Why does Potassium Nitrate crystallise when cooled and saturated?
Because no more KNO3 will dissolve in a saturated solution and when a small amount of the solvent available evaporates the excess KNO3 must come out of solution; i.e., it crystallizes into a solid.
Potassium nitrate (KNO3) crystallizes when cooled and saturated because of a phenomenon called solubility. Solubility is the ability of a substance, in this case, potassium nitrate, to dissolve in a solvent, such as water, at a given temperature. When a solvent becomes saturated, it means that it has dissolved the maximum amount of solute it can hold at that particular temperature.
Here's an explanation of why potassium nitrate specifically crystallizes when cooled and saturated:
1. Understanding solubility: Potassium nitrate is highly soluble in water, meaning it can easily dissolve in water. This is because water molecules are polar, and potassium nitrate is an ionic compound that readily dissociates into potassium ions (K+) and nitrate ions (NO3-) in water.
2. Temperature effect: As you cool down a saturated solution of potassium nitrate, you lower the energy level of the system, causing the water molecules to move more slowly. This reduced kinetic energy decreases the solvent's ability to keep potassium nitrate ions in solution.
3. Supersaturation: As the temperature decreases, the solubility of potassium nitrate decreases as well. This creates a situation where there is now an excess amount of dissolved potassium nitrate in the water, leading to a supersaturated solution. In other words, the solution contains more solute (potassium nitrate) than it would normally hold at that temperature.
4. Nucleation and crystal formation: At this point, the solution is unstable due to the excess amount of potassium nitrate dissolved. Any disturbance in the solution, such as scratching the container or adding a seed crystal, can trigger the crystallization process. When a crystallization nucleus, like a seed crystal or a dust particle, is introduced, the excess solute molecules start to gather around it and form orderly arrangements, creating solid crystals of potassium nitrate.
In summary, when a saturated solution of potassium nitrate is cooled, the decrease in temperature reduces the solubility of the compound, leading to a supersaturated solution. This supersaturation causes the excess potassium nitrate to crystallize out of the solution, resulting in the formation of solid potassium nitrate crystals.