Video Transcript
Why does heating a solution, and evaporating some of the solvent, induce
crystallization?
When answering this question, it will be helpful to imagine what is occurring within
a solution using visuals. So, let us start with a drawing and a discussion about the two parts of a solution,
the solute and solvent. The solute is the chemical in the solution that is being dissolved. For example, if we were to make a salt water solution, we would first start with the
sodium chloride crystals as the solute. The other component of a solution is the solvent, which is the chemical that
dissolves the solute to form a solution.
In this example, water is our solvent. When mixed, the solvent, which is water, will dissolve the solute, or salt
crystals. When the solution contains less than the maximum amount of solute dissolved in a
solvent at a given temperature, an unsaturated solution occurs, whereas a saturated
solution occurs when the solution contains the maximum amount of solute dissolved in
a solvent at a given temperature.
So, for this example, we will begin with a saturated or an unsaturated salt
solution. If we were to take this solution and heat it, some of the water would evaporate. The salt solution would still have the same amount of solute, which is sodium
chloride, but less of the solvent, which in this case is water. Since there is less solvent, the concentration would increase. At this point, since the solution is still hot, we may only see a few small crystals
form. This is because the majority of solutions can dissolve more solute at a higher
temperature. This fact explains why only a few small crystals will form while the solution is
still hot.
Now, let us see what happens to the salt solution when heat is removed and the
solution is allowed to cool down. As the solution cools down, more salt crystals will form as the excess solute
precipitates out. This is because the solution can no longer hold all of the solute. We call this a supersaturated solution. A supersaturated solution is a solution that contains more than the maximum amount of
solute than should be able to be dissolved in a solvent at a given temperature. This means that some dissolved solute turns into solid crystals as the temperature
and the capacity of the solvent to hold solute decreases.
When reflecting back on the question “Why does heating a solution, and evaporating
some of the solvent, induce crystallization?,” the correct answer is the solution
becomes supersaturated and, upon cooling, causes crystals to form.
