4.8.2.1Test for hydrogen
The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.
This is the test you meet whenever a metal reacts with water or with an acid, because hydrogen is the gas given off in both.
4.8.2.2Test for oxygen
The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
Note the difference from the hydrogen test: a burning splint for hydrogen, held at the open end; a glowing splint for oxygen, inserted into the tube. Getting the two the wrong way round is the commonest error in this sub-topic.
4.8.2.3Test for carbon dioxide
The test for carbon dioxide uses an aqueous solution of calcium hydroxide — limewater. When carbon dioxide is shaken with or bubbled through limewater, the limewater turns milky, or cloudy.
This is the test used whenever a carbonate reacts with an acid, and whenever a carbonate is heated until it decomposes.
- Limewater
- an aqueous solution of calcium hydroxide, used to test for carbon dioxide; it turns milky when carbon dioxide is bubbled through it
4.8.2.4Test for chlorine
The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas, the litmus paper is bleached and turns white.
The paper must be damp — chlorine has to dissolve before it can bleach — so "damp" is part of the answer, not a detail of technique.
These four tests are used throughout the rest of the specification, and recognising where each one belongs is usually what a question is really asking.
Hydrogen appears whenever a metal reacts with water or with a dilute acid, and at the cathode when a solution of a reactive metal's compound is electrolysed. Oxygen appears at the anode in the electrolysis of aqueous solutions without halide ions, and when hydrogen peroxide decomposes. Carbon dioxide appears whenever a carbonate meets an acid, and when a metal carbonate is heated until it decomposes. Chlorine appears at the anode when a chloride solution is electrolysed.
Worked example
A student adds dilute hydrochloric acid to a white solid. The mixture fizzes. Describe how to find out whether the gas is hydrogen or carbon dioxide, and say what each result would mean.
- Both gases would cause fizzing, so the fizzing alone does not identify it.
- Collect the gas in a test tube.
- Test with a burning splint: if it burns rapidly with a pop, the gas is hydrogen, so the solid contained a metal.
- Otherwise bubble the gas through limewater: if the limewater turns milky, the gas is carbon dioxide, so the solid was a carbonate.
- Carry out one test per sample of gas — a burning splint would consume the gas needed for the second test.
Answer: A pop with a burning splint means hydrogen, from a metal reacting with the acid. Limewater turning milky means carbon dioxide, from a carbonate reacting with the acid.
Common mistake
Litmus paper in chlorine may first turn red, because chlorine solution is acidic, but the result that identifies chlorine is that the paper is then bleached white. An acid alone turns the paper red without bleaching it.
Before you practise
- Hydrogen: a burning splint at the open end of the test tube — the gas burns rapidly with a pop.
- Oxygen: a glowing splint inserted into the tube — the splint relights.
- Carbon dioxide: shake with or bubble through limewater — it turns milky.
- Chlorine: damp litmus paper — it is bleached and turns white.
- In every case, give both the test and the positive result; naming only one loses half the marks.