Reversible reaction
A reversible reaction is one in which the products can react together to reform the reactants, so it can go both forwards and backwards depending on the conditions. In an equation the single arrow is replaced by the double half-arrow . The forward reaction goes left to right; the backward reaction goes right to left. In the two 4SD0 examples, changing a condition — heating versus cooling, or removing versus adding water — decides which direction dominates.
Hydrated copper(II) sulfate
Blue hydrated copper(II) sulfate contains water of crystallisation, . Heating drives the water off as vapour, leaving white anhydrous (forward, endothermic): blue → white. Adding water to the white solid turns it blue again and releases heat (backward, exothermic). Hydrated = contains water of crystallisation; anhydrous = none. This colour change is the chemical test for water — white anhydrous copper(II) sulfate turns blue.
Heating ammonium chloride
Solid ammonium chloride decomposes on heating into two colourless gases, ammonia and hydrogen chloride; on cooling higher up the tube they recombine to white solid ammonium chloride: . The white solid seems to move up the tube — it disappears at the hot base (forward) and reappears near the cool top (backward), clear evidence the reaction is reversible.
Drawn from real examiner reports.
Physical test ≠ chemical test
A boiling point of is a PHYSICAL test and scores zero when a chemical test for water is asked. The chemical test is: add the liquid to white anhydrous copper(II) sulfate — it turns blue; or to blue anhydrous cobalt(II) chloride — it turns pink. Learn which colour change goes with which reagent, and give both the starting and final colour.
Nov24 Q7b
Water present ≠ water pure
Turning anhydrous copper(II) sulfate white → blue only shows water is present — any aqueous solution does the same. It does NOT prove the water is pure. For purity you need a physical measurement: it boils at exactly and freezes at at normal pressure. Keep the two separate: colour change = present; fixed boiling point = pure.
Jun23 Q9biii
Name water vapour, not water
When the copper(II) sulfate is heated, the water leaves as a gas — write water vapour or steam, not just "water". Saying the substance given off is "water" loses the mark. Likewise it is not "evaporated off" or lost by combustion; it is driven off as vapour when the water of crystallisation is removed by heating.
Nov24 Q7ai
Use ⇌, not a single arrow
A reversible reaction must be written with the double half-arrow , not a single arrow . A single arrow says the reaction goes one way only, which loses the mark — the whole point of this topic is that the products can reform the reactants. Use in both required examples (copper(II) sulfate and ammonium chloride).
Hydrated vs anhydrous
Hydrated means the compound contains water of crystallisation (blue copper(II) sulfate, ); anhydrous means it has no water of crystallisation (white ). Heating removes the water (hydrated → anhydrous); adding water reverses it. Do not swap the two terms or their colours.
NH₃ + HCl, not the elements
When ammonium chloride decomposes, the products are the molecular gases ammonia () and hydrogen chloride () — not the elements , and . Balance it as , and remember the gases recombine on cooling to reform the solid.
Include state symbols
Reversible-reaction equations are marked on state symbols: solid gas(es). Put (s) on the solids and (g) on the water vapour, and on ammonia and hydrogen chloride. Dropping the state symbols, or marking the water vapour as (l), loses easy marks.
Name specific colours and gases
Vague statements such as "it changes colour" or "a gas forms" score nothing. Name the specific colour change (blue ↔ white) and the named gases (ammonia, hydrogen chloride, water vapour). Examiners want the exact species and colours, not a generic description.
Pair each direction with an observation
Structure answers as forwards-does-X, backwards-does-Y, each tied to an observation. Copper(II) sulfate: heat → blue turns white, vapour given off; add water → white turns blue and the tube gets hot. Ammonium chloride: heat → solid vanishes and gases form; cool → solid reforms up the tube.
Some chemical reactions can go both ways: the products can react together to reform the reactants. These are reversible reactions, shown by the reversible symbol instead of the single arrow . This topic covers the idea of a reversible reaction and the two required examples: the dehydration of hydrated copper(II) sulfate and the heating of ammonium chloride.
| Term | Mark-scheme definition (two elements) |
|---|---|
| Reversible reaction | A reaction in which the products can react to reform the reactants — it can go both forwards and backwards (shown by ) |
| Hydrated | A substance that contains water of crystallisation (water chemically built into the crystal) |
| Anhydrous | A substance that contains no water of crystallisation |
| Water of crystallisation | Water molecules that form part of the crystal structure of a compound |
| Forward reaction | The reaction that goes left to right as written |
| Backward (reverse) reaction | The reaction that goes right to left as written |
Relative formula masses (Edexcel data sheet, : Cu , Cl ):
Define a reversible reaction.
A student has a colourless liquid and wants to show that it contains water.
Describe a chemical test the student could use, and state the result that would show water is present. Explain why this test does not prove the water is pure.
