1. What’s that gas?

2.  Many chemical reaction produce a gas as one of the products.  To identify a particular gas, we have to collect it.  The way we do this depends on: 1. the density of the gas – is it heavier or lighter than air? 2. the solubility of the gas in water – is it soluble or insoluble?  Gas can be collected in gas syringe, but it is easier to identify a gas if we collect it in a test tube (put a bung on the tube!)  There are three ways we can collect a gas in a test tube:  downward displacement – for gases lighter than air (e.g. hydrogen, ammonia)  upward displacement – for gases heavier than air (e.g. carbon dioxide, chlorine, hydrogen chloride)  downward displacement of water – for gases which are insoluble or slightly soluble in water (e.g. hydrogen, oxygen)

4. Identifying hydrogen  You put a lighted splint at the mouth of the test tube.  If the gas is hydrogen it burns with a squeaky “pop”.  The hydrogen reacts with oxygen in the air to cause a small explosion when a flame or spark is present. Identifying oxygen  You put a glowing splint into the test tube.  If the gas is oxygen the splint will relight.  The splint is made of wood and wood is a fuel.  Fuels burn better in oxygen than in air because there is no nitrogen to dilute the oxygen.  The splint will burn much better in pure oxygen – so much so that the glowing splint will relight.

5. Using the litmus test for ammonia  We call tell if a gas is acidic or alkaline by holding a piece of damp litmus paper at the mouth of the test tube.  If the gas is alkaline it will turn red litmus paper blue.  The gas is certainly ammonia if there is a strong sharp smell as well.  If the given off gas in a reaction is acidic, it will turn damp blue litmus paper red. Identifying carbon dioxide  If we think that a gas given off is carbon dioxide, we can bubble it through limewater – limewater will turn milky or cloudy.

6.  A simpler way is to test carbon dioxide is to simply put a drop of limewater on the end of a flattened glass rod and hold it above the reaction mixture (take care that drop does not fall off!).  Limewater is a colourless solution of calcium carbonate.  When carbon dioxide bubble through it, a fine precipitate of calcium carbonate is formed: Ca(OH) 2(aq) + CO 2(g) → CaCO 3(s) + H 2 O (l) calcium hydroxide + carbon dioxide → calcium carbonate + water  Carbon dioxide is acidic oxide and it reacts with a base to form a salt and water.  If you bubble the carbon dioxide through the limewater for too long, the limewater goes colourless again.  This is because the calcium carbonate dissolves to form soluble calcium hydrogencarbonate.

8. Identifying chlorine  Chlorine is poisonous green gas.  If you think chlorine is going to be released, you should carry out the test in a fume cupboard.  Put damp litmus paper or universal indicator paper at the mouth of the test tube.  The indicator paper turns white – it is bleached.