Testing for Common Gases

Section 1 of 7Why we test gases

Colourless gases look alike

• Colourless gases look alike Carbon dioxide, hydrogen and oxygen are all colourless, so we cannot tell them apart by sight.
  • A chemical test gives a clear, observable result that identifies each one.
• The gas is usually collected in a test tube before the test is carried out.

Tests versus properties

• Tests versus properties A test is a procedure with a positive result that only one gas gives.
  • A property such as smell or colour describes a gas but is not an acceptable way to identify it.
• For example, saying that chlorine smells like a swimming pool is a property; it would not earn the mark for a test.

Writing the result

• Writing the result Always state the reagent or tool, how it is used, and exactly what is seen.
  • A good answer reads'the gas relights a glowing splint, so it is oxygen'.

Section 2 of 7The four tests in order of use

Gas	Test	Positive result
Carbon dioxide, CO2	Bubble the gas through limewater (calcium hydroxide solution)	Limewater turns milky or cloudy
Hydrogen, H2	Hold a lit splint at the mouth of a test tube of the gas	A squeaky pop is heard
Oxygen, O2	Place a glowing splint inside a test tube of the gas	The glowing splint relights
Chlorine, Cl2	Hold damp litmus paper in the gas	Litmus paper is bleached white

Section 3 of 7Carrying out the carbon dioxide test

1

Pour a small volume of clear limewater into a test tube or boiling tube.

2

Pass the gas into the limewater through a delivery tube so that it bubbles through the liquid.

3

Watch the limewater. If the gas is carbon dioxide, a white precipitate of calcium carbonate forms and the limewater turns cloudy or milky.

4

Write the result clearly: the limewater turns milky, so the gas is carbon dioxide. Equation: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).

Section 4 of 7Hydrogen and oxygen splint tests

Splint tests

Hydrogen: lit splint

• Hydrogen: lit splint Hold a burning splint at the open end of a test tube of gas.
  • Do not push it deep inside, because the burning needs air from outside.
• Hydrogen burns rapidly with oxygen from the air, giving the squeaky pop. Water is the product: 2H₂ + O₂ → 2H₂O.

Oxygen: glowing splint

• Oxygen: glowing splint Light a splint, blow out the flame so that it only glows, then lower it into the gas.
• Oxygen supports burning so strongly that the glowing splint relights.
  • Sometimes it only glows brighter, but in an exam say that it relights.

Memory aid

• Memory aid A lit splint goes with Hydrogen (H) and a glowing splint goes with Oxygen (O).

Section 5 of 7Chlorine and safety

Chlorine test

• Chlorine test Chlorine is a pale green-yellow gas.
  • Damp litmus paper is held in the gas;
    • it may turn red for a moment because chlorine reacts with water to form acids,
  • and then it is bleached white.
• Blue litmus, red litmus or universal indicator paper can all be used, but the paper must be damp.

Handling chlorine

• Handling chlorine Chlorine is toxic, so any experiment that makes it must be done in a fume cupboard or a well-ventilated space.

Why extinguishing a splint is not enough

• Why extinguishing a splint is not enough A burning splint is put out by carbon dioxide, but also by nitrogen and other gases with no oxygen.
  • This is therefore not a valid test for carbon dioxide.

Section 6 of 7Key terms

Term	Meaning
Limewater	A solution of calcium hydroxide used to test for carbon dioxide; it turns milky when carbon dioxide is bubbled through it.
Glowing splint	A wooden splint that has been lit and then blown out so only the end glows; used to test for oxygen.
Squeaky pop	The small explosion heard when hydrogen burns with a lit splint at the mouth of a tube.
Bleach	To remove colour; chlorine does this to damp litmus paper, turning it white.
Precipitate	An insoluble solid formed in a reaction in solution; calcium carbonate makes limewater cloudy.
Fume cupboard	An enclosed, ventilated workspace that removes toxic gases such as chlorine.

Section 7 of 7Exam technique

1

Mixing up the splint testshydrogen needs a lit (burning) splint and gives a pop, while oxygen needs a glowing splint and makes it relight.

2

Saying that a gas that puts out a lit splint must be carbon dioxide. Other gases also do this, so only the limewater test counts.

3

Using dry litmus paper for chlorine. The paper must be damp for the bleaching to be seen.

4

Writing that chlorine 'smells like a swimming pool' as an identification test. A smell is a property, not a test.

5

Pushing the burning splint deep into the tube for the hydrogen test instead of holding it at the mouth.