Question

The reason behind heating then cooling KNO₃, SiO₂, and CuSO₄5H₂O is primarily for:

A) Increasing their density
B) Achieving a particular color
C) Altering their chemical properties
D) Facilitating dissolution

Answer 1

Heating then cooling substances like KNO₃, SiO₂, and CuSO₄⋅5H₂O primarily facilitates their dissolution by changing solubility with temperature, allowing the creation of supersaturated solutions.

Step-by-step explanation:

The primary reason behind heating then cooling substances such as KNO₃, SiO₂, and CuSO₄⋅5H₂O is to facilitate their dissolution. Temperature changes the solubility of compounds; by heating a solvent, the molecules within it gain more kinetic energy, collide with the solute more frequently and with greater force, consequently increasing the rate of dissolution. Upon cooling a saturated solution, certain compounds can remain dissolved, creating a supersaturated solution which is more concentrated than would be possible at equilibrium solubility at the cool temperature.

This concept is widely applied in practices such as fractional crystallization to separate compounds based on their solubility and in the creation of hand warmers that work by exploiting the temperature dependence of solubility. Additionally, this method impacts physical properties such as color. For instance, the blue hydrated form of CuSO₄ becomes white when dehydrated after heating, due to the loss of waters of crystallization.