Comprehensive science worksheet for 7th graders covering the fundamentals of physical and chemical changes.

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 the transformation of one or more substances into new substances with different chemical properties. The key feature is that new products are formed. For example, when iron rusts, it forms iron oxide — a new substance.

- 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 form solid crystals from a solution. It is a physical change because no new substances are formed — the chemical composition remains the same. Also, it is reversible, as the crystals can be dissolved again in water to form a solution.

For example: When salt solution evaporates, salt crystals form. These can be redissolved in water.

> ✔ 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 one or more substances (called reactants) are transformed into one or more different substances (called products) through the breaking and forming of chemical bonds.

Example: Burning of wood produces ash, smoke, and gases — this is a chemical reaction.

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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 process and weakens the metal over time.

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

It is an example of a chemical change and oxidation.

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

This is a physical change because no new substance is formed — only the state changes.

Example: Formation of sugar crystals from a sugar syrup.

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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.

First example (given):
Zinc displaces copper from copper sulfate solution.

Second example (to complete):
Magnesium displaces hydrogen from hydrochloric acid.

Explanation:
In a displacement reaction, a more reactive element replaces a less reactive one from its compound.

Example:
$$
Mg + 2HCl \rightarrow MgCl_2 + H_2
$$

Here, magnesium (more reactive than hydrogen) displaces hydrogen from hydrochloric acid.

✔ So, the completed sentence could be:

> Two substances react, and one of the substances displaces a part of the other substance.
> 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 substances transform into new substances via chemical changes.
- Rusting: Oxidation of iron in presence of water and air forming rust.
- Crystallisation: Formation of crystals from a solution; a reversible physical change.

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

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