AQA GCSE Chemistry 8462: Chemical analysis – Study Guide

This study guide teaches Topic 8, Chemical analysis (sections 4.8.1 to 4.8.3), of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 with exams from June 2018 (version 1.1). The topic is assessed on Paper 2 (1 hour 45 minutes, 100 marks, 50% of the GCSE), which covers Topics 6 to 10 and can draw on ideas from sections 4.1 to 4.3. Everything in this topic is for both Foundation and Higher tier: the specification marks no part of Topic 8 as Higher tier only. It contains two required practicals, 6 (chromatography) and 7 (identifying ions).

Use it with the Chemical analysis revision notes and the Chemical analysis practice questions. The course hub is AQA GCSE Chemistry and the printable checklist lists every statement.

What this topic covers

Spec	What you must be able to do	Tier
4.8.1.1	Define a pure substance; use melting and boiling point data to tell pure from impure	Both
4.8.1.2	Identify formulations from given information	Both
4.8.1.3	Explain paper chromatography; interpret chromatograms; calculate Rf values (Required practical 6)	Both
4.8.2.1–4.8.2.4	Tests for hydrogen, oxygen, carbon dioxide and chlorine	Both
4.8.3.1–4.8.3.5	Flame tests; metal hydroxide precipitates and their equations; tests for carbonates, halides and sulfates (Required practical 7)	Both
4.8.3.6–4.8.3.7	Advantages of instrumental methods; interpret flame emission spectroscopy results	Both

Pure substances (4.8.1.1)

In chemistry, a pure substance is a single element or compound, not mixed with any other substance. In everyday language “pure” can mean something with nothing added, in its natural state, such as “pure milk”. Chemically, milk is a mixture.

Pure elements and compounds melt and boil at specific temperatures. A mixture does not: impurities usually lower the melting point and make the substance melt over a range of temperatures.

Worked example 1. A pure compound melts at 134 °C. Sample P melts sharply at 134 °C. Sample Q starts to melt at 126 °C and is fully melted at 131 °C. Which is pure? P matches the known value and melts at one temperature, so P is pure. Q melts below 134 °C and over a 5 °C range, so Q is impure.

Formulations (4.8.1.2)

A formulation is a mixture that has been designed as a useful product. Each chemical in it has a particular purpose, and the components are mixed in carefully measured quantities so the product has the required properties.

Examples named in the spec: fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

To identify a formulation from given information, look for: a mixture; a stated purpose for each component; fixed, measured proportions. A paint label listing pigment (colour), binder (sticks it to the surface) and solvent (makes it spread), each in set percentages, describes a formulation. You do not need names of components in branded products.

Chromatography (4.8.1.3)

Chromatography separates mixtures and helps identify substances. It has two phases:

• the stationary phase – in paper chromatography, the paper
• the mobile phase – the solvent, which moves through the paper.

Separation depends on how each substance is distributed between the two phases. A substance that is more attracted to the solvent spends more time in the mobile phase and travels further. A substance more attracted to the paper moves less.

Rf value:

Rf = distance moved by substance ÷ distance moved by solvent

Both distances are measured from the origin (the start line), and the substance distance is to the centre of the spot. Rf is always less than 1 and has no units.

Key facts:

• Different compounds have different Rf values in a given solvent, which helps identify them. Rf values change if the solvent changes.
• A pure compound gives a single spot in all solvents. A mixture may give several spots, depending on the solvent.

Worked example 2. On a chromatogram the origin line is 1.5 cm above the bottom of the paper. The solvent front is 9.5 cm above the bottom. The centre of a spot is 5.5 cm above the bottom. Calculate Rf.

distance moved by substance = 5.5 − 1.5 = 4.0 cm
distance moved by solvent   = 9.5 − 1.5 = 8.0 cm
Rf = 4.0 ÷ 8.0 = 0.50

Give Rf to an appropriate number of significant figures – normally the same as your data (here 2 s.f.).

Worked example 3. A food colouring gives two spots, 2.0 cm and 4.8 cm from the origin. The solvent moved 8.0 cm. Reference dyes in the same solvent have Rf values: A 0.25, B 0.45, C 0.60. Which dyes are present? Rf values: 2.0 ÷ 8.0 = 0.25 and 4.8 ÷ 8.0 = 0.60. The colouring contains A and C, not B.

Required practical 6

Investigate how paper chromatography separates coloured substances and calculate Rf values. Method outline:

1. Draw the origin line in pencil (ink would itself separate).
2. Put small spots of each sample on the line.
3. Stand the paper in a container with solvent below the origin line (so the samples don’t dissolve into the solvent pool). Cover the container.
4. Remove the paper before the solvent reaches the top; mark the solvent front at once.
5. Measure distances and calculate Rf values.

