Reversible Reactions
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
What is a Reversible Reaction?
A reversible reaction is one where the products can react together to reform the original reactants - the reaction can go in both directions.
A + B ⇌ C + D (shown by the ⇌ symbol, not a single arrow)
| Aspect | Irreversible reaction | Reversible reaction |
|---|---|---|
| Symbol | single arrow → | double arrow ⇌ |
| Direction | only forward | forward and backward |
| Products | cannot reform reactants easily | can reform reactants |
Worked Example: Recognising a Reversible Reaction
- Question: which type of reaction occurs when water reacts with anhydrous cobalt(II) chloride?
- Answer: reversible - CoCl₂(s) + 6H₂O(l) ⇌ CoCl₂·6H₂O(s). Heating the hydrated (pink) form drives off the water and regenerates the anhydrous (blue) form, so the reaction goes in both directions.
Effect of Heat and Water on Hydrated Compounds
The syllabus focuses on two examples: copper(II) sulfate and cobalt(II) chloride. In both, heat drives water OUT (forward), and adding water drives the reaction back (reverse).
| Compound | Anhydrous colour | Hydrated colour | Equation |
|---|---|---|---|
| Copper(II) sulfate | white | blue | CuSO₄(s) + 5H₂O(l) ⇌ CuSO₄·5H₂O(s) |
| Cobalt(II) chloride | blue | pink | CoCl₂(s) + 6H₂O(l) ⇌ CoCl₂·6H₂O(s) |
- Heating the hydrated (coloured) solid drives off the water of crystallisation, leaving the anhydrous solid - an endothermic change
- Adding water to the anhydrous solid reforms the hydrated solid and releases heat - an exothermic change
Worked demonstration - copper(II) sulfate: heat blue CuSO₄·5H₂O crystals → water driven off, white anhydrous CuSO₄ remains (endothermic). Add a few drops of water back to the white powder → it turns blue again and the mixture gets hot (exothermic). This colour change is used as a chemical test for water.
Worked Example: Comparing the Two Salts
- Question: which statement about these salts is correct? (A) anhydrous cobalt(II) chloride turns from white to blue when added to water (B) anhydrous cobalt(II) chloride loses water on gentle heating (C) anhydrous copper(II) sulfate reacts with water to make hydrated copper(II) sulfate (D) anhydrous copper(II) sulfate turns from blue to white when water is added
- Answer: C. Anhydrous copper(II) sulfate (white) reacts with water to form hydrated copper(II) sulfate (blue) - this is used as the water test. Anhydrous cobalt(II) chloride is blue, not white (ruling out A), and it is the hydrated form (not the anhydrous form) that loses water on heating in the reverse sense described in B; D has copper sulfate's colour change backwards.
Both colour changes reverse on heating, showing the reactions are reversible in both directions
A General Example: Ammonium Chloride
NH₄Cl(s) ⇌ NH₃(g) + HCl(g) is a classic test-tube demonstration of reversibility: heating the white solid at the bottom makes it sublime into two colourless gases, which recombine as a white solid ring further up the (cooler) tube.
Common Mistakes
- ❌ Saying anhydrous cobalt(II) chloride is white
- ✓ anhydrous CoCl₂ is blue; it is the hydrated form that is pink. (It is anhydrous copper(II) sulfate that is white.)
- ❌ Thinking any colour change means a new, permanent substance has formed
- ✓ the CuSO₄/CoCl₂ colour changes are reversible - heating and adding water simply convert between the anhydrous and hydrated forms of the same salt
Interactive revision notes, videos and practice questions load below.