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Water Salinity and Density

In this experiment, discover how the density of salt water can create interesting effects in nature.

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Evan is back with another impressive water experiment that you can try at home to learn about water salinity and density.

Fresh water is less dense than salt water, so it can sit on top. In undistributed waters, they can remain as separate bodies for a long time! This is referred to as aΒ salt wedge.Β Did you know there is a pretty amazing salt wedge in the swan river? The wedge of salt water can move up to 55km upstream of the Swan River.

What you’ll need:

  • 2 plastic trays
  • A plastic clipboard (or a thin plastic chopping board)
  • 5 litres of water
  • 1 egg
  • Table salt
  • Measuring spoons
  • Mixing spoon
  • 2 different colours of food colouring
  • 5 jars with a flat rim (big enough to hold about a litre of water each)

Instructions

  1. Fill each jar with water. Place your egg in the first jar and watch how it sinks to the bottom. This is because the egg is more dense than the water around it.
  2. In the same jar, start adding large scoops of salt into the water. You may need to stir the salt around to ensure it dissolves into the water. Keep adding salt until the egg floats to the top. When this happens, the water is now more dense than the egg.
  3. Next, let’s see what happens when fresh water and salt water meet. Grab two more jars of fresh water, and place one of the jars onto one of your plastic trays. Add a few drops of one colour of food colouring to the first jar, then add a different colour to the second jar.
  4. To the first jar on your plastic tray, add a spoonful of salt and stir. Place the plastic board on top of the second jar, and hold tight to flip the jar over. With the board still in place underneath the second jar, place it on top of the first jar and ensure the openings of both jars are aligned. Carefully remove the board from between the jars.
  5. Notice how the colours contained within each jar of water don’t mix: this is because the “fresh” water in in the second jar is less dense than the salty water in the first.

In nature, this effect is known as a “halocline”. When a river of freshwater meets the ocean, the fresh water flows on top of the salty water, until forces like wind and waves mix them together.

  1. Let’s see what happens when we put fresh water on the bottom and salt water on top. Repeat Steps 3-4, this time placing the jar of salt water on top. Notice how the denser, salty water rushes to the bottom of the other jar and the colours mix together.

 

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