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