Tests for gases: hydrogen, oxygen, carbon dioxide and chlorine
Each of the four common gases has its own test. Hydrogen burns with a pop when you hold a lit splint at the mouth of the tube. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper white. AQA prints all four results in its specification, and Edexcel and OCR list the same four tests.
The four tests and their results
Pick a gas in the tester to see what you do and what you should observe. Here is the same set as a list:
- Hydrogen: hold a burning splint at the open end of a test tube of the gas. The hydrogen burns fast with a pop.
- Oxygen: put a glowing splint into a test tube of the gas. The splint bursts back into flame.
- Carbon dioxide: bubble the gas through limewater, or shake the two together. The limewater turns milky, which means cloudy.
- Chlorine: put a piece of damp litmus paper into the gas. The paper loses its colour and turns white.
Limewater is a solution of calcium hydroxide in water. Bleached means the colour has been removed.
Burning splint or glowing splint
These two are easy to swap. A burning splint has a flame, and it is the test for hydrogen. A glowing splint has no flame, only a hot tip, and it is the test for oxygen.
One way to keep them apart: oxygen is the gas that helps things burn, so it brings the glowing splint back to life. Hydrogen is the gas that burns, so it needs a flame to set it off.
Where you meet each gas
An acid and a metal react to make a salt and hydrogen. The reactivity series tells you which metals react fastest. AQA says an acid and a metal carbonate make a salt, water and carbon dioxide. Edexcel puts its hydrogen and carbon dioxide tests in its topic on acids, and the pH scale page covers that topic.
Edexcel sets the oxygen test in its topic on the atmosphere, next to how early plants released oxygen by photosynthesis. Its chlorine test sits with the Group 7 elements, the halogens. OCR Twenty First Century places its chlorine test in the topic on clean water, where chlorine kills microorganisms.
What each board says
The gas tests are in every GCSE course here, including Combined Science. AQA prints the method and the result for all four gases, in both 8462 and 8464. Edexcel lists a test for each gas in both 1CH0 and 1SC0, and names limewater for carbon dioxide. It does not print the results.
OCR Gateway groups all four gases in one statement and shares it with its Combined Science course. OCR Twenty First Century groups oxygen, hydrogen and carbon dioxide together. It tests chlorine separately and names blue litmus paper for it.
Separate chemistry adds tests for ions, such as the flame test colours for metal ions. Edexcel's GCSE Chemistry course also adds a test for ammonia gas.
Common mistakes
- Swapping the splints. Hydrogen needs a burning splint and oxygen needs a glowing one.
- Writing that the gas turns milky. The limewater is what changes.
- Using dry litmus paper for chlorine. AQA describes the test with damp paper.
- Saying oxygen gives a pop. The pop is the result for hydrogen.
After the gases, the next set of lab tests identifies metal ions by colour. Try it in Flame Test Lab. The chemistry hub lists the other pages in this set.
Questions people ask
What is limewater?
A solution of calcium hydroxide in water. It turns milky when carbon dioxide passes through it.
Should the litmus paper be red or blue for chlorine?
OCR Twenty First Century names blue litmus paper. AQA says damp litmus paper without naming a colour, and the result is that the paper goes white.
Are the gas tests in Combined Science?
Yes. AQA and Edexcel include them in Combined Science, and OCR Gateway shares its gas test statement with its Combined Science course.