Identification of ions and gases
Cambridge IGCSE Chemistry (0620) · Unit 12: Experimental techniques and chemical analysis · 11 flashcards
Identification of ions and gases is topic 12.3 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 12 — Experimental techniques and chemical analysis , alongside Experimental design and Separation and purification. In one line: Paper chromatography is used to separate mixtures of soluble colored or colorless substances using a suitable solvent. It relies on the different solubilities and affinities of the substances for the stationary (paper) and mobile (solvent) phases.
This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical). Past papers from 2022 to 2025 show this topic across undefined questions worth 83 marks (around 1.3% of all Chemistry marks in those years).
The deck below contains 11 flashcards — 3 definitions, 1 key concept and 2 identification cards — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.
Paper chromatography used for
Paper chromatography is used to separate mixtures of soluble colored or colorless substances using a suitable solvent. It relies on the different solubilities and affinities of the substances for the stationary (paper) and mobile (solvent) phases.
What the Cambridge 0620 syllabus says
Official 2026-2028 specThese are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.
- Describe Describe how paper chromatography is used to separate mixtures of soluble coloured substances, using a suitable solvent
- Interpret Interpret simple chromatograms to identify: (a) unknown substances by comparison with known substances (b) pure and impure substances
- Describe Describe how paper chromatography is used to separate mixtures of soluble colourless substances, using a suitable solvent and a locating agent Supplement
- State State and use the equation for R: distance travelled by substance R= distance travelled by solvent Supplement
What is paper chromatography used for?
Paper chromatography is used to separate mixtures of soluble colored or colorless substances using a suitable solvent. It relies on the different solubilities and affinities of the substances for the stationary (paper) and mobile (solvent) phases.
How can paper chromatography identify unknown substances?
Unknown substances can be identified by comparing their chromatograms to those of known substances run under identical conditions. Substances with matching Rf values are likely the same.
How can paper chromatography indicate the purity of a substance?
A pure substance will produce a single spot on the chromatogram. An impure substance will produce multiple spots, indicating the presence of different components.
What is a locating agent, and when is it used?
A locating agent is a substance used to make colorless substances visible on a chromatogram. It reacts with the colorless substances to produce colored spots. Ninhydrin is a common locating agent for amino acids.
State the equation for calculating the Rf value.
Rf = (distance travelled by substance) / (distance travelled by solvent). The Rf value is a characteristic constant for a given substance under specific chromatographic conditions.
A spot travels 3.0 cm on a chromatogram, and the solvent front travels 5.0 cm. Calculate the Rf value.
Rf = (distance travelled by substance) / (distance travelled by solvent) = 3.0 cm / 5.0 cm = 0.6. Remember to include units for working, but the Rf value itself is dimensionless.
Why is it important to use the same conditions when comparing chromatograms?
Rf values are affected by factors such as the solvent used, the temperature, and the type of paper. To accurately compare, all variables except the unknown substance must be constant.
What do the state symbols (s), (aq), (l), and (g) represent?
(s) represents solid, (aq) represents aqueous (dissolved in water), (l) represents liquid, and (g) represents gas. All state symbols must be correct in chemical equations.
In a reaction, 25.0 cm³ of 0.400 mol/dm³ HCl reacts completely. How many moles of HCl are used?
Moles of HCl = (Volume in dm³/1000) x Concentration = (25.0/1000) x 0.400 = 0.0100 mol.
If 0.0100 mol of HCl reacts according to the equation 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂, how many moles of CO₂ are produced?
From the balanced equation, 2 moles of HCl produce 1 mole of CO₂. So, moles of CO₂ = 0.0100 mol HCl / 2 = 0.00500 mol.
If 0.00500 mol of CO₂ gas is produced, what volume does it occupy at room temperature and pressure (RTP), assuming 1 mole of gas occupies 24000 cm³?
Volume of CO₂ = Moles of CO₂ x Molar volume at RTP = 0.00500 mol x 24000 cm³/mol = 120 cm³.
Key Questions: Identification of ions and gases
What is paper chromatography used for?
Paper chromatography is used to separate mixtures of soluble colored or colorless substances using a suitable solvent. It relies on the different solubilities and affinities of the substances for the stationary (paper) and mobile (solvent) phases.
What is a locating agent, and when is it used?
A locating agent is a substance used to make colorless substances visible on a chromatogram. It reacts with the colorless substances to produce colored spots. Ninhydrin is a common locating agent for amino acids.
What do the state symbols (s), (aq), (l), and (g) represent?
(s) represents solid, (aq) represents aqueous (dissolved in water), (l) represents liquid, and (g) represents gas. All state symbols must be correct in chemical equations.
Tips to avoid common mistakes in Identification of ions and gases
- ● List out the common gases (O2, N2, CO2, NH3, Cl2, etc.) and note their distinctive physical and chemical properties.
- ● When identifying ions, write down the possible cation AND the possible anion separately before deciding on the compound.
- ● Think about which substance was loaded into the burette at the start of the experiment.
- ● When naming apparatus, make sure your answer provides a specific detail to distinguish it from similar items.
- ● Tie ion charge to periodic table position: Group 1 elements form 1+ ions, Group 6 elements usually form 2- ions, etc.
More topics in Unit 12 — Experimental techniques and chemical analysis
Identification of ions and gases sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Key terms covered in this Identification of ions and gases deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
Related Chemistry guides
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