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Changes Around Us: Physical and Chemical - Are Changes Permanent?

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A Physical Change is a change in which a substance undergoes a change in its physical properties like shape, size, colour, or state. It is generally reversible, and no new substance is formed. For example, the melting of ice: H2O(s)⇌H2O(l)H_2O(s) \rightleftharpoons H_2O(l).

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A Chemical Change is a change in which one or more new substances are formed. These changes are usually permanent and irreversible. Examples include burning of magnesium ribbon or the curdling of milk.

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Signs of a chemical change include the evolution of gas (e.g., CO2CO_2), change in colour, production of heat or light, and the formation of a precipitate.

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Rusting is a chemical change that affects iron objects. It occurs when iron (FeFe) reacts with oxygen (O2O_2) and water vapour (H2OH_2O) in the air to form iron oxide (Fe2O3⋅xH2OFe_2O_3 \cdot xH_2O).

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Galvanization is the process of depositing a layer of a metal like zinc (ZnZn) or chromium (CrCr) on iron to prevent it from rusting.

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Crystallization is a physical process used to obtain large crystals of pure substances from their solutions. Since no new substance is formed, it is a physical change.

📐Formulae

2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO

MgO+H2O→Mg(OH)2MgO + H_2O \rightarrow Mg(OH)_2

Fe+CuSO4 (Blue)→FeSO4 (Green)+Cu (Brown deposit)Fe + CuSO_4 \text{ (Blue)} \rightarrow FeSO_4 \text{ (Green)} + Cu \text{ (Brown deposit)}

CH3COOH+NaHCO3→CH3COONa+H2O+CO2↑CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2 \uparrow

CO2+Ca(OH)2→CaCO3↓+H2OCO_2 + Ca(OH)_2 \rightarrow CaCO_3 \downarrow + H_2O

4Fe+3O2+xH2O→2Fe2O3⋅xH2O (Rust)4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O \text{ (Rust)}

💡Examples

Problem 1:

Classify the following as physical or chemical changes: (i) Photosynthesis, (ii) Digestion of food, (iii) Tearing of paper, (iv) Burning of coal.

Solution:

(i) Chemical, (ii) Chemical, (iii) Physical, (iv) Chemical.

Explanation:

Photosynthesis and digestion involve the formation of new substances and are irreversible (chemical). Tearing paper only changes its size (physical). Burning coal produces carbon dioxide and ash (chemical).

Problem 2:

When carbon dioxide is passed through lime water, it turns milky. Write the chemical equation and explain why.

Solution:

The lime water turns milky due to the formation of Calcium Carbonate (CaCO3CaCO_3).

Explanation:

The reaction is: CO2+Ca(OH)2→CaCO3+H2OCO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O. The white precipitate of CaCO3CaCO_3 makes the lime water appear milky.

Problem 3:

Explain why rusting of iron objects is faster in coastal areas than in deserts.

Solution:

Coastal areas have higher humidity (moisture) and salt content in the air.

Explanation:

Rusting requires both oxygen and water: 4Fe+3O2+xH2O→2Fe2O3⋅xH2O4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O. In coastal areas, the high moisture content and the presence of salts accelerate the process of oxidation, whereas deserts have dry air with very little moisture.