A compound is a pure substance formed when two or more different elements are chemically combined in a fixed proportion. The elements lose their original properties and form a completely new substance with different properties.

Binary compounds contain only two different elements.

Examples of binary compounds:

Properties of compounds:

• Cannot be separated by physical methods (filtering, evaporation)
• Have properties different from the elements that form them
• Elements are combined in fixed proportions by mass
• Heat is usually absorbed or released during formation

A mixture consists of two or more substances that are physically combined but not chemically bonded. The components retain their original properties and can be separated by physical methods.

Types of Mixtures

Homogeneous mixtures (solutions): The components are completely mixed and appear as one substance.

• Sugar dissolved in water
• Salt dissolved in water

Heterogeneous mixtures: The components can be seen separately and are not uniformly distributed.

• Sand mixed with iron filings
• Oil floating on water

Solutions

A solution is a homogeneous mixture of a solvent and one or more solutes.

• Solvent: The substance that dissolves (usually water)
• Solute: The substance that gets dissolved

Types of solutions:

1. Unsaturated solution: Can still dissolve more solute at a given temperature

2. Saturated solution: Holds the maximum amount of solute it can dissolve at a given temperature; adding more solute will not dissolve

3. Supersaturated solution: Holds more solute than it can normally dissolve (prepared by heating and then slowly cooling)

The method used to separate a mixture depends on the properties of its components.

1. Magnetic Separation

Uses a magnet to separate magnetic materials from non-magnetic ones.

Example: Separating iron filings from powdered sulphur.

2. Filtration

Separates a solid from a liquid using a filter paper or porous material.

Example: Filtering chalk powder from water.

3. Decantation

Pouring off a liquid carefully to leave solid sediment behind.

Example: Separating muddy water from sand after sedimentation.

4. Evaporation

Heating a solution to convert the solvent to gas, leaving the solid solute behind.

Example: Obtaining salt crystals from salt solution.

5. Simple Distillation

Heating a liquid to produce vapour, then cooling the vapour to get a pure liquid.

Example: Obtaining pure water from salty water.

6. Fractional Distillation

Separates miscible liquids with different boiling points using a fractionating column.

Example: Separating ethanol (boiling point 78.4°C) from water (boiling point 100°C).

7. Sublimation

A solid changes directly to gas on heating, then condenses back to solid on cooling.

Example: Separating iodine from sand; ammonium chloride from impurities.

8. Layer Separation (Separating Funnel)

Separates immiscible liquids (liquids that do not mix).

Example: Separating kerosene from water.

Problem: You have a mixture of iron filings and sulphur powder. How would you separate them?

Solution:

1. Use magnetic separation — pass a magnet through the mixture
2. The iron filings are attracted to the magnet and are removed
3. The sulphur powder remains behind
4. This works because iron is magnetic and sulphur is not

In everyday life in Tanzania, separation methods are used when preparing maziwa (milk) — allowing fresh milk to stand causes cream (fat) to separate as a layer on top, which can be skimmed off using layer separation principles. Similarly, when cooking wali (rice), the water is drained off after boiling, demonstrating decantation. These simple kitchen practices show how chemistry is part of daily life.