Salty Sink or Float


Salty Sink or Float

By Christopher Earnhardt, Science Educator

salty sink or swim experiment

If you’ve ever gotten an accidental mouthful of ocean water, you know that our flippered friends live in some pretty salty stuff! Scientists call the measurement of how much salt is dissolved into a given volume of water its salinity. Very salty bodies of water, like the ocean, have a high salinity while lakes, rivers and streams have extremely low to zero salinity. But just what is the difference between the super-salty oceans and the not-so-salty lakes and rivers? What do those differences mean for the creatures who live there? With this simple science experiment, you can find out!


  • Water
  • Table Salt
  • 3 Drinking Glasses
  • Food Coloring
  • Stirring Spoon


Grab your three (3) drinking glasses and set one aside. Pour an equal amount of room temperature water into the other two glasses, enough that they won’t overflow when combined in the third glass. Add ½ tsp of salt to one glass and stir until it is completely dissolved. To tell the two glasses of water apart, pick two colors of food coloring and add a few drops to both glasses. Now, form a hypothesis: What will happen when you combine the water into one glass? Pour the glasses one at a time, and do not stir.

The Science

So… what happened? Even though both glasses of water were poured into the third glass, they didn’t mix! This isn’t some magic trick… it’s science! Water with high salinity is denser than water with low salinity which causes it to sink to the bottom with the plain water resting on top. If you then stir the mixture, the water from each cup will have the same salinity, and the colors will blend.

Further Exploration

  • Can you think of places in the world where salt water and fresh water may mix together? What kind of ecosystem does that create?
  • Some animals can only survive in salt water while others need fresh. List animals that can exist in either or both!

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