Class 7 Physical and chemical changes Worksheet - Free Printable

Let's solve the problems step by step and provide clear explanations.

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1. Choose the correct answer.

#### A. What is depositing a layer of zinc on iron called?
Answer: b. Galvanisation

Explanation:
Galvanisation is the process of coating iron or steel with a layer of zinc to prevent rusting. This protective layer of zinc acts as a barrier against moisture and oxygen, which are necessary for rust formation. It is commonly used in construction and manufacturing to protect metal structures.

> ✔ Correct answer: b. Galvanisation

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#### B. A chemical change always gives rise to:
Answer: b. at least one new product

Explanation:
A chemical change involves a transformation where the original substances react to form new substances with different chemical properties. For example, burning wood produces ash, smoke, and gases — all new products.

- Option a (at least two new products) is not always true — sometimes only one new product forms.
- Options c and d (water, carbon dioxide) are specific examples, not general outcomes of all chemical changes.

> ✔ Correct answer: b. at least one new product

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#### C. Crystallisation is an example of:
Answer: a. a reversible physical change

Explanation:
Crystallisation is the process where dissolved solute particles come together to form solid crystals. It is a physical change because:
- No new substance is formed.
- The substance can be reversed (e.g., dissolving the crystals back into solution).

For example, sugar crystals can dissolve in water again — this is reversible.

> ✔ Correct answer: a. a reversible physical change

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2. Define.

#### a. Chemical reaction
Definition:
A chemical reaction is a process in which two or more substances (elements or compounds) interact to form one or more new substances with different chemical properties. This involves breaking and forming chemical bonds.

> Example: Burning of magnesium in air to form magnesium oxide.

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#### b. Rusting
Definition:
Rusting is a chemical reaction in which iron reacts with oxygen and water to form hydrated iron(III) oxide, commonly known as rust. It is a slow oxidation process that weakens iron over time.

> Formula:
Iron + Oxygen + Water → Rust (Fe₂O₃·xH₂O)

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#### c. Crystallisation
Definition:
Crystallisation is the process of forming solid crystals from a solution, melt, or gas. It occurs when dissolved particles lose energy and arrange themselves into a regular, repeating pattern.

> Example: Formation of salt crystals from seawater evaporation.

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3. Complete the second example.

The sentence says:

> "Two substances react, and one of the substances displaces a part of the other substance."

This describes a displacement reaction, where a more reactive element replaces a less reactive one in a compound.

#### Example 1 (given):
Zinc displaces copper from copper sulfate solution.
→ Zn + CuSO₄ → ZnSO₄ + Cu

#### Example 2 (to complete):
Magnesium displaces hydrogen from hydrochloric acid.
→ Mg + 2HCl → MgCl₂ + H₂

> Explanation: Magnesium is more reactive than hydrogen, so it pushes hydrogen out of the acid, producing magnesium chloride and hydrogen gas.

✔ Completed example:
Magnesium displaces hydrogen from hydrochloric acid.

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✔ Final Answers Summary:

#### 1. Multiple Choice:
- A. b. Galvanisation
- B. b. at least one new product
- C. a. a reversible physical change

#### 2. Definitions:
- Chemical reaction: A process where new substances are formed due to chemical changes.
- Rusting: Oxidation of iron in the presence of oxygen and water to form rust.
- Crystallisation: Formation of solid crystals from a solution or melt; a physical change.

#### 3. Complete the example:
- Magnesium displaces hydrogen from hydrochloric acid.

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