DIY Science: Desalination Experiment

Most of the water on earth (96.5%, in fact!) is saltwater, but so many plants and animals – including you and me – rely on freshwater for survival. We need to do our best to protect our sources of freshwater because it is energetically expensive to remove salt from water. See for yourself — give the experiment below a try!

Screen Shot 2020-08-18 at 2.56.04 PM
Click here to watch a video of the experiment in action.

Materials:

Photo 1

  • Bowl
  • Spoon or whisk
  • Large pot
  • Salt
  • Water
  • Spoon
  • Boil safe mug
  • Tinfoil
  • Ice
  • Stovetop

Procedure:

  • Mix salt into a bowl of warm water and stir until it is combined. Water will look cloudy.

Photo 2

  • Pour the saltwater into the pot.
  • Place a boil safe mug into the bottom of a large pot being careful to avoid getting any in the mug.

Photo 3

  • Cover the pot with foil so the edges are sealed – but leave the top slack so it is a little concave.
  • Place a handful of ice cubes in the concave park of the tin foil.

Photo 4

  • Turn on the stove and bring the water to a boil.
  • Let it simmer for 10-15 minutes. Do NOT let the water boil to the point the pot is dry!
  • Turn off the heat and let it sit for 20-30 minutes.
  • Using a paper towel, absorb the melting ice cube water.
  • Carefully remove the tin foil.
  • You will now find water in the mug. The water in the mug will have far less salt than the water in the pot.
  • Here is how you test that water: Grab 4 clear containers, preferably tall skinny containers. Pour the water from the mug into one of the clear container and add red food coloring. Pour water from the pot into another clear container and add blue food coloring.

Photo 5

  • In the third container add 1 inch of blue water and in the fourth container add 1 inch of the red water.
  • Using a pipette (or just carefully and slowly), pour red water onto the blue water. Then pour blue water onto the red water.

Results:

Saltwater (Blue) is denser than freshwater (Red). When you add saltwater to freshwater, they mix. When you add freshwater to saltwater, they stay separated because the less dense freshwater sits on top of the saltwater.

Summary:

Why is it energetically expensive to convert saltwater to freshwater? Think about all the work you put into this experiment. You probably only ended up with somewhere around 5-10ml of freshwater, even after letting it boil for 15 minutes. It takes a lot of energy to turn the water into vapor and then cool that vapor into liquid and capture the liquid in the mug. Imagine that on an industrial scale!

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