Testing for water: identify the colour change
Use two anhydrous reagents to identify water without confusing presence, purity and safe drinking.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Distinguish anhydrous from hydrated substances.
- Read a starting colour and final colour as an observation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does anhydrous mean here?
What you will learn
- Describe the colour changes of anhydrous cobalt(II) chloride and copper(II) sulfate with water.
- Explain what a positive water test establishes and what it leaves unknown.
Name the starting reagent and both colours
Anhydrous cobalt(II) chloride is blue and turns pink when hydrated by water. Anhydrous copper(II) sulfate is white and turns blue when hydrated by water. Start with the anhydrous reagent in the stated test; an already hydrated, coloured sample does not supply the same clear starting comparison.
State both the reagent and the direction of change. Saying it turns blue is ambiguous unless the starting substance is named: the copper(II) sulfate test goes white to blue, whereas cobalt(II) chloride goes blue to pink. These are observations from a supplied or supervised test, rather than an instruction to handle cobalt salts or unknown liquids outside a laboratory.
Presence is one question, purity is another
A positive test supports the presence of water in the stated sample and conditions. A solution containing dissolved salt can still contain water and give a positive result. Therefore the result does not show that the entire sample is pure water, nor does it identify every dissolved substance.
Melting-point and boiling-point comparisons address a different school-level purity question in the next lesson. Chemical and microbial drinking-water checks require appropriate assessment; the two colour tests alone do not establish safety. Keep the conclusion proportional to the observation: water present rather than no other substances present.
Sources for this lesson
Pause and explain
A salt solution gives the stated positive water test. What follows?
Worked example
A supplied liquid turns white anhydrous copper(II) sulfate blue. State the supported conclusion and whether dissolved salt is excluded.
Show the worked solution
- The specified change matches hydration of the anhydrous reagent.
- It supports the presence of water in the liquid.
- A salt solution can give the same result, so dissolved salt is not excluded.
Answer Water is present; purity and absence of dissolved salt are not established.
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 a positive water test with anhydrous cobalt(II) chloride and contrast the copper(II) sulfate test.
Give me a hint
Keep each starting colour attached to its reagent.
Compare my reasoning
- Cobalt(II) chloride starts blue and changes to pink.
- Copper(II) sulfate starts white and changes to blue.
- Both changes support water present under the stated suitable test conditions.
CoCl₂ blue → pink; anhydrous CuSO₄ white → blue; both indicate water present.
Look for these in your work
- Both reagent identities appear.
- Starting and final colours are in the correct order.
A report gives copper(II) sulfate as blue to white when water is added. Correct the water-test description and state its conclusion.
Give me a hint
Adding water hydrates the initially anhydrous reagent.
Compare my reasoning
- The anhydrous copper(II) sulfate starting sample is white.
- Hydration by water turns it blue, not the reverse.
- This supports water present, without showing all impurities absent.
Correct to white anhydrous CuSO₄ turning blue; water present does not establish purity.
Look for these in your work
- The reversed observation is corrected.
- The conclusion is limited to the test's purpose.
Both distilled water and a supplied salt solution produce the positive colour test. A student calls them equally pure. Explain the error and name another school-level property comparison needed.
Give me a hint
The same detection result can occur in different mixtures.
Compare my reasoning
- Both samples can contain water and hydrate the reagent.
- Dissolved salt means one need not be pure even though the test is positive.
- Compare melting and boiling behaviour under stated pressure for the school purity question, while recognising this is not a full safety assessment.
The tests detect water in both; they do not equate purity. Compare melting/boiling behaviour under stated conditions.
Look for these in your work
- Presence and composition are separated.
- A relevant additional property comparison is named.
Common mistakes
- Giving cobalt(II) chloride the copper(II) sulfate colour sequence.
- Omitting the anhydrous starting reagent.
- Treating water present as proof of pure water.
- Calling a colour test a drinking-water safety certificate.
What can you explain now?
Give the two required anhydrous-reagent colour changes for water.
Compare with the explanation
Cobalt(II) chloride: blue to pink. Copper(II) sulfate: white to blue.
The starting reagents and both colour directions distinguish the tests.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall each anhydrous reagent and its two colours, then explain why a positive test cannot establish pure or safe drinking water.
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.