Analysing substances

Flame tests are used to detect the presence of a particular metal ion in a compound. Metal ions change the colour of a flame when they are heated in it. Different metal ions give different colours to the flame - so flame tests can be used to identify the presence of a particular metal in a sample.

This is how you would carry out a typical flame test:

1. dip a clean flame test loop in the sample solution

2. hold the flame test loop at the edge of a Bunsen burner flame

3. observe the changed colour of the flame, and decide which metal it indicates

4. clean the loop in acid and rinse with water, then repeat steps 1 to 3 with a new sample

Colour of the flame;

• Lithium; Crimson
• Sodium; Yellow
• Possasium; Lilac
• Calcium; Red
• Barium; Green

Using precipitation to identify metal ions

Some reactions form a precipitate - this is an insoluble solid formed in the reaction. Precipitates often appear as small particles suspended in a solution.  A precipitate may be formed when a few drops of sodium hydroxide are added to a solution of a metal compound. For example, a blue precipitate of copper(II) hydroxide forms when sodium hydroxide solution is added to copper(II) sulfate solution:

copper(II) sulfate + sodium hydroxide → copper hydroxide + sodium sulfate

CuSO_4(aq) + 2NaOH_(aq) → Cu(OH)_2(s) + Na₂SO_4(aq)

The colour and the properties of the precipitate can be used to identify the metal ion present.

Calcium, magnesium and aluminium

Calcium, magnesium and aluminium all form white precipitates when reacted with sodium hydroxide.

However, it is possible to identify whether the white precipitate is due to the presence of aluminium ions, as adding excess sodium hydroxide causes a precipitate of aluminium hydroxide to dissolve. This does not happen for the precipitates formed by calcium and magnesium ions.

Metal ion   Colour of precipitate   What happens when excess sodium hydroxide is added? Aluminium                  White           Aluminium hydroxide precipitate dissolves Calcium         White            No change Magnesium         White           No change

The distinctive colour of the precipitate formed when particular transition metal ions react with sodium hydroxide, allows them to be identified.

You need to know the colours for the metal ions below.

Transition metal ion       Colour of precipitate Copper (II)             Blue Iron (II)               Green Iron (III)             Brown

Testing for carbonate ions;Metal carbonates contain carbonate ions, CO₃^2-. The presence of carbonate ions can be confirmed using a two-step experiment:

Step 1: Carbonates react with dilute acids to produce carbon dioxide and water

For example:

magnesium carbonate + sulfuric acid → magnesium sulfate + carbon dioxide + water

MgCO_3(s) + H₂SO_4(aq) → MgSO_4(s) + CO_2(g) + H₂O(l)

Step 2: Collect the gas given off and bubble it through limewater

Limewater is calcium hydroxide solution. It turns cloudy white if carbon dioxide is bubbled through it:

calcium hydroxide + carbon dioxide → calcium carbonate + water

Ca(OH)_2(aq) + CO_2(g) → CaCO_3(s) + H₂O(l)

The presence of the white precipitate confirms that carbonate ions were originally present in step 1.

The halogens are the elements in Group 7 of the periodic table. They include chlorine, bromine and iodine. Their ions are called halide ions. You can find out more about these by studying Trends within the periodic table

You can test to see if a solution contains chloride ions, bromide ions or iodide ions using silver nitrate solution. To do this:

1. a few drops of dilute nitric acid are added to the solution
2. a few drops of silver nitrate solution are then added
3. the colour of any precipitate formed is recorded

the table below summarises the colours of each precipitate

Halide ion      Precipitate formed     Colour of precipitate Chloride, Cl– Silver chloride, AgCl     White Bromide, Br– Silver bromide, AgBr     Cream Iodide, I– Silver iodide, AgI     Yellow

You can test to see if a solution contains sulfate ions SO₄^2- using barium chloride solution. To do this:

1. a few drops of dilute hydrochloric acid are added to the solution
2. a few drops of barium chloride solution are then added

The presence of a white precipitate of barium sulfate shows the presence of sulfate ions in the solution.

For example:

barium chloride + sodium solution → barium sulfate + sodium chloride

BaCl_2(aq) + Na₂SO_4(aq) → BaSO_4(s) + 2NaCl_(aq)

Titrations are used to determine the volumes of acid and alkali needed to react together to produce a neutral solution. Titrations are carried out using a piece of apparatus called a burette, along with a suitable indicator. A titration is carried out using a number of steps:

1. A pipette is used to accurately measure a volume of an alkali, often 25 cm³. A pipette filler is used to draw solution into the pipette safely. The alkali is emptied into a conical flask.
2. A few drops of a suitable indicator are then added to the conical flask. This will show a change of colour when the acid and alkali have neutralised one another and the titration is complete.
3. The acid is placed in a burette and the starting volume of acid is read against the scale marked on the burette.
4. The acid from the burette is added to the conical flask, and the flask is swirled to mix its contents. When the acid in the burette has almost run in, it is added one drop at a time. Eventually, a colour change shows that the correct amount has been added to react completely with the alkali in the conical flask.
5. The volume of acid added from the burette is noted. The titration results can then be used to calculate the concentration of the acid or alkali (if the concentration of the other is known).   (Universal indicator is unsuitable for titrations because it has a range of colours. Phenolphthalein is often used instead. It changes from pink in alkali to colourless in acid.)