Tests for gases (4.8.2)

Gas	Test	Positive result
Hydrogen	burning splint held at the open end of a test tube of gas	burns rapidly with a pop
Oxygen	glowing splint inserted into the gas	splint relights
Carbon dioxide	shake with or bubble through limewater (calcium hydroxide solution)	limewater turns milky (cloudy)
Chlorine	damp litmus paper put into the gas	litmus is bleached and turns white

Note the difference: burning splint for hydrogen, glowing splint for oxygen. The litmus for chlorine must be damp.

Identifying ions (4.8.3)

Flame tests (4.8.3.1)

Flame tests identify some metal ions (cations). A clean wire loop is dipped in the sample and held in a flame.

Metal ion	Flame colour
Lithium, Li⁺	crimson
Sodium, Na⁺	yellow
Potassium, K⁺	lilac
Calcium, Ca²⁺	orange-red
Copper, Cu²⁺	green

If a sample contains a mixture of ions, some flame colours can be masked – the strong yellow of sodium can hide lilac potassium, for example. That limits flame tests to single compounds.

Metal hydroxides (4.8.3.2)

Add sodium hydroxide solution to a solution of the compound.

Metal ion	Precipitate	In excess NaOH
Aluminium, Al³⁺	white	dissolves
Calcium, Ca²⁺	white	stays
Magnesium, Mg²⁺	white	stays
Copper(II), Cu²⁺	blue	stays
Iron(II), Fe²⁺	green	stays
Iron(III), Fe³⁺	brown	stays

Aluminium, calcium and magnesium all give white precipitates. Adding excess sodium hydroxide picks out aluminium. Telling calcium from magnesium needs a flame test: calcium gives orange-red; magnesium gives no distinctive flame colour.

Worked example 4. Write a balanced equation for iron(III) chloride reacting with sodium hydroxide. Iron(III) is Fe³⁺, so the hydroxide needs three OH⁻: Fe(OH)3. Three Cl on the left need 3NaCl on the right.

FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl
ionic:  Fe³⁺ + 3OH⁻ → Fe(OH)3

You are not expected to write equations for the aluminium precipitate dissolving (sodium aluminate).

Carbonates, halides and sulfates (4.8.3.3–4.8.3.5)

Ion	Test	Positive result
Carbonate, CO3²⁻	add dilute acid; bubble gas through limewater	fizzing; limewater turns milky (carbon dioxide)
Chloride, Cl⁻	add dilute nitric acid, then silver nitrate solution	white precipitate (silver chloride)
Bromide, Br⁻	as above	cream precipitate (silver bromide)
Iodide, I⁻	as above	yellow precipitate (silver iodide)
Sulfate, SO4²⁻	add dilute hydrochloric acid, then barium chloride solution	white precipitate (barium sulfate)

The acid is added first to remove carbonate ions, which would also form a precipitate and give a false result. Use nitric acid for halides – hydrochloric acid would add chloride ions. Hydrochloric acid is fine for the sulfate test.

Required practical 7

Use chemical tests to identify the ions in unknown single ionic compounds, covering flame tests, hydroxide precipitates, carbonates, halides and sulfates. Identify the cation (flame test or NaOH), then the anion (acid/limewater, silver nitrate or barium chloride). Example: green flame and blue precipitate with NaOH means Cu²⁺; white precipitate with acidified barium chloride means SO4²⁻; the compound is copper(II) sulfate.

Instrumental methods (4.8.3.6–4.8.3.7)

Instrumental methods detect and identify elements and compounds. Compared with the chemical tests above, they are:

• accurate
• sensitive – they work on very small samples
• rapid.

Flame emission spectroscopy analyses metal ions in solution. The sample is put into a flame and the light given out is passed through a spectroscope. The output is a line spectrum. Each metal ion gives its own pattern of lines, so comparing the sample’s lines with a reference set identifies the ions present – even in a mixture, where a flame test colour would be masked. The intensity of the lines can be used to measure concentration.

Worked example 5. Standard sodium solutions give these emission readings:

Concentration (mg/dm³)	0	5	10	15	20
Intensity (arbitrary units)	0	12	24	36	48

A water sample gives an intensity of 30. Find its sodium concentration. Intensity rises by 12 for every 5 mg/dm³, which is 2.4 units per mg/dm³. Concentration = 30 ÷ 2.4 = 12.5 mg/dm³. On a graph, read across from 30 to the line and down to the axis.

Common errors

• Measuring Rf distances from the bottom of the paper instead of the origin line.
• Measuring to the top of a spot instead of its centre.
• Drawing the origin line in pen.
• Mixing up glowing splint (oxygen) and burning splint (hydrogen).
• Saying chlorine “turns litmus red”. It bleaches it white.
• Writing “precipitate forms” without the colour. The colour is what identifies the ion.
• Forgetting that only aluminium hydroxide dissolves in excess sodium hydroxide.
• Acidifying a chloride test with hydrochloric acid.
• Saying a pure substance “has nothing added” in a chemistry answer. Use the chemical definition.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, GCSE exams June 2018 onwards, version 1.1, published by AQA – section 4.8 Chemical analysis.