Pure Substances vs Mixtures

In chemistry, matter can be classified as either a pure substance or a mixture.

Pure Substance

Made of only one type of element or compound. It has a fixed composition and definite physical properties (e.g. melting/boiling points).

Examples: Distilled water (H₂O), pure gold (Au), table salt (NaCl)

Mixture

Contains two or more substances that are physically combined but not chemically bonded. The components can be separated by physical methods.

Examples: Salt water, air, soil, trail mix

Key Difference

In a mixture, the components retain their individual properties and can be separated by physical means. In a pure substance, the composition is uniform and cannot be separated by physical methods (only chemical reactions can break compounds into elements).

Types of Mixtures

Comparing Mixture Types

Property	Solution	Suspension	Colloid
Particle Size	< 1 nm	> 1000 nm	1 – 1000 nm
Appearance	Transparent	Cloudy / opaque	May appear cloudy
Settles on Standing?	No	Yes	No
Filtered?	Cannot be filtered	Can be filtered	Cannot be filtered
Examples	Salt water, vinegar	Muddy water, sand in water	Milk, fog, mayonnaise

Separation Techniques

Since the components of a mixture are not chemically bonded, they can be separated using physical methods. The technique you choose depends on the physical properties of the substances in the mixture (e.g. particle size, boiling point, solubility).

Filtration

Separates an insoluble solid from a liquid by passing the mixture through filter paper. The solid particles are too large to pass through and remain as the residue, while the liquid passes through as the filtrate.

Use when: You need to separate a solid that does NOT dissolve in a liquid (e.g. sand from water, tea leaves from tea).

Evaporation

Separates a dissolved solid from a solution by heating. The liquid (solvent) evaporates, leaving the solid behind. This method is useful when you want to keep the solid.

Use when: You want to recover a dissolved solid from a solution (e.g. obtaining salt from salt water).

Distillation

Separates substances based on different boiling points. The mixture is heated until one component evaporates, then the vapour is cooled and condensed back into a liquid in a separate container. This allows you to collect both the solid and the liquid.

Use when: You want to recover the liquid from a solution (e.g. pure water from salt water) or separate two liquids with different boiling points.

Chromatography

Separates dissolved substances based on how they travel through a medium (e.g. filter paper) at different rates. The solvent moves up the paper, carrying different components to different heights depending on their solubility and attraction to the paper.

Use when: You need to separate and identify components of a mixture of dissolved substances (e.g. different dyes in ink, food colouring).

Choosing the Right Technique

Decision Flow

Key Vocabulary

Solute

The substance that dissolves in a solvent (e.g. salt in salt water).

Solvent

The substance that does the dissolving (e.g. water in salt water). Water is the "universal solvent".

Residue

The solid material that remains trapped in the filter paper during filtration.

Filtrate

The liquid that passes through the filter paper during filtration.

Soluble

A substance that can dissolve in a given solvent (e.g. sugar is soluble in water).

Insoluble

A substance that cannot dissolve in a given solvent (e.g. sand is insoluble in water).

Worked Examples

1

How would you separate sand from water?

Step 1: Identify the mixture type. Sand is an insoluble solid mixed with water (a liquid).

Step 2: For an insoluble solid in a liquid, use filtration.

Answer: Pour the mixture through filter paper in a funnel. The sand (residue) stays in the filter paper, and the water (filtrate) passes through.

2

How would you obtain pure water from salt water?

Step 1: Salt water is a solution (salt dissolved in water).

Step 2: We want to collect the liquid (water), not the solid. This means we need distillation, not just evaporation.

Answer: Use distillation. Heat the salt water until the water evaporates (boils at 100 °C). The water vapour travels through a condenser, cools, and is collected as pure liquid water. The salt remains behind.

3

A student wants to find out how many dyes are in a black felt-tip pen. What technique should they use?

Step 1: The ink contains multiple dissolved substances (dyes) mixed together.

Step 2: To separate and identify dissolved substances, use chromatography.

Answer: Place a spot of the ink on chromatography paper, dip the bottom edge in solvent (water). As the solvent travels up the paper, different dyes will be carried different distances, appearing as separate coloured spots.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Which separation technique would you use to obtain salt from salt water?

Evaporation Filtration Chromatography Magnetism

Question 2

A student stirs sand into a beaker of water. This is an example of a:

Solution Suspension Colloid Pure substance

Question 3

In filtration, what is the name of the liquid that passes through the filter paper?

Residue Solute Filtrate Distillate

Question 4

Which technique is best for separating the different coloured dyes in ink?

Distillation Filtration Evaporation Chromatography

Question 5

What is the key difference between distillation and evaporation?

Distillation collects the liquid; evaporation only keeps the solid Distillation uses cold water; evaporation uses hot water They are exactly the same process Evaporation can separate insoluble solids; distillation cannot

Mixtures and Separation Techniques