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A reversible reaction is one where the products can react to reform the original reactants, shown with the ⇌ arrow. Free notes and 10 questions answered.
Understanding Reversible Reactions is part of Reversible reactions and dynamic equilibrium in GCSE AQA Higher Chemistry.
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A reversible reaction is one where the products can react to reform the original reactants, shown with the ⇌ arrow. Free notes and 10 questions answered.
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Use this lesson to stay inside Reversible reactions and dynamic equilibrium while connecting the detail back to the wider Topic 6: The rate and extent of chemical change topic.
Read the core explanations from Understanding Reversible Reactions before testing yourself from memory.
Most reactions you've seen go one way only - like burning magnesium or adding acid to metal. The reactants turn into products and that's it. But some reactions can run backwards too.
Picture heating blue copper sulphate crystals. They turn white and release water vapour. Add water back to the white powder and it turns blue again. The reaction has reversed itself.
We show reversible reactions with a special double arrow: ⇌. This tells you the reaction can go both ways, unlike the single arrow → which shows one-way reactions.
Let's look at some reversible reactions you can actually see happening. The key is understanding what the forward and backward reactions are.
Worked example: Copper Sulfate Hydration - A Visible Reversible Reaction
Blue copper sulphate crystals are heated, then water is added back. Explain what happens using a chemical equation.
Answer: $CuSO_4 \cdot 5H_2O(s) \rightleftharpoons CuSO_4(s) + 5H_2O(g)$
Another common example is the thermal decomposition of ammonium chloride. When heated, it breaks down into ammonia and hydrogen chloride gases. Cool it down and the white solid reforms.
Here's where reversible reactions get interesting. You can control which way they go by changing the conditions. It's like having a switch that determines the direction.
Think about the copper sulphate example. Heat pushes the reaction forward (blue to white). Adding water pushes it backward (white to blue). The reaction responds to what you do to it.
The main conditions that affect reversible reactions are temperature, pressure, and concentration. Change any of these and the reaction will shift to counteract your change.
Worked example: Predicting Direction Changes
For the reaction: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ + heat. Predict what happens when: a) temperature increases b) pressure increases
Answer: a) Higher temperature: less ammonia formed (backward reaction favoured) b) Higher pressure: more ammonia formed (forward reaction favoured)
When a reversible reaction reaches equilibrium, something fascinating happens. Both reactions keep going, but at exactly the same rate. It looks like nothing's happening, but there's constant activity at the molecular level.
Picture a busy train station. People arrive at the same rate they leave. The number of people stays constant, but individuals are constantly moving. That's dynamic equilibrium.
At equilibrium, you can't tell the reaction is still happening by looking at it. The concentrations don't change. But if you could see individual molecules, you'd see them constantly switching between reactants and products.
Reversible reactions aren't just lab curiosities - they're the basis of major industrial processes. The Haber process for making ammonia is probably the most important reversible reaction in the world.
Industrial chemists use Le Chatelier's principle to maximise ammonia production. They use high pressure (favours forward reaction) but moderate temperature (compromise between rate and yield).
Exam precision note: In Understanding Reversible Reactions - Reversible reactions - Reversible reactions and dynamic equilibrium, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
A reversible reaction can move in both directions, and at equilibrium the forward and reverse reactions continue at the same rate. That is why the amounts of reactants and products stay constant even though particles are still reacting. A general equilibrium can be written as A + B ⇌ C + D.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Reversible reactions are the basis of major industrial processes, such as the Haber process for making ammonia. Chemists use Le Chatelier's principle to control their direction, using high pressure and a moderate temperature to maximise ammonia production.
A reversible reaction is a chemical reaction where the products can react together to reform the original reactants, so both forward and backward reactions can occur. Reversible reactions are shown with the double arrow ⇌, written generally as A + B ⇌ C + D.
When ammonium chloride is heated, it breaks down into ammonia and hydrogen chloride gases. When these gases cool, they react to reform the white solid, so the thermal decomposition of ammonium chloride is reversible.
A reversible equation is written with the double arrow ⇌ instead of the single arrow →, showing the reaction can go both ways. For example, CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g), or in general A + B ⇌ C + D.
An irreversible reaction goes one way only, such as burning magnesium, where reactants turn into products and that's it. In a reversible reaction the products can react together to reform the original reactants, shown with the ⇌ symbol.
Heating blue hydrated copper sulfate is a reversible reaction: CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O. Heating drives off water to leave white anhydrous copper sulfate, and adding water turns it blue again. The thermal decomposition of ammonium chloride is another example.
The ⇌ symbol means a reaction is reversible and can go both ways, unlike the single arrow →, which shows a one-way reaction. The forward and backward reactions can both occur under the right conditions.
Changing the conditions, such as temperature, pressure or concentration, controls which direction a reversible reaction goes. In the copper sulfate example, heating pushes the reaction forward (blue to white) and adding water pushes it backward (white to blue).
When blue copper sulphate crystals are heated, they turn white and release water vapour: CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(g). Heating drives off the water, leaving white anhydrous copper sulphate.
White anhydrous copper sulphate turns blue when water is added because the reverse reaction reforms the blue crystals: CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s). The blue colour comes from water molecules attached to copper ions.