Water: Chemical Tests (Cambridge (CIE) IGCSE Combined Science): Revision Note

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 copper(II) sulfate 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

Purity 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 used in practical chemistry rather than tap water because it contains fewer chemical impurities

• Tap water contains dissolved ions and other impurities

  • These impurities could interfere with chemical reactions and affect results

• Distilled water is made by heating water to form vapour and then condensing it back to a liquid

  • This removes most dissolved impurities