Identifying Substances
Most of this lesson covers tests that only appear on the separate GCSE Chemistry course. For Combined Science you need the gas tests for hydrogen, oxygen, carbon dioxide and chlorine. The other tests are useful background if you go on to study Chemistry further.
Why We Need Chemical Tests
Chemists often need to identify unknown substances. This might be to check whether a water supply is safe, to identify minerals in rocks, to verify the purity of a manufactured product, or to analyse an unknown sample in a forensic investigation. For Combined Science you only need the four gas tests. These tests fall into two broad categories: traditional wet chemistry tests that rely on visible observations, and modern instrumental methods that use machines for faster, more accurate results.
Flame Tests
A flame test identifies metal ions by the colour they produce when heated in a Bunsen burner flame. Different metal ions emit light of different colours because their electrons absorb energy and then release it as visible light at characteristic wavelengths. To carry out a flame test: dip a clean nichrome wire loop in concentrated hydrochloric acid (to remove previous residues), then dip it into the sample, and hold it in a blue Bunsen flame. The wire must be cleaned thoroughly between tests to avoid contamination from previous samples, which would give misleading results.
Flame test colours: Lithium — crimson red. Sodium — yellow. Potassium — lilac. Calcium — orange-red. Copper — green / blue-green.
Tests for Gases
You must be able to test for four gases. Each test produces a unique, easily identifiable result:
Hydrogen (\(H_2\)) — hold a burning splint near the mouth of the test tube; a squeaky pop confirms hydrogen. The pop is caused by the hydrogen igniting and reacting with oxygen in the air: \(2H_2 + O_2 \rightarrow 2H_2O\). Oxygen (\(O_2\)) — insert a glowing splint into the gas; it relights because oxygen supports combustion. Carbon dioxide (\(CO_2\)) — bubble the gas through limewater (calcium hydroxide solution); it turns milky or cloudy because insoluble calcium carbonate is formed: \(Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O\). Chlorine (\(Cl_2\)) — hold damp litmus paper near the gas; it is first turned red (because chlorine is acidic) and then bleached white because chlorine is a powerful bleaching agent.
Testing for Metal Ions Using Sodium Hydroxide
Adding sodium hydroxide solution (NaOH) to solutions containing metal ions produces coloured precipitates. The precipitate forms because the metal ions react with hydroxide ions to produce an insoluble metal hydroxide. The colour of the precipitate identifies the metal ion present. These tests are on the separate Chemistry course, not on Combined Science.
Calcium ions (\(Ca^{2+}\)) — white precipitate of calcium hydroxide. Copper(II) ions (\(Cu^{2+}\)) — blue precipitate of copper hydroxide. Iron(II) ions (\(Fe^{2+}\)) — green precipitate of iron(II) hydroxide; the equation is \(Fe^{2+} + 2OH^- \rightarrow Fe(OH)_2\). Iron(III) ions (\(Fe^{3+}\)) — brown precipitate of iron(III) hydroxide; the equation is \(Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3\). Aluminium ions (\(Al^{3+}\)) — white precipitate that dissolves in excess NaOH to form a colourless solution (aluminium hydroxide is amphoteric). Magnesium ions (\(Mg^{2+}\)) — white precipitate that does not dissolve in excess NaOH. This difference between aluminium and magnesium is important: both give white precipitates, but only aluminium’s dissolves in excess.
Testing for Negative Ions
You can test for certain negative ions (anions) using specific reagents:
Carbonate ions (\(CO_3^{2-}\)) — add dilute acid (such as hydrochloric acid) to the sample. If carbonates are present, the acid reacts to produce carbon dioxide gas, which can be tested by bubbling it through limewater — it turns milky if \(CO_2\) is present. Sulfate ions (\(SO_4^{2-}\)) — first add dilute hydrochloric acid to remove any carbonate ions that might interfere, then add barium chloride solution. A white precipitate of barium sulfate (\(BaSO_4\)) confirms sulfate ions are present. Halide ions — first add dilute nitric acid (to remove other ions that might interfere), then add silver nitrate solution. Chloride ions (\(Cl^-\)) give a white precipitate of silver chloride, bromide ions (\(Br^-\)) give a cream precipitate of silver bromide, and iodide ions (\(I^-\)) give a yellow precipitate of silver iodide.
Halide tests with silver nitrate: chloride \(\rightarrow\) white precipitate, bromide \(\rightarrow\) cream precipitate, iodide \(\rightarrow\) yellow precipitate. Always acidify with dilute nitric acid first to prevent interference from other ions.
When identifying an unknown ionic compound, work systematically. First dissolve the sample in water. Use a flame test to identify the metal ion (cation). Then use precipitate tests with NaOH to confirm the metal ion. Next, test for the negative ion (anion) using the appropriate reagent — silver nitrate for halides, barium chloride for sulfates, or dilute acid for carbonates. Record all observations carefully and compare with known results.
Instrumental Methods
Instrumental methods of analysis are faster, more sensitive and more accurate than traditional chemical tests. Gas chromatography separates the components of a mixture by passing it through a column; each component travels at a different rate, producing separate peaks on a chromatogram. The number of peaks tells you how many components are in the mixture, and the retention time identifies each substance. Mass spectrometry gives the relative molecular mass of a substance by measuring the mass-to-charge ratio of its ions. Gas chromatography linked with mass spectrometry (GC-MS) is particularly powerful because it both separates and identifies components in a single analysis. These methods are used in forensic science, environmental monitoring, drug testing and the pharmaceutical industry.
Instrumental methods can detect very small quantities of a substance (high sensitivity), making them invaluable when only a trace amount is available. They are fast and can be automated for processing many samples. They give accurate, quantitative results rather than just qualitative observations. However, the equipment is expensive to purchase and maintain, and requires trained operators to interpret the data correctly. Despite these costs, instrumental analysis has largely replaced traditional tests in professional laboratories.