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Supersaturated Solutions

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How does a hot or a cold pack work?

In the picture above, a thermal pack is applied to the back. Small packs can be used either for heating or cooling, depending on the material used. A heat pack contains a supersaturated solution of material such as sodium acetate. The solution is clear until a small metal trigger is activated. The sodium acetate then crystallizes out of solution and generates heat in the process.

Supersaturated Solutions

Watch a video:

http://www.youtube.com/watch?v=_NsX6EkqmsQ

Some solutes, such as sodium acetate, do not recrystallize easily. Suppose an exactly saturated solution of sodium acetate is prepared at 50°C. As it cools back to room temperature, no crystals appear in the solution, even though the solubility of sodium acetate is lower at room temperature. A supersaturated solution is a solution that contains more than the maximum amount of solute that is capable of being dissolved at a given temperature. The recrystallization of the excess dissolved solute in a supersaturated solution can be initiated by the addition of a tiny crystal of solute, called a seed crystal. The seed crystal provides a nucleation site on which the excess dissolved crystals can begin to grow. Recrystallization from a supersaturated solution is typically very fast.

Summary

  • A supersaturated solution can recrystallize when a seed crystal is added to the solution.

Practice

Read the materials and watch the video at the link below, then answer the questions:

http://commons.wikimedia.org/wiki/File:Handwarmer-crystallisation.ogv

  1. Name a compound that can form supersaturated solutions.
  2. Is the recrystallization process fast or slow?
  3. What else happened when recrystallization took place?

Review

  1. How do we know a solution is supersaturated?
  2. How can we cause recrystallization of a supersaturated solution?
  3. What does the seed crystal do?

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