The Fundamental Unit of Life | Class 9 Science Notes

5.1 How Can We Classify Mixtures?

Mixtures are combinations of two or more substances where each substance retains its own properties. They can be classified based on the uniformity of their composition into homogeneous and heterogeneous mixtures. A homogeneous mixture, also called a solution, has a uniform composition throughout. For example, a well-stirred mixture of sugar dissolved in water tastes equally sweet in every sip, indicating uniform distribution of sugar molecules. Other examples include vinegar (acetic acid in water) and aerated drinks like soda (carbon dioxide dissolved in water). Such mixtures do not show visible particles and remain uniform over time.

In contrast, a heterogeneous mixture has a non-uniform composition. For instance, a mixture of sand and water shows visible sand particles that settle at the bottom over time. The mixture of oil and water is also heterogeneous as the two liquids do not mix and form separate layers. The classification is important because the method of separation depends on the type of mixture.

To explore this, an activity involves preparing three mixtures: salt in water (homogeneous), chalk powder in water (heterogeneous), and milk in water (appears homogeneous but is a colloid). Observations include visibility of particles, light scattering (Tyndall effect), and filtration results. Salt solution remains clear with no residue on filtration, chalk suspension shows visible particles and residue, and milk shows light scattering but no residue, indicating different mixture types.

This classification helps in understanding the nature of mixtures and guides the choice of separation techniques.

📊 Diagram: Fig. 5.1: Homogeneous mixture; Fig. 5.2: Heterogeneous mixture; (a), (b), (c): Passing laser light through various mixtures (salt and water, chalk powder and water, milk and water).

🧪 Activity: Activity 5.1: Prepare mixtures of salt in water, chalk powder in water, and milk in water; observe particle visibility, light scattering using laser pointer, and filtration residue.

🔗 Connection: This section introduces the types of mixtures, leading to the next section on solutions, their components, and concentration.