Presentation on theme: "What’s that gas?. 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."— Presentation transcript:
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)
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.
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.
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.
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.