Testing water purity

The purity of water can be tested by evaporating it (to dryness) on an evaporating dish or by measuring its boiling point.

Evaporation

If dissolved salts are present, then evaporating the water will leave them behind. The purer the water, the less solid residue will be left. Pure water will leave no residue.

Testing its boiling point

Pure substances have precise melting and boiling points. Pure water will melt at 0°C and boil at 100°C. The water vapour will also condense at 100°C. If impurities are present:

  • the melting point goes down
  • the boiling point goes up

The greater the amount of dissolved substances present, the more the melting point or boiling point changes from the temperature expected for pure water.