IGCSE Chemistry (0620)

Identifying a substance from observations

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

Heating a hydrated salt releases its water of crystallisation as water vapour, which condenses as colourless liquid in a cooler part of the apparatus.
Common mistake: Students may wrongly guess another product if they overlook the role of water of crystallisation.
Recognise that effervescence with acid and a gas turning limewater milky is specific evidence for carbonate ions, not just any gas or reaction with acid.
Common mistake: Students often confuse this with other gas tests or ions.
Recognising that a reaction producing a gas is complete when gas evolution (bubbling/effervescence) stops, not when solids disappear or the solution becomes clear, as gas production directly indicates ongoing reaction.
The flame test identifies metal ions by their characteristic flame colour, so the correct conclusion links the observed yellow flame specifically to the presence of sodium ions, not to potassium or other elements.
Recognise that when a carbonate reacts with an acid, visible bubbling (effervescence) occurs due to gas release, and the solid carbonate gradually disappears as it reacts, not just dissolving without bubbles or remaining unchanged.
Recognise that a reducing agent (like sulfite ions) will decolourise acidified potassium manganate(VII) by reducing the purple MnO4– ions to a colourless state, rather than expecting a colour change to another colour or no change.

Suggest the identity of the colourless liquid X.

Try it first, then show the working

Answer: Water.

18 exam-style questions on Identifying a substance from observations, each marked to the scheme.