What are atoms, elements and compounds, and how do we represent them?
Atoms, elements and compounds; chemical symbols and formulae; mixtures and the separation techniques used to separate them.
A focused answer to AQA GCSE Chemistry 4.1.1, covering atoms, elements, compounds and mixtures, how chemical formulae represent substances, and the techniques used to separate mixtures.
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What this dot point is asking
AQA wants you to define atoms, elements and compounds, interpret chemical symbols and formulae, distinguish mixtures from compounds, and describe the physical methods used to separate mixtures: filtration, crystallisation, simple and fractional distillation, and chromatography. The crucial distinction tested is between a compound (chemically bonded, fixed ratio, new properties) and a mixture (not bonded, separable physically).
Atoms and elements
An atom is the smallest part of an element that can exist. An element is a substance made of only one type of atom. There are about 100 elements, each represented by a chemical symbol such as for oxygen or for sodium (from the Latin natrium). The elements are arranged in the periodic table.
Compounds
When elements react, their atoms join to form compounds. A compound contains two or more elements chemically combined in fixed proportions, shown by a formula such as (water) or (carbon dioxide). The small numbers (subscripts) tell you the ratio of atoms, so every water molecule has exactly two hydrogen atoms to one oxygen atom.
Chemical reactions are shown by equations. In a balanced symbol equation there must be the same number of each type of atom on both sides, because no atoms are made or lost.
Mixtures and separation
A mixture is two or more substances (elements or compounds) that are not chemically bonded together. Because there are no chemical bonds between the parts, a mixture can be separated by physical methods that do not involve chemical reactions:
- Filtration: separates an insoluble solid from a liquid.
- Crystallisation: evaporates the solvent so a soluble solid forms crystals.
- Simple distillation: separates a liquid (the solvent) from a dissolved solid by boiling then condensing the liquid.
- Fractional distillation: separates two or more liquids with different boiling points.
- Chromatography: separates substances dissolved in a solvent that move at different rates.
The method chosen depends on the differences in properties between the parts: solubility, boiling point, or particle size.
Try this
Q1. State the difference between an element and a compound. [2 marks]
- Cue. An element has one type of atom; a compound has two or more elements chemically bonded.
Q2. Name the technique used to separate an insoluble solid from a liquid. [1 mark]
- Cue. Filtration.
Q3. Name the technique used to separate two miscible liquids with different boiling points. [1 mark]
- Cue. Fractional distillation.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20184 marksA student has a mixture of sand and salt (sodium chloride). Describe a method to obtain pure, dry samples of both the sand and the salt from the mixture, naming the separation techniques used.Show worked answer →
A 4-mark Paper 1 practical-skills question on separating a mixture.
Add water and stir to dissolve the salt (1 mark). Filter the mixture: the insoluble sand is the residue on the filter paper, the salt solution passes through as the filtrate; wash and dry the sand (1 mark). Evaporate or crystallise the salt solution to obtain the salt: heat to drive off water, leaving salt crystals (1 mark). The techniques used are filtration (sand) and crystallisation or evaporation (salt) (1 mark).
Markers reward naming both techniques and matching each to the correct component.
AQA 20213 marksExplain the difference between a compound and a mixture, using iron and sulfur as your example. Describe how the properties differ before and after iron and sulfur are heated together to react.Show worked answer →
A 3-mark question contrasting compounds and mixtures.
A mixture of iron and sulfur is not chemically bonded, so the iron can still be removed with a magnet and each element keeps its own properties (1 mark). When heated, the iron and sulfur react to form the compound iron sulfide, in which the elements are chemically bonded in a fixed ratio (1 mark). The compound has completely different properties: it is not magnetic and cannot be separated by physical means, only by a chemical reaction (1 mark).
Markers reward the bonded-versus-not-bonded distinction and a clear before-and-after property contrast.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)