Water: Chemical Tests (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

Chemical tests for water

• The presence of water is commonly tested for using anhydrous cobalt(II) chloride or anhydrous copper(II) sulfate

Cobalt(II) chloride

• Anhydrous cobalt(II) chloride, CoCl₂, is blue
• Hydrated cobalt(II) chloride, CoCl₂•6H₂O is pink
• So, anhydrous cobalt(II) chloride can be used to test for water
  • This test is usually done with cobalt chloride paper

anhydrous cobalt(II) chloride + water hydrated cobalt(II) chloride

CoCl₂ (s) + 6H₂O (l)  CoCl₂•6H₂O (s)

• The presence of water causes a colour change from blue to pink 

Cobalt chloride paper changes from blue to pink in the presence of water

Copper(II) sulfate

• Anhydrous copper(II) sulfate, CuSO₄ is white
• Hydrated copper(II) sulfate, CuSO₄•5H₂O, is blue
• So, anhydrous cobalt(II) chloride can be used to test for water

anhydrous copper(II) sulfate + water hydrated copper(II) sulfate

CuSO₄ (s) + 5H₂O (l)  CuSO₄•5H₂O (s)

• The presence of water causes a colour change from white to blue 

Anhydrous copper(II) sulfate changes from white to blue in the presence of water

Testing for purity

• Pure substances boil and melt 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 boiling and melting points as they consist of different substances that melt or boil at different temperatures
• Therefore, boiling and melting point data can 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

What is distilled water?

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