STUDENT SPACE · TOPIC 3

Water tests and purity

Distinguish detecting water from showing that water is pure.

The essentials

Anhydrous copper(II) sulfate changes from white to blue in the presence of water.
Pure water melts at 0 °C and boils at 100 °C at standard atmospheric pressure.
Tap water contains dissolved substances even when it is safe to drink.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

A liquid boils steadily at 100 °C at standard pressure. Is this better evidence of purity than a water test? Explain.

Need a hint for Question 1?

Ask whether the test shows identity or purity. Write both the starting and final colour.

Check Question 1

Yes. A fixed boiling point matching pure water is evidence of purity; a chemical water test only detects its presence.

State the initial and final colours in the cobalt(II) chloride test.

Need a hint for Question 2?

Compare dry and wet cobalt(II) chloride paper.

Check Question 2

Blue to pink.

Explain why distilled water is preferred for preparing a reagent solution.

Need a hint for Question 3?

Dissolved impurities can interfere with reactions.

Check Question 3

It contains fewer chemical impurities that could interfere with the reaction or result.

Use Lesson 3 in your worksheet for written practice. Your teacher will discuss those answers in class.