Chemistry
Testing for gases
Hydrogen, oxygen, or CO₂? Three classic tests reveal invisible gases.
What you need first
Opening a bottle of sparkling water hisses, dough rises while baking, bubbles climb up during . Gases form everywhere, but they are invisible and all look the same: like nothing. So how do you know which gas you are dealing with? Chemistry has developed clever identification tests in which each gas shows an unmistakable reaction.
The pop test and the glowing splint test
The pop test detects hydrogen: when the teacher holds a burning wooden splint to the mouth of a test tube of hydrogen, there is a whistling pop. The hydrogen burns in a flash with the oxygen in the air, forming water. A mixture of hydrogen and oxygen like this is called oxyhydrogen, and igniting it delivers the pop that gives the test its name. The glowing splint test detects oxygen: a glowing splint, one that is only smouldering, bursts brightly back into flame in pure oxygen, because oxygen strongly fans combustion. Without enough oxygen the splint stays dark.
No fire: it does not work without heat
Fire needs all three sides. Extinguishing means taking one side away.
The limewater test
(CO₂) does not burn and does not fan a flame either, so it needs a test of its own: limewater. This is a clear, colourless solution. If a gas containing CO₂ is passed through it, the solution turns milky and cloudy because tiny particles of chalk form. Chalk contains carbon and oxygen, exactly the building blocks that CO₂ brings along. The small 2 in the formula, the subscript, says that one carbon atom goes with two oxygen atoms, so a CO₂ has 3 atoms. Hydrogen and oxygen, by contrast, leave the limewater clear. This is how you can show, for example, that exhaled air and the gas in fizzy drinks contain CO₂.
Why these tests belong in the classroom
The pop test and the glowing splint test involve open flames and reactive gases. Experiments like these belong strictly in the classroom, with a teacher, safety goggles, and proper equipment. Your job is a different one, and it is at least as important: understand the tests, interpret the observations correctly, and instantly recognise in your teacher's demonstration which gas has just given itself away.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
When a burning splint is brought close, there is a whistling pop. Which gas was in the test tube?
A glowing splint bursts brightly into flame in the test tube. Which gas has been detected?
A gas is passed through clear limewater, which then turns milky. Which gas is it?
Match each gas to the signal that reveals it.
How many atoms are there in one CO₂ molecule?
In the pop test, hydrogen burns in a flash with the oxygen in the air, forming ….
Where this leads