CHEMISTRY · Stage 7 · Chapter 2: Elements, Compounds and Mixtures
Chemistry 2.9 What’s in a mixture? — Pure Substances vs Mixtures
5 min read 10 parts Lesson 9 of 13
Lesson summary
2.9 What's in a mixture? — Pure Substances vs Mixtures — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define a mixture, distinguish pure substances from mixtures, interpret particle diagrams and choose suitable separation methods.Mixture = two or more substances…
- Properties of a mixture
- Easy separation
- Other examples
- Possible diagrams
- Limitation
- Pure substance (chemistry)
Section 1 of 10Mixtures
- Mixture = two or more substances that are not joined to each other and are just mixed up.
- Salt and water, stones and rice, and sand and salad are mixtures.
- Properties of a mixture
- the substances are not joined together, they keep their own properties, the amounts can be changed and they are often easy to separate.
- Easy separation
- stones can be picked out of rice, sand washed off salad and water evaporated from salt.
Section 2 of 10What mixtures contain
- Elements
- a jar of iron and copper nails is a mixture of two elements.
- Compounds
- toothpaste is a mixture of compounds.
- Both
- air is a mixture of elements and compounds (carbon dioxide and water).
- (nitrogen, oxygen and argon)
- air is a mixture of elements and compounds (carbon dioxide and water).
- Other examples
- sea water is a mixture of water, salts and dissolved gases, and alloys are mixtures that are mainly metals.
Section 3 of 10Useful mixtures
- Scientists mix exactly the right amounts of substances to make paints, medicines and deodorants.
- Alloys such as brass are mixtures of metals.
Section 4 of 10Particle diagrams of mixtures
- Each circle is one atom, and atoms of different elements have different colours.
- Possible diagrams
- a mixture of two elements as single atoms, a mixture of two elements where one exists as molecules, a mixture of two compounds and a mixture of an element and a compound.
- Strength
- the diagrams clearly show the atoms in the elements and compounds.
- Limitation
- they are not to scale.
Section 5 of 10Pure substances
- Pure substance (chemistry) = a substance that contains only one element or only one compound.
- Everyday use
- shop labels say pure when they mean natural, so pure orange juice is actually a mixture of water, sugar, acids and flavour molecules.
Section 6 of 10Testing for purity
- Melting and boiling points
- pure substances have fixed ones, while mixtures melt and boil over a range of temperatures.
- Pure water boils at 100 °C and freezes at 0 °C, but salt water boils above 100 °C and freezes below 0 °C.
- Chromatography
- a pure substance gives one spot and a mixture gives more than one.
Section 7 of 10Separating mixtures
- Principle
- the substances in a mixture are not bonded, so they can be separated by physical methods that use a difference in physical properties.
| Method | Used to separate | Property used |
|---|---|---|
| Filtration | Insoluble solid from liquid | Particle size |
| Evaporation | Dissolved solid from solution | Solvent evaporates |
| Crystallisation | Pure crystals from solution | Solubility changes with temperature |
| Simple distillation | Solvent from solution | Boiling point |
| Fractional distillation | Liquids with different boiling points | Boiling point |
| Chromatography | Dissolved colours | Solubility |
| Magnet | Magnetic metals | Magnetism |
Section 8 of 10Combining methods
- Salt from sand and salt
- add water to dissolve the salt, filter to remove the sand, then evaporate the water from the filtrate to leave salt crystals.
- Good answers
- name the method, the property it uses and what ends up as residue and filtrate.
Section 9 of 10Chromatography and distillation
- Chromatography
- spots of ink are placed on a pencil line on filter paper, and the paper stands in solvent below the line.
- The solvent rises and carries the dyes at different rates, so they separate.
- Simple distillation
- a solution is heated, the solvent boils off, is cooled in a condenser and is collected as pure liquid, leaving the solute behind.
- Fractional distillation
- a column separates liquids with different boiling points, such as ethanol (78 °C) and water (100 °C), and is used in industry to separate crude oil.
Section 10 of 10Why purity matters
- Safety
- impurities in a drug can be harmful, so manufacturers test melting points and use chromatography to check products are pure.
- Water
- water companies filter and treat water so it meets safety standards.
- Comparing waters
- evaporating samples leaves a little solid from tap water, a lot from sea water and none from distilled water.
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