Reversible reactions and hydrated salts
Use changes in conditions to explain the opposite directions of the required hydration examples.
Section C6.3 · Reversible reactions and equilibrium
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish physical and chemical changes without using reversibility as the only rule.
- Recognise that some crystals contain chemically combined water.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does ⇌ indicate in a chemical equation?
What you will learn
- Use ⇌ to show that a reaction can proceed in both directions.
- Describe heating and adding water for hydrated copper(II) sulfate and cobalt(II) chloride.
A reversible reaction can form reactants again
A reversible chemical reaction can proceed forwards to form products and backwards to form the original reactants under appropriate conditions. Use the symbol ⇌ to show the two directions. It does not mean every mixture has equal amounts of the substances or that the reaction is necessarily at equilibrium; those are separate questions considered in the Extended lesson.
Conditions such as heating or adding water can favour different directions in the school hydration examples. Reversing a chemical reaction still involves changing substances. Therefore the ability to reverse an observed change is not enough to classify it as physical.
Track the colour and the water together
Hydrated copper(II) sulfate is blue. On heating in the stated classroom model it loses its combined water and becomes white anhydrous copper(II) sulfate. Adding water to the anhydrous form restores the blue hydrated form. The model can be shown as hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water, with heating favouring water removal and water addition favouring hydration.
Hydrated cobalt(II) chloride is pink, whereas its anhydrous form is blue. Heating the hydrated form gives the blue anhydrous form, and adding water restores the pink hydrated form. Do not swap the two salts' colour pairs: copper sulfate is blue/white, cobalt chloride is pink/blue. These are recognition and interpretation examples for supervised teaching, not a practical instruction to heat these substances at home.
Sources for this lesson
Pause and explain
Which colour pair belongs to copper(II) sulfate in the stated model?
Worked example
A blue hydrated copper(II) sulfate sample becomes white after heating. Water is then added and the sample becomes blue. Explain the two directions and represent them with ⇌.
Show the worked solution
- Heating removes the chemically combined water and gives white anhydrous copper(II) sulfate.
- Adding water restores the blue hydrated substance.
- Write hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water; the two condition changes favour opposite directions.
Answer Heating: blue to white by water loss; adding water: white to blue by hydration.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the changes when hydrated copper(II) sulfate is heated and water is later added to its anhydrous form.
Give me a hint
Pair each direction with the water change and its colour evidence.
Compare my reasoning
- Heating removes combined water: blue hydrated salt becomes white anhydrous salt.
- Water addition reverses the hydration change: white becomes blue.
- Use ⇌ to represent the opposing chemical directions under the appropriate conditions.
Heat: blue to white and water loss. Add water: white to blue and hydration.
Look for these in your work
- Both colour observations are correct.
- Each condition is linked with the matching water change.
Give the hydrated and anhydrous colours of cobalt(II) chloride and predict the effect of heating its hydrated form.
Give me a hint
Keep cobalt chloride separate from the copper sulfate example.
Compare my reasoning
- Hydrated cobalt(II) chloride is pink.
- The anhydrous form is blue.
- Heating the hydrated form removes combined water and gives a pink-to-blue change in the stated model.
Hydrated pink; anhydrous blue; heating hydrated salt changes pink to blue.
Look for these in your work
- The correct salt is identified.
- The predicted direction matches loss of hydration water.
A report says the blue-to-white copper sulfate reaction cannot be chemical because water addition reverses it. Correct the classification rule and identify the evidence for reversibility.
Give me a hint
Chemical changes can be reversible; track the hydrated and anhydrous substances.
Compare my reasoning
- Hydrated and anhydrous copper sulfate differ in their chemically combined water.
- Heating and water addition produce opposing changes between those forms.
- The restoration supports reversibility, but reversibility does not make a chemical reaction physical.
It is a reversible chemical hydration/dehydration change; reversibility alone is not the physical-change definition.
Look for these in your work
- The flawed classification rule is replaced.
- The opposing conditions are used as evidence.
Common mistakes
- Treating ⇌ as proof of equal quantities or equilibrium.
- Calling every reversible chemical reaction a physical change.
- Confusing copper sulfate's blue/white pair with cobalt chloride's pink/blue pair.
What can you explain now?
What colour change is expected when water is added to anhydrous cobalt(II) chloride in the stated hydration model?
Compare with the explanation
Blue to pink
The blue anhydrous form becomes pink on hydration; copper sulfate has the different white-to-blue change.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Write both salts' colour pairs from memory and predict heating and water-addition changes. Explain what ⇌ establishes and what it does not establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.