Atoms, Elements & Compounds

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CognitoAtoms, elements and compounds

CognitoEquations and state symbols

Key facts & methods

Elements and compounds

An element is a pure substance made of only one type of atom. Cannot be broken down further by chemical reactions.
A compound is a pure substance made of two or more elements chemically combined in fixed ratios.
Compounds have different properties from the elements they are made from.
A mixture contains two or more substances not chemically combined — can be separated by physical means.
Chemical formula shows the ratio of atoms: H₂O = 2 hydrogen, 1 oxygen. NH₃ = 1 nitrogen, 3 hydrogen.

Chemical equations

Word equation: names of reactants → names of products.
Symbol equation: chemical formulae, must be balanced (same number of each atom on each side).
Balancing: only change the numbers (coefficients) in front of formulae — never change the subscripts.
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Example: 2H₂(g) + O₂(g) → 2H₂O(l)

Separation techniques for mixtures

Filtration: separates insoluble solid from liquid. Residue stays on filter paper; filtrate passes through.
Evaporation/crystallisation: separates dissolved solid from solution by heating.
Simple distillation: separates liquid from dissolved solid using different boiling points.
Fractional distillation: separates liquids with close boiling points (e.g. ethanol and water, or crude oil fractions).
Chromatography: separates dissolved substances with different solubilities.

Exam questions — 4 questions · 10 marks · AQA 8462 style

3 marksBalancing equationsAQA 8462 P1 style

Balance the following symbol equation: __ Al + __ O₂ → __ Al₂O₃. Include state symbols if: aluminium is solid, oxygen is gas, aluminium oxide is solid. (3 marks)

Hint: Balance Al first (Al₂O₃ has 2 Al), then O (Al₂O₃ has 3 O). Scale up as needed. Add state symbols last.

+30 XP

1 markCompound definitionAQA 8462 style

Which of the following is a compound?

A Oxygen (O₂)

B Salt water

C Water (H₂O)

D Air

3 marksSeparation methodAQA 8462 style

A student has a mixture of sand and salt in water. Describe how they could obtain pure, dry salt from this mixture. (3 marks)

Hint: Two stages: (1) remove the insoluble sand by filtration, (2) remove the water from the salt solution by evaporation.

+30 XP

1 markState symbolsAQA 8462 style

In the equation: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l), what does (aq) mean?

A The substance is a gas

B The substance is dissolved in water (aqueous solution)

C The substance is a pure liquid

D The substance is a solid

Atoms, Elements & Compounds

Elements and compounds

Chemical equations

Separation techniques for mixtures

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