Analysis of Substances

(GCSE Chemistry)

We can use a number of tests in order to work out which halogen gases are produced in reactions. There are four main tests for four main gases; chlorine, hydrogen, oxygen and carbon dioxide;

1. To test for hydrogen we use what's called the 'squeaky pop' test. We place the gas into a test tube and then place a lit splint at the mouth of the tube. If a 'squeaky pop' noise is produced, we know hydrogen is present
2. To test for oxygen we test to see if a splint can be re-lit (oxygen is required for fire to be present). Place the gas into a test tube and then place a glowing (as if you've just blown it out) splint at the mouth of the tube. If the splint relights, oxygen is present.
3. To test for chlorine, we test t see a change in litmus paper. Place a strip of litmus paper into the mouth of the test tube and watch how it changes (or doesn't) change colour. If the strip changes colour from red to white, we know chlorine gas is present.
4. To test for carbon dioxide, we use lime water (aqueous calcium carbonate) and place the bubbles (often through a tube) into the solution. If the solution turns cloudy, we know carbon dioxide is present.

(Maria O'Swalt)

Similarly, we use different techniques in the identification of different negative (non-metal) ions. It's important to know how to test for carbonate, sulphate and halide ions;

1. When testing for carbonate ions we often use the knowledge that; metal carbonate + acid = salt + water + carbon dioxide. Using the test for carbon dioxide (as above) we can use an acid and run the produced gas through lime water. If the lime water goes cloudy we then know that carbon dioxide is present and therefore carbonate ions are in the solution.
2. We can test for sulphate ions also using reactions. We know that barium ions react with sulphate ions to produce barium sulphide (a salt) so we can react the solution with dilute hydrochloric acid and barium chloride to produce a white precipitate ( if sulphate ions present)
3. We can also test for halide ions of iodide, chloride and bromide. Firstly, we add in dilute nitric acid to the solution so that if carbonate ions are present they react and are removed (otherwise results would be less clear). Next, we add silver nitrate to form different coloured precipitates.

• If the precipitate is chloride, it will be white
• If the precipitate is bromide, it will be cream
• If the precipitate is iodide, it will be yellow

Furthermore, we can test for the presence of metal ions using flame tests that produce different colours. We firstly use an inoculating wire loop and dip this in dilute hydrochloric acid and heat over a Bunsen burner (to remove any impurities that might affect the results/colour of the flame produced). Next you should dip the loop in the different precipitates/ solutes and  heat this over the Bunsen burner. Different colours will be produced by the presence of different metal ions and it's important to know which colours indicate which ions;

(Martin Adams)

• If copper ions are present, the flame is GREEN
• If sodium ions are present, the flame is YELLOW
• If potassium ions are present, the flame is LILAC
• If calcium ions are present, the flame ORANGE
• If lithium ions are present, the flame is CRIMSON 

Some colours are stronger than others and so might overpower/ hide the colours of other metal ions so it's good to remember that no chemistry (or science for that matter!) is perfect or 100% accurate

Colourful! (Marliese Brandsma)