4.8.2.1,4.8.2.2,4.8.2.3,4.8.2.4 - Identifying hydrogen, oxygen, carbon dioxide and chlorine

Hydrogen and oxygen tests

Hydrogen burns, whereas oxygen supports the burning of another substance, so their splint tests use different starting conditions.

To test for hydrogen, hold a burning splint at the open end of a test tube containing the gas. Hydrogen burns rapidly with a characteristic pop sound. The reaction releases energy as hydrogen reacts with oxygen from the air to form water:

The sound is the observation used to identify hydrogen. Bubbles from a metal–acid reaction suggest a gas is being made, but bubbles themselves do not identify it. A suitable small sample and a teacher-approved procedure are essential because hydrogen is flammable; do not attempt to ignite a large collected volume.

To test for oxygen, first light a splint and blow out the flame so the end is still glowing. Insert it into the test tube of gas. The glowing splint relights in oxygen. Oxygen is not being described as a fuel: it supports the splint's combustion. A burning splint and a glowing splint are therefore not interchangeable details.

A splint that simply goes out does not identify a gas: several gases do not support combustion, and an inadequate sample can also give no positive result. Use the characteristic positive observation, and keep the apparatus away from other flammable materials.

The carbon dioxide test

Carbon dioxide is identified by the cloudy solid it forms when it reacts with limewater.

Limewater is an aqueous solution of calcium hydroxide. It is initially clear. Shake a small sample of carbon dioxide with limewater, or bubble the gas through it using a delivery tube. A positive result is that the limewater turns milky or cloudy. Calling it simply “a colour change” is imprecise: the cloudiness comes from a white insoluble solid, a precipitate of calcium carbonate.

The solid calcium carbonate scatters light, so the liquid looks cloudy. This connects the visible observation to a reaction rather than to the gas's own colour. The gas can be produced by reacting a carbonate with dilute acid; testing the product then provides evidence for the gas's identity.

Use fresh limewater and a controlled amount of gas. Bubbling a large excess of carbon dioxide can eventually make the cloudiness disappear through a further reaction; the initial milky result is the school identification test. A clear result from a faulty delivery tube does not show that carbon dioxide is absent. Check that gas actually enters the limewater and that the reagent is suitable. Prevent suck-back by following the supervised apparatus procedure, rather than letting hot apparatus cool while still connected to the liquid.