5. Separation Techniques: Filtration, Evaporation & Crystallisation

1. Key Terminology

• Mixture: Two or more different substances mixed together that are not chemically joined.

• Insoluble Solid: A solid that cannot dissolve in a liquid (e.g., sand in water).

• Soluble Solid: A solid that can dissolve in a liquid (e.g., salt in water).

• Solute: The substance that dissolves (e.g., salt).

• Solvent: The liquid the solute dissolves into (e.g., water).

• Solution: The mixture formed when a solute dissolves in a solvent (e.g., salt water).

2. Filtration

• Purpose: Separates an insoluble solid from a liquid.

• Process: * Filter paper is placed inside a funnel.

  • The paper has tiny holes that allow the liquid to pass through but are too small for the solid particles.

  • The mixture is poured through, leaving the solid on the paper and the liquid in the container below.

3. Evaporation

• Purpose: A quick way to separate a soluble solid from a liquid.

• Process: * The solution is placed in an evaporating dish and heated (usually with a Bunsen burner).

  • The solvent evaporates, making the solution more concentrated.

  • Eventually, all the solvent disappears, leaving dry crystals of the solid behind.

• Limitation: Cannot be used if the solid is susceptible to thermal decomposition (breaking down when heated).

4. Crystallisation

• Purpose: A gentler way to separate a soluble solid from a liquid, especially if the solid decomposes easily.

• Process:

  • The solution is heated gently (often using a water bath) until some solvent evaporates and crystals begin to form.

  • Heating is stopped, and the solution is left to cool. More crystals form as the temperature drops because solids are less soluble in cold liquids.

  • The remaining liquid is filtered out, leaving the crystals behind.

  • The crystals are then dried in a warm place or a drying oven.