Telling One Gas From Another
- Chemistry
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Gases are different substances even when they all look like nothing.
- Each common gas has a specific test with a clear yes-or-no result.
- A test is only evidence if a different gas would have given a different answer.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Catching something you cannot see
A reaction makes bubbles, but how do you catch a gas you cannot see? Fill a jar with water, turn it upside down in a bowl of water, and run a tube from the reaction up into it. The gas bubbles up and pushes the water down ahead of it, so you end up with a jar of gas and a clear view of exactly how much you collected. This method only works for gases that do not dissolve in water, which is worth knowing: some do, and for those a different method is needed.
One test each
So now you have a jar of gas. Which gas is it? Each of the common ones has its own test. Bubble it through limewater: if the clear liquid turns milky, it is carbon dioxide. Lower in a glowing splint: if it bursts back into flame, it is oxygen. Bring in a lit splint: if you hear a squeaky pop, it is hydrogen. Each test gives a plain yes or no, and that is exactly what makes it useful. A test that every gas passed would tell you nothing whatever.
Curriculum links: NGSS MS-PS1-2