A-Level Chemistry: Metal-Aqua Ions

Test-tube reactions of metal-aqua ions are a classic part of Year 13 inorganic chemistry: examiners expect precise colours, correct complex formulae and balanced equations, and they like to ask why the 3+ ions behave differently from the 2+ ions. This material works through that content ion by ion and reagent by reagent.

It starts with acidity. In water, iron(II) and copper(II) form [M(H₂O)₆]²⁺ ions, while aluminium and iron(III) form [M(H₂O)₆]³⁺ ions. You explain why the 3+ ions are more acidic, in terms of their greater charge/size ratio polarising the O–H bonds of the co-ordinated water molecules, and write the hydrolysis equation in which a hexaaqua ion donates a proton to water.

Next come the three bases used in the laboratory. With a few drops of sodium hydroxide each ion gives a neutral hydroxide precipitate: blue for copper(II), green for iron(II) (turning brown as air oxidises it), brown for iron(III) and white for aluminium. You learn why aluminium hydroxide is amphoteric, dissolving in excess alkali to give a colourless solution and in acid to re-form the aqua ion, while iron(III) hydroxide does not. With aqueous ammonia you separate its two roles: in small amounts it acts as a base and gives the same precipitates; in excess it acts as a ligand only with copper, giving the deep blue [Cu(NH₃)₄(H₂O)₂]²⁺ ion. With sodium carbonate the 2+ ions precipitate as carbonates, whereas the more acidic 3+ ions give hydroxide precipitates and bubbles of carbon dioxide, a difference you are asked to explain and to show in balanced equations.

The material also covers ligand substitution by chloride ions: concentrated hydrochloric acid turns blue copper(II) solution yellow-green as [CuCl₄]²⁻ forms, with a change of co-ordination number from 6 to 4 and of shape from octahedral to tetrahedral. Finally you use described test results to identify unknown ions, which is the skill the qualitative-analysis practical builds.

Four formats are offered. The 12-question quiz mixes explanation, observation and equation questions. The flashcards give each ion with each reagent, plus the key terms amphoteric, hydrolysis and ligand substitution. The written work has eight questions to answer by hand, including full equations and an identification task, each with a reference answer. The oral exam lets you practise explaining the observations aloud to an examiner, one question at a time.

The content is based on the A-level chemistry content on reactions of ions in aqueous solution, as set out for example in the AQA A-level specification, which limits it to these four ions; other boards cover the same reactions with some differences in the ions listed. It is revision practice written by Zestly, not an exam board resource.

• Explain why [M(H₂O)₆]³⁺ ions are more acidic than [M(H₂O)₆]²⁺ ions and write hydrolysis equations
• Describe and write equations for the reactions of Fe²⁺, Cu²⁺, Fe³⁺ and Al³⁺ aqua ions with sodium hydroxide, ammonia and sodium carbonate
• Explain the amphoteric behaviour of aluminium hydroxide
• Distinguish ammonia acting as a base from ammonia acting as a ligand
• Describe the ligand substitution of copper(II) by chloride ions, including the change in co-ordination number and shape
• Identify an unknown metal ion from test-tube observations

Practice material written by Zestly, based on the A-level chemistry content on reactions of ions in aqueous solution (for example AQA A-level Chemistry 7405, section 3.2.6, and ligand substitution in section 3.2.5).