Chemistry · Unit 16

Chemical Analysis

Identifying substances

Analysis answers the question that comes before every other one: what is actually in this sample?

The unit covers flame tests, tests for ions and gases, chromatography, and the instrumental methods - mass spectrometry, infrared, NMR - that replaced most wet chemistry in modern laboratories.

This unit breaks down into 20 short steps and 120 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 120
Difficulty: 1-3

What this unit covers

Flame Tests
Testing for Ions
Testing for Gases
Chromatography
Instrumental Analysis

Where this fits

Best after Acids, Bases and pH and Redox Reactions.

Where people slip

A negative result is informative. Ruling out an ion narrows the possibilities as usefully as confirming one, and good analysis is built from both.

How the unit is structured

Chemical Analysis runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Chromatography

Fill the blankLevel 2
1. Rf value = distance moved by the spot / distance moved by the ____.
- solventcorrect
- paper
- pencil
- beaker
The Rf value compares how far a spot travels with how far the solvent front travels.
Guess the numberLevel 2
2. On a chromatogram a spot moves 3 cm while the solvent moves 6 cm. What is the Rf value?
Answer: 0.5
Rf = distance moved by spot / distance moved by solvent = 3 / 6 = 0.5.
Multiple choiceLevel 2
3. In paper chromatography, what is the mobile phase?
- The solventcorrect
- The paper
- The pencil line
- The spot of sample
The mobile phase is the solvent, which moves up the paper carrying the dissolved substances.
Odd one outLevel 2
4. Which of these is NOT a chromatography technique?
- Fractional distillationcorrect
- Paper chromatography
- Thin-layer chromatography
- Gas chromatography
Fractional distillation separates liquids by boiling point; paper, thin-layer and gas chromatography are all chromatography methods.
Put in orderLevel 2
5. Put the steps of running a paper chromatogram in the correct order.
Answer: Draw a baseline in pencil -> Add a spot of the sample to the baseline -> Stand the paper in a small amount of solvent -> Let the solvent rise up the paper -> Mark how far the solvent front has travelled -> Calculate the Rf value
You draw a pencil baseline, spot the sample, let the solvent rise, then measure distances to find the Rf value.
Sort into groupsLevel 2
6. Sort each example into pure substance or mixture.
Answer: Distilled water = Pure substance; Air = Mixture; Sea water = Mixture; Pure copper = Pure substance; Crude oil = Mixture; Sodium chloride crystals = Pure substance
A pure substance contains only one type of substance, while a mixture contains two or more.

Flame Tests

Multiple choiceLevel 1
7. What colour flame does lithium produce in a flame test?
- Crimson redcorrect
- Yellow
- Lilac
- Green
Lithium compounds burn with a crimson (deep red) flame in a flame test.
Fill the blankLevel 2
8. A pale apple-green flame colour in a flame test indicates the presence of ____ ions.
- bariumcorrect
- lithium
- sodium
- calcium
Barium ions give a characteristic pale apple-green flame colour.
Match the pairsLevel 2
9. Match each metal to the flame colour it produces.
Answer: Lithium = Crimson red; Sodium = Yellow; Potassium = Lilac; Copper = Green
Lithium is crimson, sodium is yellow, potassium is lilac and copper is green.
Odd one outLevel 2
10. Three of these flame-test pairings are correct. Which is the ODD one out (the incorrect pairing)?
- Calcium - greencorrect
- Lithium - crimson red
- Sodium - yellow
- Potassium - lilac
Calcium gives an orange-red flame, not green, so that pairing is the odd one out.
Picture questionLevel 2
11. 🔥 A metal salt burns with this bright yellow flame, like an orange street lamp. Which metal is it?
- Sodiumcorrect
- Potassium
- Copper
- Lithium
Sodium produces the bright yellow flame seen in older sodium street lamps.
Fact or fibLevel 3
12. In flame emission spectroscopy, a greater light intensity at a characteristic wavelength means a higher concentration of that metal ion.
Answer: True
The intensity of the emitted light is proportional to concentration, so flame emission spectroscopy can measure how much of an ion is present.

