C3-5 : Chemical Analysis
Identifying Group 1 and Group 2 metals is a piece of cake when we burn them, as they tend to have unique flames which we can associate with the elements. We call these tests flame tests. We perform a flame test by putting a small amount of a compound to be tested in a platinum wire loop which has been dipped in hydrochloric acid, and then we hold the substance over a blue Bunsen flame. The flame should show a particular colour which can be used to identify the unknown substance.
- lithium will burn a bright red flame
- sodium will burn a golden yellow flame
- potassium will burn a lilac flame
- calcium will burn a brick red flame
- barium will burn a green flame
Testing for Positive Ions
Another test for unknown substances is to test the reactions with sodium hydroxide solution. Aluminium, calcium and magnesium ions all form a white precipitate when they react with sodium hydroxide (NaOH). Hence adding it, if a white precipitate forms, we know its one of those three. To find out which one, we can add more and more NaOH – because eventually aluminium ions dissolve in it. If it dissolves, it’s aluminium, otherwise it’s either calcium or magnesium. To find out which of those two it is, we can use their flame tests – calcium burns with a brick red flame, whereas magnesium produces no special flame.
Other metal ions produce coloured precipitates when they have NaOH added to them. Adding sodium hydroxide solution to:
- a substance with copper(II) ions produces a light blue precipitate
- a substance with iron(II) ions produces a “dirty” green precipitate
- a substance with iron(III) ions produces a red-brown precipitate
As well as this, NaOH can be used to detect if ammonium ions ( NH4+ ) are present in an unknown substance. Ammonium ions react with NaOH to form ammonia and water:
NH4+ (aq) + OH- (aq) NH3 (aq) + H2O (l)
To test for ammonium ions, we add NaOH to a solution of an unknown substance. If ammonium ions are present, ammonia (as well as water) forms. When we warm the solution, ammonia is then given off as a gas. We can detect ammonia gas using damp red litmus which should turn blue as ammonia is an alkaline gas.
Testing for Negative Ions
There are a number of different tests we can use to detect negative ions. Each type of negative ion has its own test…
Testing for carbonates: If we add a dilute acid (e.g. hydrochloric) to a carbonate, it fizzes and produces carbon dioxide gas. We can test for carbon dioxide gas using limewater, and if it fizzes and produces the gas, we know it’s a carbonate. There are two particular metal carbonates which are giveaways, however, which makes it slightly easier to detect them. For example, copper carbonate is a green substance which when heated decomposes to give …