Solar Desalination

  • How can water be purified using solar energy?
  • What type of water can be purified? (highly concentrated solutions with lots of salt or just a little bit of salt in water, what about oil in water mixture?)

Recommended Items:

  • TDS (total dissolved solids) meter
  • Two bowls one large the other small.

Concepts used:

  • Solutions, Evaporation (water cycle)


There are two types of mixtures, heterogeneous mixtures and homogenous mixtures. Heterogeneous mixtures are mixtures that have different substances or phases in them. For example oil sitting on top of water: in this case there are two substances separated from one another and this is a heterogeneous mixture. Homogenous mixtures are solutions, which are uniform throughout. For example salt (NaCl) dissolved in water is evenly distributed throughout the mixture, it is a homogenous solution. In a solution you have a solute and a solvent, the solute is the substance in the smaller amount that is dissolved into the solvent, the substance in larger amount.

One way to separate water from a mixture is through evaporation and condensation which is related to the water cycle. When water heats to above its boiling point, it evaporates into a gas, and then when the temperature cools, this gas water condenses into a liquid.   Because different substances such as water and salt have different boiling points it is possible to separate the two by heating salt water to evaporate the water without evaporating (or making a gas vapor of) the salt.

In this experiment, different concentrations (amount of salt) in water are tested to determine what amount can be purified. Use two different sized bowls; put salt water in the large bowl. Then place an empty small bowl in the larger one, and cover the stack with plastic (make sure the plastic only touches the sides of the large bowl).   Place a small pebble in the center of the plastic wrap so that water can drip into the small bowl as it evaporates and recondenses on the surface of the plastic wrap. Allow the bowls to sit out in the sun and periodically measure the water’s TDS with the TDS meter in the small bowl. Be careful not to uncover the bowls or the evaporated water will escape into the air outside the bowl/wrap stack.

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