Chemical Tests for Water

• The presence of water is commonly tested for using anhydrous cobalt(II) chloride or anhydrous copper(II) sulfate
• Anhydrous cobalt(II) chloride, CoCl2, is blue
• Hydrated cobalt(II) chloride, CoCl26H2O is pink

  • 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)
• 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 with cobalt chloride paper

  A) cobalt chloride

  B) water

  
• Anhydrous copper(II) sulfate, CuSO4 is white
• Hydrated copper(II) sulfate, CuSO45H2O, is blue
• 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 with anhydrous copper (II) sulfate

  A) pipette

  B) water

  C) anhydrous

  D) hydrated

  
• 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 C
• Impurities tend to decrease the melting point of water

  • So, impure water will melt at temperatures below 0 C
• 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 typically used because it contains more impurities which could interfere with chemical reactions