greasing creates a barrier against moisture, while galvanisation involves coating iron with zinc to prevent rust.

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

physical
Physical changes involve alterations in physical properties such as shape, size, or state, without changing the substance's chemical identity.

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

chemical reactions
Medicines are created through a series of chemical reactions that transform raw materials into effective therapeutic agents.

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?

When magnesium oxide is dissolved in water, it creates a new substance. This process can be represented by the following equation:

Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide (Mg(OH)₂)

Magnesium hydroxide is classified as a base, indicating that magnesium oxide transforms into a new substance when mixed with water.

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 copper sulphate solution (CuSO₄), the following occurs:

• Iron displaces copper from the solution because iron is more reactive.
• The solution changes colour from blue to green, indicating the formation of iron sulphate (FeSO₄).
• A brown deposit of copper forms on the iron nail.

The reaction can be summarised 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 galvanisation involves applying a protective layer of metal, such as zinc or chromium, onto iron. This method helps to:

• Prevent rusting by blocking moisture and oxygen from reaching the iron.
• Extend the lifespan of iron products, such as pipes and ships.
• Reduce maintenance costs associated with rust damage.

For example, iron pipes used in homes are often galvanised to ensure they remain rust-free and functional.

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

Formation of manure from leaves is a chemical change because:

• The composition of the resulting manure is different from that of the original leaves.
• New substances are created during the decomposition process.
• This transformation involves a series of chemical reactions.

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

A slice of an apple turns brown when left uneaten due to a process called oxidation. This occurs because:

• The apple's cells are damaged when cut.
• Enzymes in the apple react with oxygen in the air.
• This reaction produces new substances that have a brown colour.

Thus, the brown colour is a sign of chemical change occurring in the apple.

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

Tearing a piece of paper is a physical change because:

• Only the shape and size of the paper are altered.
• No new substance is created during this process.

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

Spoiling of food is a chemical change because it involves:

• Unwanted quality changes, such as staling and discoloration.
• Development of off-flavours and unpleasant odours.
• Formation of new substances that cannot be reverted to their original state.

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

Ozone acts as a natural shield against harmful ultraviolet radiation from the sun. It provides protection by:

• Absorbing ultraviolet radiation, which prevents it from reaching the Earth's surface.
• Breaking down into oxygen, which is less harmful.
• Reducing the risk of damage to humans and other life forms.

If ozone did not absorb this radiation, it would cause significant harm to all living organisms.

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?

Crystallisation is the process of forming large crystals of pure substances from their solutions. It is a type of physical change where:

• Substances dissolve in a liquid.
• As the solution cools, crystals begin to form.
• This method is often used to obtain pure substances.

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

Rusting occurs more rapidly during the rainy season due to the increased moisture in the air. This high humidity accelerates the rusting process because:

• Both oxygen and water are essential for rusting.
• Higher moisture levels lead to more water vapour, which promotes rust formation.
• Iron articles exposed to humid conditions are more likely to develop rust.

Therefore, the combination of moisture and oxygen during the rainy season significantly increases the rate of rusting.

Q.20. What is a physical change?

A physical change occurs when a substance alters its physical properties without forming a new substance. Key characteristics include:

• Changes can involve shape, size, colour, and state of the substance.
• These changes are generally reversible.
• No new substances are created during a physical change.