Water: Chemical Tests | Chemistry for Grade 11 (IGCSE) PDF Download

Chemical tests for water

Using cobalt(II) chloride

• Cobalt(II) chloride changes color from blue to pink when water is added. This test is commonly conducted with cobalt chloride paper. The chemical equation for this process involves anhydrous cobalt(II) chloride reacting with water to form hydrated cobalt(II) chloride.
• The equation is:
  anhydrous cobalt(II) chloride + water ⇌ hydrated cobalt(II) chloride
  CoCl₂ (s) + 6H₂O (l) ⇌ CoCl₂.6H₂O (s)
• Test for water using cobalt chloride paper which turns pink in the presence of water:
  

Using Copper(II) Sulfate

• Anhydrous copper(II) sulfate changes from white to blue in the presence of water.
• The chemical equation for this reaction is:
  anhydrous copper(II) sulfate + water ⇋ hydrated copper(II) sulfate
  CuSO₄ (s) + 5H₂O (l) ⇋ CuSO₄.5H₂O (s)
• Test for water using anhydrous copper(II) sulfate which turns blue in the presence of water:
  

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 °C
• Impurities tend to decrease the melting point of water, so impure water will melt at temperatures below 0 °C

Distilled Water

• Distilled water is created by heating water to form a vapor, then condensing it back into a liquid state.
• It is characterized by a very low presence of impurities, making it highly pure.
• In practical chemistry, distilled water is preferred due to its high purity levels.
• Tap water, on the other hand, contains more impurities which could potentially disrupt chemical reactions, hence it is not commonly used in labs.