Mixtures
Lajoy Tucker
Teacher
Introduction
A mixture consists of two or more elements or compounds that are not chemically combined together.
The chemical properties of each substance in a mixture remain unchanged because no chemical bonds are formed or broken between them.
Mixtures are common in everyday life, e.g. air (a mixture of gases), sea water (water + salts), and alloys like brass (copper + zinc).
Key Features of Mixtures
Feature | Description |
Chemical bonding | No new chemical bonds are formed between substances. |
Properties | Each component keeps its own chemical properties. |
Separation | Can be separated by physical methods (no chemical reactions). |
Separating Mixtures – Physical Methods
Process | Used to Separate | Principle / How It Works | Example |
Filtration | Insoluble solid from a liquid | The solid cannot pass through the filter paper | Sand from water |
Crystallisation | Soluble solid from a solution | The solvent is evaporated, leaving crystals of solute.
| Salt from salt solution |
Simple distillation | Solvent from a solution | The solvent boils, evaporates, then condenses into a separate container.
| Water from salt water |
Fractional distillation | Two or more liquids with different boiling points | The mixture is heated; liquids boil and condense at different temperatures
| Ethanol from water, or crude oil fractions.
|
Chromatography | Substances dissolved in a solvent (e.g. dyes, inks) | Different substances move at different speeds through the stationary phase.
| Colours in ink, food dyes |
Chromatography is covered in detail in Topic 8
When selecting a method, consider:
Whether the substances are solids, liquids, or gases.
Whether they are soluble or insoluble.
Whether their boiling points or solubilities are different.
Question 1:
Explain why mixtures can be separated by physical processes.
Answer:
Because the substances in a mixture are not chemically bonded, so their physical properties (like boiling point or solubility) can be used to separate them.
Question 2:
Which separation technique would you use to obtain pure water from seawater, and why?
Answer:
Use simple distillation, because water has a lower boiling point than the dissolved salts and can be evaporated and condensed separately.
Question 3:
Describe how you could separate a mixture of sand and salt.
Answer:
1. Add water to dissolve the salt.
2. Filter the mixture – sand remains as residue.
3. Evaporate the filtrate to crystallise the salt.
Summary
A mixture contains substances not chemically joined together.
The properties of each substance stay the same.
Mixtures can be separated using physical methods like filtration, crystallisation, distillation, and chromatography.
Always choose a separation method based on physical differences such as boiling point, particle size, or solubility.
No answer provided.