Test for gases

Most gases are clear, colourless and odourless.

So it's not easy to distinguish between them with a simple chemical test.

Test for Hydrogen

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)

_{Test for Oxygen}

_{Oxygen has a similar density to air, so needs to be contained in a "right-side up" container, preferably w}ith a bung.

_{Oxygen supports burning but only 21% of dry air is Oxygen.}

_{So, if we light a splint and shake out the flame so that the end is still glowing the only reason the flame does not reappear is that 79% of the air around it is not Oxygen.}

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

_{This increases the rate of reaction, providing enough heat to re-light the flame.}

_{Test for Carbon Dioxide}

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

_{So the test for Carbon Dioxide must be something that onlyCarbon 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, it's not necessary to re-dissolve it to prove the gas is Carbon Dioxide.}

_{Ca(OH)2(aq) + CO2(g) --> CaCO3(s) + H2O(l)}

_{Test for Ammonia}

_{Ammonia is a very soluble, alkaline gas.}

_{So we can test for Ammonia by putting damp red Litmus paper in the gas}

_{The gas dissolves and the Litmus paper goes blue.}

_{Test for Chlorine}