Water: Chemical Tests (Cambridge O Level Chemistry)

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 hydrated cobalt(II) chloride

CoCl₂ (s) + 6H₂O (l)  CoCl₂.6H₂O (s)

                     blue            pink

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 + water hydrated copper(II) sulfate

CuSO₄ (s) + 5H₂O (l)  CuSO₄.5H₂O (s)

                            white             blue

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