MYP Chemistry: Chemical Tests for Gases & Ions FAQ

State the exact test method, then the exact positive observation — both parts earn separate marks. For example, 'insert a glowing splint; it relights' is two marks, not one. Never just name the gas or ion without describing what you'd actually see or hear happen.

Gas and ion tests are usually assessed under MYP Sciences Criterion A (Knowing and Understanding), often through short-answer or multiple-choice questions in unit tests and the eAssessment. Command terms like 'state', 'describe' and 'identify' each demand a different level of detail, so match your answer length to the verb used.

The most common are 'state', 'describe', 'identify' and 'deduce'. 'State' needs a brief, precise fact. 'Describe' needs full detail — method plus result. 'Identify' means naming the substance from evidence given. 'Deduce' means using a result (like a precipitate colour) to work out an unknown ion.

There are four gases examined repeatedly at MYP 4-5: oxygen, carbon dioxide, hydrogen and ammonia. Each has one specific test and one specific positive result, and mixing these up is the single most common error I see in mock papers every year.

Insert a glowing splint into the gas. If oxygen is present, the splint relights. This works because oxygen supports combustion more strongly than normal air, reigniting the smouldering wood. Don't confuse this with the hydrogen test, which uses a lit — not glowing — splint.

Bubble the gas through limewater (a dilute solution of calcium hydroxide). If carbon dioxide is present, the limewater turns cloudy or milky white due to insoluble calcium carbonate forming. This is a two-mark answer: the method and the specific colour change.

Hold a lit splint at the mouth of the test tube. Hydrogen gives a squeaky pop as it ignites rapidly. The sound, not a flame colour, is the key observation examiners want you to name — many students write 'it burns', which loses the mark.

For cations like ammonium, add sodium hydroxide and warm gently — ammonia gas released turns damp red litmus blue. For anions like carbonate, add dilute acid and test for carbon dioxide with limewater. Each ion test pairs a specific reagent with a specific colour or gas change.

Add dilute nitric acid, then a few drops of silver nitrate solution. Chloride gives a white precipitate, bromide a cream precipitate, iodide a yellow precipitate. The nitric acid step matters — it removes other ions that could give a false-positive precipitate.

Almost always it's incomplete answers — naming the gas without describing the exact observation, or vice versa. Examiners award marks for method and result separately, so a half-answer caps your score even when you clearly know which gas it is.

Quiz your child with quick flashcard-style prompts — 'test for oxygen?', 'observation for carbonate?' — since this topic rewards fast recall over deep problem-solving. According to the IB's MYP: From Principles into Practice guide, MYP Sciences assessment increasingly favours applied, evidence-based responses over rote definitions, so ask them to explain why each test works, not just what happens.

A concise, colour-coded reference table beats a textbook chapter for this topic — students need quick recall, not extended reading. On RevisionPrep, the MYP Chemistry Revision Notes and Topical Worksheets group all the gas and ion tests together with practice questions in the exact command-term style used in class assessments.