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Materials Around Us - Properties: Appearance, Hardness, and Solubility

Grade 6CBSE

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

🔑Concepts

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Materials are classified based on their physical properties such as appearance, hardness, and solubility to make their study convenient.

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Appearance: Materials can be classified as lustrous (shiny) or non-lustrous (dull). Metals like Iron (FeFe), Copper (CuCu), Aluminum (AlAl), and Gold (AuAu) generally show lustre. They may lose their shine due to the action of air and moisture forming an oxide layer, such as Fe2O3Fe_2O_3.

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Hardness: Materials that can be compressed or scratched easily are called 'soft' (e.g., cotton or sponge). Materials that are difficult to compress are called 'hard' (e.g., Iron or Diamond).

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Solubility (Solids): Substances that completely disappear or dissolve in water are 'soluble' (e.g., Salt NaClNaCl, Sugar C12H22O11C_{12}H_{22}O_{11}). Those that do not mix even after vigorous stirring are 'insoluble' (e.g., Sand SiO2SiO_2, Sawdust).

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Solubility (Liquids): Some liquids mix completely with water and are called 'miscible' (e.g., Vinegar, Lemon juice). Liquids that do not mix and form a separate layer are 'immiscible' (e.g., Mustard oil, Kerosene).

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Solubility (Gases): Some gases are soluble in water, such as Oxygen (O2O_2), which is essential for the survival of aquatic organisms. Others, like Nitrogen (N2N_2), are less soluble.

📐Formulae

Density(ρ)=Mass(m)Volume(V)\text{Density} (\rho) = \frac{\text{Mass} (m)}{\text{Volume} (V)}

Solubility=Mass of SoluteMass of Solvent×100\text{Solubility} = \frac{\text{Mass of Solute}}{\text{Mass of Solvent}} \times 100

💡Examples

Problem 1:

Why do copper (CuCu) vessels lose their shine over time and how can it be restored?

Solution:

Copper reacts with moist CO2CO_2 and O2O_2 in the air to form a green layer of basic copper carbonate [CuCO3⋅Cu(OH)2][CuCO_3 \cdot Cu(OH)_2].

Explanation:

This layer makes the surface appear dull. Rubbing the surface with sand paper or using acidic substances (like lemon juice) removes the layer, revealing the underlying lustrous metal.

Problem 2:

Classify the following as soluble or insoluble in water: (a) Chalk powder, (b) Sugar, (c) Sand, (d) Copper Sulphate (CuSO4CuSO_4).

Solution:

(a) Insoluble, (b) Soluble, (c) Insoluble, (d) Soluble.

Explanation:

Sugar and CuSO4CuSO_4 dissolve at a molecular/ionic level in H2OH_2O because the attractive forces between the solute and water molecules are strong enough. Chalk (CaCO3CaCO_3) and Sand (SiO2SiO_2) do not dissolve and settle down at the bottom.

Problem 3:

What happens when you mix Vinegar (CH3COOHCH_3COOH) and Cooking oil in two separate glasses of water?

Solution:

Vinegar is miscible, while Cooking oil is immiscible.

Explanation:

Vinegar dissolves completely in H2OH_2O because it is polar. Cooking oil is non-polar and less dense than water, so it forms a separate layer on the surface of the H2OH_2O.