Mixtures

• A mixture consists of two or more elements or compounds that are not chemically combined together.

• Think of it like a fruit salad – you have different fruits (elements or compounds) all mixed up, but they are still individual fruits.

• In a mixture, the chemical properties of each substance remain unchanged. For example, if you mix iron powder and sulfur powder, the iron is still magnetic and the sulfur is still yellow – they haven't reacted to form a new substance like iron sulfide.

• Making a mixture does not involve a chemical reaction, and no new substances are made.

Separating mixtures

• Unlike compounds, which require chemical reactions to break them down into elements, mixtures can be separated using physical processes. 

• These methods exploit the different physical properties of the substances in the mixture (like boiling point, solubility, or particle size).

• These physical separation processes do not involve chemical reactions and no new substances are formed during the separation.

Filtration

• Used to separate: An insoluble solid from a liquid.

• How it works: The mixture is poured through a filter medium (like filter paper) that has small holes. The liquid particles are small enough to pass through the holes (this is the filtrate), while the larger solid particles are caught on the filter paper (this is the residue).

• Example: Separating sand from water. The sand (insoluble solid) stays on the filter paper, and the water (liquid) passes through.

Crystallisation

• Used to separate: A soluble solid from a solution (a solid dissolved in a liquid).

• How it works: The solution is heated, usually in an evaporating basin, to evaporate most of the solvent, leaving a more concentrated solution. Heating is stopped, and the solution is left to cool. As it cools, the solubility of the solid decreases, and pure crystals of the solid start to form. These crystals can then be collected by filtration and dried. Sometimes, the solvent is completely evaporated, but this may result in a less pure solid.

• Example: Obtaining pure salt crystals from salty water. The water evaporates, and salt crystals form as the solution cools.

Simple distillation

• Used to separate: A solvent (liquid) from a solution (a soluble solid dissolved in a liquid), or to separate liquids with significantly different boiling points.

• How it works: The solution is heated. The solvent, which has a lower boiling point than the dissolved solid, evaporates and turns into a gas (vapour). The vapour rises and passes into a condenser, where it is cooled (usually by cold water flowing around an inner tube). The cooling causes the vapour to condense back into a liquid, which is collected in a separate beaker. The dissolved solid is left behind in the original flask.

• Example: Obtaining pure water from salt solution or separating water (boiling point 100°C) from ethanol (boiling point 78°C), though fractional distillation is better for liquids with closer boiling points.

Fractional distillation

• Used to separate: A mixture of liquids with different boiling points, especially when the boiling points are relatively close together.

• How it works: Similar to simple distillation, but a fractionating column is placed between the flask and the condenser. The mixture is heated, and both liquids will start to evaporate. As the vapours rise up the fractionating column, they cool. The substance with the higher boiling point will condense sooner and fall back into the flask. The substance with the lower boiling point will continue to rise up the column, reaching the condenser where it cools and is collected. The fractionating column provides a large surface area (often with glass beads or rings) for repeated evaporation and condensation cycles, which helps to achieve a better separation.

• Example: Separating ethanol from water, or separating crude oil into its different fractions (like petrol, diesel, kerosene).

Chromatography

• Used to separate: Mixtures of soluble substances that are coloured (like inks or food colourings), or colourless substances that can be visualised later. It separates substances based on their different solubilities in a solvent (the mobile phase) and their different attractions to a stationary material (the stationary phase, often paper or a thin layer of solid).

• How it works (Paper Chromatography): A spot of the mixture is placed on a pencil line (the baseline) drawn on chromatography paper. The edge of the paper is dipped into a solvent in a beaker, making sure the solvent level is below the baseline. As the solvent moves up the paper (the stationary phase) by capillary action, it dissolves the substances in the spot (the mobile phase). Substances that are more soluble in the solvent and less attracted to the paper will travel further up the paper. Substances that are less soluble and more attracted to the paper will not travel as far. This separates the mixture into different spots at different heights.

• Example: Separating the different coloured dyes in black ink. Different spots of colour will appear on the chromatogram (the developed chromatography paper).

Choosing the right technique