Most gases are clear, colourless and odourless.

So it's not easy to distinguish between them by looking at them or smelling them (which would be dangerous!)

So, we need a simple chemical test that can show which gas is which visually.

Hydrogen is less dense than air and explosive when mixed with Oxygen.

A test-tube filled with hydrogen can be safely held up-turned since no Hydrogen can escape by sinking into denser air.

Light a splint and swiftly turn the test tube upwards a little towards the flame to allow the Hydrogen to mix with air.

The explosion should be detected as a squeaky pop.

Hydrogen + Oxygen --> Water Vapour

2H_2(g) + O_2(g) -->2H₂O_(g)

Oxygen has a similar density to air, so needs to be contained in a 'rightside-up' container, preferably with a bung.

Oxygen supports burning. But only 21% of air is Oxygen.

So, if we light a splint and shake out the flame so that the end is still glowing the only reason that it doesn't burst back into flame is that 79% of the air around it isn't Oxygen

Placing the gloiwing splint in pure Oxygen means that every gas molecule that collides with the glowing ember is Oxygen.

And this increases the rate of reaction providing enough heat to relight the flame.

_{Carbon Dioxide puts out fires. But so do most other gases.}

_{So the test for Carbon Dioxide must be something that only Carbon Dioxide does.}

_{Only Carbon Dioxide turns Lime-water milky white.}

_{The white colour is a precipitate of Calcium Carbonate ( like chalk).}

_{If enough Carbon Dioxide bubbles through the Lime-water the white precipitate re-dissolves.}

_{But since no other gas causes a white precipitate (solid), it's not necessary to re-dissolve it to prove the gas is Carbon Dioxide.}

Ca(OH)_2(aq) + CO_2(g) --> CaCO_3(s) + 2H₂O_(l)

_{Chlorine is a soluble, green gas}

_{But it can look yellow in small amounts and it may be very difficult to see at all.}

_{But it dissolves to form an acidic solution.}

_{So, Damp blue indicator paper will dissolve the gas and turn red.}

_{But the dissolved Chlorine is also a bleach.}

_{So, the indicator paper will soon go white}