Water: Chemical Tests (Cambridge O Level Chemistry)

Revision Note

Test yourself
Alexandra Brennan

Last updated

Chemical Tests for Water

Chemical tests for water

Using cobalt(II) chloride:

  • Cobalt(II) chloride turns blue to pink on the addition of water. This test is usually done using cobalt chloride paper
  • The equation is:

anhydrous cobalt(II) chloride + water rightwards harpoon over leftwards harpoon hydrated cobalt(II) chloride

CoCl2 (s) + 6H2O (l) begin mathsize 14px style rightwards harpoon over leftwards harpoon end style CoCl2.6H2O (s)

                     blue            pink

eFKSRmiO_10-1-1-cocl2-test

Test for water using cobalt chloride paper which turns pink in the presence of water

Using copper(II) sulfate:

  • Anhydrous copper(II) sulfate turns white to blue on the addition of water
  • The equation is:

anhydrous copper(II) sulfate + waterrightwards harpoon over leftwards harpoon hydrated copper(II) sulfate

CuSO4 (s) + 5H2O (l) begin mathsize 14px style rightwards harpoon over leftwards harpoon end style CuSO4.5H2O (s)

                            white             blue

wMZ_a~Ru_10-1-1-cuso4-test

Test for water using anhydrous copper(II) sulfate which turns blue in the presence of water

Examiner Tip

Make sure you learn the symbol equations.

Purity of Water

Testing for purity

  • Pure substances melt and boil at specific and sharp temperatures
  • Water has a boiling point of 100 °C and a melting point of 0 °C
  • Mixtures have a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures
  • Melting and boiling points data can therefore be used to determine the purity of water
  • Impurities tend to increase the boiling point of water, so impure water will boil at temperatures above 100 oC
  • Impurities tend to decrease the melting point of water, so impure water will melt at temperatures below 0 oC

Distilled water

  • Distilled water is water that has been heated to form a vapour, and then condensed back to a liquid
  • It contains very few impurities 
  • Distilled water is used in practical chemistry because of its high purity
  • Tap water contains more impurities which could interfere with chemical reactions so is typically not used

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.