This guide covers the key concepts of mixtures, solutions, composition, and separation techniques. Use these notes and tips to build a strong understanding for your O/L examination.

1. Types of Mixtures & Solutions

Short Notes & Key Concepts

• Mixture: A substance containing two or more components that are not chemically combined. Their individual properties are retained, and they can be separated by physical methods.

• Homogeneous Mixture (Solution): A mixture where the components are uniformly distributed, and the composition is the same throughout. You cannot see the individual parts.

  • Examples: Salt water, sugar solution, clean air, brass (an alloy).

• Heterogeneous Mixture: A mixture where the components are not uniformly distributed. You can often see the different parts.

  • Examples: Muddy water, fruit salad, concrete, oil and water.

• Key Terms for Solutions:

  • Solute: The substance that dissolves (present in a smaller amount). Example: Salt in salt water.

  • Solvent: The substance that does the dissolving (present in a larger amount). Example: Water in salt water.

  • Solution = Solute + Solvent

• Solubility: The maximum mass of a solute that can dissolve in 100g of a solvent at a specific temperature.

  • Factors Affecting Solubility of Solids:

    1. Temperature: Solubility of most solids increases as temperature increases.

    2. Nature of Solute & Solvent: The chemical properties matter. The rule is "like dissolves like":

       • Polar solutes (like salt, sugar) dissolve in polar solvents (like water).

       • Non-polar solutes (like grease, oil) dissolve in non-polar solvents (like kerosene, petrol).

  • Factors Affecting Solubility of Gases:

    1. Temperature: Solubility of gases decreases as temperature increases (e.g., warm soda goes flat quickly).

    2. Pressure: Solubility of gases increases as pressure increases (e.g., CO₂ is dissolved in soda water under high pressure).

2. Expressing the Composition of Mixtures

• Concentration (mol dm⁻³): This is the most important for O/L Chemistry. It is the amount of solute (in moles) dissolved in a unit volume (1 dm³ or 1 Litre) of the solution.

  • Formula: Concentration (C) = Amount of moles (n) / Volume of solution (V in dm³)

• Mass Fraction (m/m): The ratio of the mass of one component to the total mass of the mixture. It has no units.

  • Formula: Mass fraction = Mass of component / Total mass of mixture

• Volume Fraction (V/V): The ratio of the volume of one component to the total volume of the mixture. Used for liquids or gases. It has no units.

  • Formula: Volume fraction = Volume of component / Total volume of mixture

3. Separation Techniques

The technique you use depends on the physical properties of the components in the mixture.

Exam Tips & Tricks

1. Know Your Definitions: Be ready to define key terms like mixture, solution, solute, solvent, solubility, and concentration.

2. Master the Separation Principles: For any separation technique, you MUST know the physical property it relies on (e.g., boiling point, particle size, solubility). This is a very common exam question.

3. Draw the Setups: Be able to draw and label the apparatus for Filtration and Simple Distillation. These are frequently tested.

4. Concentration Calculations: Practice simple calculations using C = n/V. Remember that 1 dm³ = 1000 cm³.

5. Identify the Technique: You will often be given a scenario (e.g., "how would you separate sand and salt?") and asked to describe the steps. This usually involves more than one technique (e.g., dissolve the salt in water, filter out the sand, then evaporate the water to get the salt).

Important Points to Remember

• Filtration is for insoluble solids. Evaporation/Distillation is for soluble solids.

• Simple Distillation separates a liquid from a solid. Fractional Distillation separates a liquid from another liquid.

• The dark spot in chromatography separates because each colour travels up the paper at a different speed.

• In industry, Fractional Distillation is vital for refining crude oil, and Crystallization is key for making sugar and salt.