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.
Watch a video:
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.
- A supersaturated solution can recrystallize when a seed crystal is added to the solution.
Read the materials and watch the video at the link below, then answer the questions:
- Name a compound that can form supersaturated solutions.
- Is the recrystallization process fast or slow?
- What else happened when recrystallization took place?
- How do we know a solution is supersaturated?
- How can we cause recrystallization of a supersaturated solution?
- What does the seed crystal do?