Why does ammonium chloride precipitate?
Ammonium chloride does not dissolve in water infinitely, but only until it reaches a certain limit called saturation point. At room temperature (around 25 °C or 77 °F), we can dissolve about half of the NH4Cl vial contents. However, by heating up the solution we “force” all the ammonium chloride to dissolve.
As we let the vial cool down gradually, process reverses, and excessive NH4Cl precipitates. And because of gradual temperature change, ammonium chloride manages to form beautiful spiky snowflakes (not the powder it was initially).
Interestingly, similar process occurs when real snowflakes form: at high altitudes air cools down gradually, and when it reaches 0 °C [32 °F] water vapors gather together into snowflakes. It is not by chance that snow is also called “precipitation”!
How does water temperature influence solubility of compounds?
It is important to divide the answer into two parts.
On the one hand, increasing the temperature helps us dissolve compounds faster. Indeed, we can tell that from our everyday experience. It is not a problem to dissolve six teaspoons of sugar in a cup of hot tea (tea is normally brewed at 85-95 °C or 185-203 °F). However, in order to dissolve the same amount of sugar in water at room temperature, you’ll need all your scientific insistence: to agitate water with a spoon for a very long time. In fact, when we increase water temperature, particles of a compound start to move faster, which makes dissolving process easier.
On the other hand, we may reach the compound saturation limit at room temperature. For example, we can dissolve only 35.9 gram of cooking salt (NaCl) per 100 mL of water at room temperature. Hence, no matter what, we cannot dissolve more salt at that temperature. As a rule, solubility increases as we rise temperature. Therefore, we can dissolve more salt in boiling water (3.5 gram more). Same happens in our experiment with ammonium chloride. However, its solubility increases more significantly, from 38.3 grams to 74.1 grams per 100 mL of water (almost twofold increase!)
Turns out, there are some compounds which solubility decreases with raise of water temperature, for instance, lithium carbonate Li2CO3.
Why it is ammonium chloride we need for the experiment?
As noted earlier, ammonium chloride solubility is twice as much as in cold. This property allows us to create beautiful snowfall in a vial. Moreover, each compound would form crystals of different, unique shape. Finding the right compound that features snowflake appearance crystals is true challenge.
For instance, sodium chloride solubility in hot water is only 10% higher than in cold. Such difference is insufficient for conducting a spectacular experiment.