Mixtures and Separation Techniques

What Is a Pure Substance?

In chemistry, a pure substance has a very specific meaning — it contains only one type of element or compound. A pure substance has a sharp, definite melting point and boiling point.

Exam tip: "Pure" in everyday language (e.g. "pure orange juice") means uncontaminated. In chemistry, "pure" means only one substance is present.

Identifying Purity by Melting/Boiling Point

• A pure substance melts and boils at a sharp, fixed temperature
• A mixture melts and boils over a range of temperatures
• Adding an impurity lowers the melting point and raises the boiling point

For example:

• Pure water melts at exactly $0°\text{C}$ and boils at exactly $100°\text{C}$ (at 1 atm)
• Salt water melts below $0°\text{C}$ and boils above $100°\text{C}$

Solutions

A solution forms when a solute dissolves in a solvent:

$\text{Solute} + \text{Solvent} = \text{Solution}$

• Solute — the substance that dissolves (e.g. salt, sugar)
• Solvent — the liquid that does the dissolving (e.g. water)
• Solution — the resulting mixture (e.g. salt water)

A solution is a homogeneous mixture — it has the same composition throughout.

Other Types of Mixtures

3.1 Filtration

Purpose: Separates an insoluble solid from a liquid.

How it works:

1. Pour the mixture through filter paper in a funnel
2. The solid particles are too large to pass through and remain as the residue
3. The liquid passes through as the filtrate

Examples:

• Separating sand from water
• Separating precipitates from reaction mixtures

3.2 Evaporation and Crystallisation

Purpose: Separates a dissolved solid (solute) from a solution.

Evaporation:

1. Heat the solution until all the solvent evaporates
2. The solid remains behind
3. Quick but may decompose the solid if overheated

Crystallisation (preferred method):

1. Heat the solution gently to evaporate some solvent
2. Test by dipping a glass rod — if crystals form on the rod, stop heating
3. Leave to cool slowly — crystals form as the solution becomes saturated
4. Filter to collect crystals and pat dry with filter paper

Key point: Crystallisation produces better-quality, larger crystals than rapid evaporation.

3.3 Simple Distillation

Purpose: Separates a solvent from a solution (e.g. obtaining pure water from salt water).

How it works:

1. Heat the solution in a flask — the solvent evaporates
2. Vapour passes into a condenser (cooled by cold water)
3. Vapour condenses back to liquid and is collected
4. The solute remains in the flask

Key temperatures:

• The thermometer reads the boiling point of the solvent
• For pure water: thermometer stays at $100°\text{C}$

3.4 Fractional Distillation

Purpose: Separates a mixture of miscible liquids with different boiling points.

How it works:

1. Heat the mixture — the liquid with the lowest boiling point evaporates first
2. Vapour rises through a fractionating column (packed with glass beads)
3. The column provides a temperature gradient — cooler at top, hotter at bottom
4. Only the vapour of the substance with the lowest boiling point reaches the top
5. It passes into the condenser and is collected
6. Temperature rises and the next liquid is collected

Example: Separating ethanol (bp $78°\text{C}$) from water (bp $100°\text{C}$)

3.5 Chromatography

Purpose: Separates dissolved substances based on how they interact with a stationary phase and a mobile phase.

Paper Chromatography Method:

1. Draw a pencil line (not pen — pencil is insoluble) near the bottom of chromatography paper
2. Place spots of the substances to be tested on the line
3. Place the paper in a beaker with a shallow layer of solvent (mobile phase)
4. The solvent level must be below the pencil line
5. As the solvent rises by capillary action, it carries the dissolved substances with it
6. Different substances travel different distances depending on their solubility in the mobile phase and their attraction to the paper (stationary phase)

Interpreting Chromatograms:

The $R_f$ value identifies substances:

$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$

• $R_f$ values are always between 0 and 1
• Each substance has a unique $R_f$ value in a given solvent
• If two spots have the same $R_f$ value, they are likely the same substance
• A pure substance produces one spot; a mixture produces multiple spots

This is a common required practical for GCSE Chemistry.

Method:

1. Draw a pencil baseline $1\text{ cm}$ from the bottom of the paper
2. Add spots of known food colourings and an unknown sample
3. Place paper in a beaker with water (solvent) just below the line
4. Cover with a lid to prevent solvent evaporating
5. When solvent nears the top, remove and mark the solvent front
6. Measure distances and calculate $R_f$ values

What to Record:

• Distance moved by each spot
• Distance moved by solvent front
• Number of spots for each sample
• $R_f$ values

• Always state the name of the separation technique and explain why it works
• In chromatography questions, always show your $R_f$ calculation clearly
• Remember the distinction between evaporation (quick, may damage crystals) and crystallisation (slow cooling, better crystals)
• When describing distillation, mention the condenser and that cooling water flows upward (counter-current)

• A pure substance contains one element or compound and has a sharp melting/boiling point
• Mixtures contain multiple substances not chemically combined
• Key separation techniques: filtration, evaporation/crystallisation, distillation (simple and fractional), chromatography
• $R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$
• Choose the technique based on the physical properties of the components