Instrumental Analysis

Build the sentenceLevel 2
13. Put the words in order to complete the rule about purity:
Answer: pure substance melts at one fixed temperature
A pure substance melts at one fixed temperature, whereas a mixture melts over a range.
Choose all that applyLevel 2
14. Which of these are advantages of instrumental analysis over simple chemical tests? Select all that apply.
- They are very sensitive and detect tiny amountscorrect
- They give results quicklycorrect
- They can work on very small samplescorrect
- They always need large samples and take days
Instrumental methods are rapid, highly sensitive and can analyse very small samples accurately.
Match the pairsLevel 2
15. Match each instrumental technique to what it mainly identifies.
Answer: Mass spectrometry = Relative molecular mass; Infrared spectroscopy = Bonds and functional groups; NMR spectroscopy = Hydrogen environments; Flame emission spectroscopy = Metal ions
Mass spectrometry gives relative molecular mass, IR shows bonds, NMR reveals hydrogen environments and flame emission identifies metal ions.
Fact or fibLevel 3
16. UV-visible spectroscopy works by absorbing light that promotes electrons to higher energy levels (electronic transitions).
Answer: True
UV-visible spectroscopy measures the absorption of ultraviolet or visible light that excites electrons to higher energy levels.
Fill the blankLevel 3
17. In flame emission spectroscopy, each metal ion produces its own characteristic ____ spectrum.
- linecorrect
- continuous
- sound
- colourless
Each metal ion gives a unique pattern of lines, called a line spectrum, that acts like a fingerprint.
Guess the numberLevel 3
18. How many different hydrogen environments (and therefore peak groups in its 1H NMR spectrum) does ethanol, CH3CH2OH, have?
Answer: 3 environments
Ethanol has three hydrogen environments: the CH3 group, the CH2 group and the O-H hydrogen.

Testing for Gases

Fact or fibLevel 1
19. Carbon dioxide turns limewater cloudy (milky).
Answer: True
Bubbling carbon dioxide through limewater turns it cloudy as insoluble calcium carbonate forms.
Multiple choiceLevel 1
20. How can you test for hydrogen gas?
- A lit splint gives a squeaky popcorrect
- It relights a glowing splint
- It turns limewater cloudy
- It bleaches damp litmus paper
Hydrogen burns quickly with a lighted splint, giving a squeaky pop.
Build the sentenceLevel 2
21. Put the words in order to describe the result of the hydrogen gas test:
Answer: lighted splint makes squeaky pop
Hydrogen is confirmed when a lighted splint makes a squeaky pop.
Match the pairsLevel 2
22. Match each gas to its identifying test result.
Answer: Hydrogen = Squeaky pop with a lighted splint; Oxygen = Relights a glowing splint; Carbon dioxide = Turns limewater milky; Chlorine = Bleaches damp litmus paper
Hydrogen pops, oxygen relights a glowing splint, carbon dioxide turns limewater milky and chlorine bleaches damp litmus.
Tap the pairsLevel 2
23. Match each gas to the test that identifies it.
Answer: Hydrogen = Squeaky pop with a lit splint; Oxygen = Relights a glowing splint; Carbon dioxide = Turns limewater cloudy; Chlorine = Bleaches damp litmus paper
Each common gas has its own simple laboratory test.
Fill the blankLevel 3
24. Limewater turns cloudy when carbon dioxide reacts with it to form insoluble ____.
- calcium carbonatecorrect
- calcium chloride
- sodium carbonate
- calcium oxide
Carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate.

Testing for Ions

Multiple choiceLevel 1
25. What happens when dilute acid is added to a carbonate?
- It fizzes, releasing carbon dioxide
- It forms a yellow precipitate
- It turns lilac
- Nothing happens
Carbonates fizz with dilute acid, releasing carbon dioxide gas.
Choose all that applyLevel 2
26. Select the TWO correct chemical tests for ions.
- Carbonates fizz with dilute acid, releasing carbon dioxide
- Sulfates give a white precipitate with barium chloridecorrect
- Chlorides give a yellow precipitate with silver nitrate
- Iodides give a white precipitate with silver nitrate
Carbonates fizz with acid, and sulfates give a white precipitate with barium chloride.
Fact or fibLevel 2
27. Silver bromide formed in the halide test is a cream-coloured precipitate.
Answer: True
Bromide ions give a cream precipitate of silver bromide with silver nitrate solution.
Fill the blankLevel 2
28. Iodide ions form a ____ precipitate when tested with silver nitrate solution.
- yellowcorrect
- white
- cream
- green
Iodide ions give a yellow precipitate of silver iodide with silver nitrate.
Match the pairsLevel 2
29. Match each ion to its test result.
Answer: Chloride ion = White precipitate; Bromide ion = Cream precipitate; Iodide ion = Yellow precipitate; Carbonate ion = Fizzes with dilute acid
Halides give coloured precipitates with silver nitrate, while carbonates fizz with acid.
Put in orderLevel 2
30. Put the steps of testing an unknown solution for iron(III) ions in the correct order.
Answer: Dissolve the sample to make a solution -> Add sodium hydroxide solution dropwise -> Observe a brown precipitate forming -> Conclude that iron(III) ions are present
Dissolve the sample, add sodium hydroxide dropwise, observe a brown precipitate, and conclude that iron(III) ions are present.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 16 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Chemical Analysis

120 questions across 20 steps. Start with step one and crawl at your own pace.

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Read about Chemical Analysis

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

How Chromatography Separates MixturesChromatography separates dissolved substances by making them travel through a material at different speeds. Attraction to the mobile and stationary phases determines how far each component moves.August 4, 2026 · 5 min read

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