Tests for gases and ions
Chemistry · GCSE · The School
The four gas tests, and the exact wording
Hydrogen: a lit splint gives a squeaky pop. Oxygen: a glowing splint relights. Carbon dioxide: limewater turns milky. Chlorine: damp litmus paper is bleached white. Marks are lost on the detail, not the idea — “a splint” is not enough, because whether it is lit or glowing is the whole test.
Flame tests name the metal
Clean a nichrome wire in acid, dip it in the sample, hold it in a blue flame: lithium is crimson, sodium yellow, potassium lilac, calcium orange-red, copper green. Cleaning matters more than it sounds, because sodium is so intensely yellow that a trace left on the wire can hide the other colours.
Sodium hydroxide identifies others by precipitate
Add sodium hydroxide solution and watch the solid that forms. Copper(II) gives blue, iron(II) green, iron(III) brown. Aluminium, calcium and magnesium all give white — so a white precipitate does not identify anything on its own, and you add excess sodium hydroxide, in which only the aluminium one redissolves.
A negative result is still a result
Limewater staying clear does not mean no gas was made; it means no carbon dioxide was detected in what reached it. Say what was tested and what was found, not what you assumed. This is the same discipline as the rest of science: the honest sentence is “no carbon dioxide detected”, not “nothing happened”.
Suppose a class is given a colourless unknown solution that holds one metal ion from this lesson. Test 1 is the flame test, done first, because sodium hydroxide contains sodium and a yellow sodium flame tends to hide other colours. The unknown burns orange-red, which matches calcium. Test 2 uses a fresh portion with sodium hydroxide, and a white precipitate forms. Aluminium, calcium and magnesium all fit, so white alone proves nothing. Adding more sodium hydroxide to the same tube leaves the white precipitate unchanged, so it is not aluminium. In the flame list only calcium burns orange-red. So only calcium fits every result. No bubbles were seen, so no gas test was needed. A full answer says fresh portions were used, names each test and what was seen, and ends: the unknown contains calcium ions.
Instrumental methods, and why they are used
Machine analysis is faster, more sensitive and works on tiny samples, which matters for forensic and medical work where there may be almost nothing to test. The trade-offs are cost, and that the machine must be calibrated against known samples. Wet tests remain on the specification because they show the chemistry rather than hiding it.