Every chemical test in SPM Chemistry, in one place

Scattered across the SPM Chemistry syllabus are dozens of small “test for…” questions, and they are among the easiest marks to secure, or to lose, because each answer is a fixed reagent and a fixed observation. This page gathers the gas tests, the test for water, and the organic tests into one reference. For each, know three things: what you add, what you do, and exactly what you see. Keep the gas tests reference beside you as you learn them.

Tests for common gases

• Hydrogen, H₂, bring a lighted wooden splinter to the mouth of the tube; it burns with a “pop” sound.
• Oxygen, O₂, insert a glowing (not flaming) splinter; it rekindles and relights.
• Carbon dioxide, CO₂, bubble the gas through limewater; the limewater turns milky (cloudy white).
• Ammonia, NH₃, hold moist red litmus paper in the gas; it turns blue. Ammonia is also pungent and gives white fumes when a rod dipped in concentrated hydrochloric acid is held near it.
• Chlorine, Cl₂, a greenish-yellow gas; hold moist blue litmus paper in it and it turns red, then is bleached white.
• Sulfur dioxide, SO₂, pungent; it turns acidified potassium dichromate(VI) paper from orange to green.

The three “splinter and limewater” tests, hydrogen, oxygen, carbon dioxide, are the ones examiners return to most often; the detail on each sits in tests for hydrogen, oxygen and carbon dioxide.

Test for water

• Physical confirmation, a pure liquid that boils at 100 °C and freezes at 0 °C.
• Chemical tests, water turns anhydrous copper(II) sulfate from white to blue, and turns blue cobalt(II) chloride paper pink. Note that these tests show a substance is water, but not that it is pure; purity needs the boiling and freezing points.

Distinguishing alkanes from alkenes

This is the classic organic test, hinging on the carbon-carbon double bond:

• Bromine water, shake the sample with brown bromine water. An alkene decolourises it quickly at room temperature (an addition reaction across the double bond). An alkane does not decolourise it under ordinary conditions.
• Acidified potassium manganate(VII), an alkene decolourises the purple solution; an alkane does not.

Either test cleanly separates a saturated from an unsaturated hydrocarbon, so state the reagent, the observation for each, and the reason (the double bond).

Testing for a carboxylic acid

• Litmus, a carboxylic acid turns moist blue litmus paper red, because it is acidic.
• With a carbonate, add sodium carbonate; effervescence of carbon dioxide is seen (which turns limewater milky), confirming an acid.
• Esterification, warm the acid with an alcohol and a little concentrated sulfuric acid; a sweet, fruity smell of ester is produced.

Testing an alcohol by oxidation

Warm the alcohol with acidified potassium manganate(VII) or acidified potassium dichromate(VI): the alcohol is oxidised to a carboxylic acid, and the oxidising agent changes colour, purple manganate(VII) is decolourised, and orange dichromate(VI) turns green. The organic groups behind these behaviours are summarised in organic functional groups.

Reading a “test for” question correctly

In the exam, a “describe a test” answer needs all three parts, and a missing one usually costs the mark:

1. Reagent or apparatus, name it precisely (“acidified potassium manganate(VII)”, not just “potassium manganate”).
2. What you do, “add”, “warm”, “bubble the gas through”, “hold moist litmus in the gas”.
3. Observation, the exact change: a colour, a gas, a precipitate, a smell. “It reacts” is never enough.

A frequent slip is naming the reagent but forgetting the observation, or describing a vague colour change instead of the specific one. Another is confusing “glowing” with “lighted” splinter, oxygen needs a glowing splinter, hydrogen a lighted one. Precision is the whole game here.

Turning tests into reliable marks

These tests are pure recall, so they respond quickly to the right method: quiz yourself from the substance to the observation and back, and rehearse writing the full three-part answer rather than just picturing it. If your child recognises the tests but keeps dropping the observation or the reagent detail, that gap closes fast with feedback on real questions. Our teachers coach SPM Chemistry online, one-to-one in English from RM50 an hour, with a paid one-hour trial to start, drilling the tests until reagent, method and observation come out complete every time.