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Salinity and the freezing point of water

You will need:

  • 2 balloons
  • Salt, funnel and a measuring spoon
  • 75mL of water per balloon
  • A freezer
  • A stopwatch

    Copyright

Instructions


1. Place a funnel into the mouth of the balloon. Add at least 1 tablespoon of salt into the balloon.

2. Carefully measure 75mL of water into balloon, tie the balloon and shake to dissolve the salt.

3. Add 75mL of water to the other balloon and tie it.

4. Place both balloons within the freezer and start the timer. Check the balloons on hourly intervals.
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Why Does This Happen?

Adding salt to water lowers it's freezing point below 00 Celsius. As ice forms in polar regions, the salt within the forming ice is rejected into the surrounding water. This causes water below the ice to exist at sub zero temperatures.

Also, water expands when it freezes - try measuring the balloons diameter with vernier callipers before freezing and afterwards once it has frozen > better than exploding cans in the freezer! Expanding causes the ice to be less dense than the surrounding liquid water, thereby the ice floats.

People use salt to melt ice on roads, an important safety concern for motorists.
 
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Reference: Brady, J. E. & Holum, J. R. (1993). Chemistry. The Study of Matter and Its Changes. John Wiley & Sons, New York
 
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