Question Video: Explaining Why Heating a Solution Induces Crystallization | Nagwa Question Video: Explaining Why Heating a Solution Induces Crystallization | Nagwa

Question Video: Explaining Why Heating a Solution Induces Crystallization Chemistry • First Year of Secondary School

Why does heating a solution, and evaporating some of the solvent, induce crystallization?

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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.

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