The Fundamental Unit of Life | Class 9 Science Notes

5.3 Methods of Separation of Homogeneous Mixtures

Homogeneous mixtures, or solutions, can be separated into their components by exploiting differences in physical properties such as solubility and boiling point. One common method is crystallization, which involves forming pure solid crystals from a saturated solution by cooling.

For example, a saturated solution of compound 'B' at 60 °C contains 287 g of solute per 100 g of water. When cooled to 40 °C, the solubility decreases to 241 g per 100 g water, so the excess 46 g of compound 'B' crystallizes out as pure solid. Crystals are solids with particles arranged in a regular geometric pattern. Natural crystals include rock salt, candy sugar, snowflakes, and frost.

Crystallization is used to separate and purify solids, especially when two solids are soluble in the same solvent but present in different quantities. The principle is based on the difference in solubility at different temperatures.

An activity demonstrates crystallization using copper sulfate. A saturated hot solution is prepared, filtered to remove impurities, and allowed to cool slowly to form large, shiny crystals. Sulfuric acid may be added to prevent unwanted reactions. Crystallization is a valuable technique in laboratories and industries for obtaining pure substances.

Other methods for separating homogeneous mixtures include distillation, which separates miscible liquids based on boiling points, and paper chromatography, which separates components based on their movement rates on paper.

📊 Diagram: Fig. 5.7: Crystals of (a) rock salt, (b) candy sugar, and (c) frost; Fig. 5.8: Steps involved in the process of crystallization (a) Preparing the saturated solution, (b) Filtering the hot saturated solution, (c) Cooling down the filtrate, (d) Crystals of copper sulfate; Fig. 5.9: Steps followed in the salt manufacturing process; Fig. 5.11: Distillation process; Fig. 5.12: Distillation set-up; Fig. 5.13: Deg-Bhapka method

🧪 Activity: Activity 5.3: Prepare copper sulfate crystals by making a saturated solution, filtering, and cooling slowly to observe crystallization.

🔗 Connection: This section introduces separation methods for homogeneous mixtures, leading to separation of heterogeneous mixtures in the next section.