Educational worksheet on physical and chemical changes for Grade VII students.

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 protect it from rusting. This is done to prevent corrosion, as zinc acts as a sacrificial protector. The term "galvanisation" comes from Luigi Galvani, an Italian scientist.

> ✘ a. Layering – Too general; not a scientific term for this process.
> ✘ c. Sublimation – When a solid turns directly into gas (e.g., dry ice).
> ✘ d. Crystallisation – Formation of crystals from a solution.

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

✔ Explanation:
In a chemical change, the original substances undergo a reaction to form new substances with different chemical properties. For example, burning wood produces ash, smoke, and gases — all new products.

> ✘ a. at least two new products – Not necessarily true; sometimes only one new product forms.
> ✘ c. water – Not always produced in chemical changes.
> ✘ d. carbon dioxide – Only occurs in certain reactions like combustion.

So, the most accurate answer is 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 forms solid crystals upon cooling or evaporation. It is a physical change because no new substance is formed — just a change in state or form.

It’s also reversible because you can dissolve the crystals back into a solution.

> ✘ b. irreversible physical change – Incorrect; it can be reversed.
> ✘ c. reversible chemical change – No new substance formed → not a chemical change.
> ✘ d. irreversible chemical change – Again, not a chemical change.

So, crystallisation is 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) react 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:
$$
2Mg + O_2 \rightarrow 2MgO
$$

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#### b. Rusting
Definition:
Rusting is a chemical change that occurs when iron reacts with oxygen and moisture in the presence of water to form hydrated iron(III) oxide, commonly known as rust.

Chemical Equation:
$$
4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3 \text{ (rust)}
$$

It is a slow oxidation process and causes deterioration of iron.

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#### c. Crystallisation
Definition:
Crystallisation is the process of forming solid crystals from a solution, melt, or vapor. It occurs when a solute precipitates out of a supersaturated solution upon cooling or evaporation.

Example: Forming sugar crystals from a sugar syrup.

This is a physical change because the chemical composition remains unchanged.

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3. Complete the second example.

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

This describes a displacement reaction.

First Example (given):
Zinc displaces copper from copper sulfate solution:
$$
Zn + CuSO_4 \rightarrow ZnSO_4 + Cu
$$

Second Example (to complete):
Magnesium displaces hydrogen from hydrochloric acid:
$$
Mg + 2HCl \rightarrow MgCl_2 + H_2
$$

✔ Explanation:
Magnesium is more reactive than hydrogen, so it displaces hydrogen gas from the acid. This is a classic single displacement reaction.

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✔ Final Answers:

#### 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 substances react to form new substances with different properties.
- Rusting: Oxidation of iron in the presence of moisture and oxygen to form rust.
- Crystallisation: Formation of solid crystals from a solution, a physical and reversible change.

#### 3. Second Example:
> Magnesium displaces hydrogen from hydrochloric acid:
> $$
> Mg + 2HCl \rightarrow MgCl_2 + H_2
> $$

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