Question

When urea is added to water the freezing point decreases. Explain why?​

Answer 1

Answer:

Freezing point depression is one of the colligative properties; it depends on the concentration of the solute, but not on its identity. A quick answer starts with the definition of freezing point; it is the point at which the solid phase and the liquid phase exist in equilibrium. The solid phase is more ordered and thus has lower entropy. The second law says entropy increase is favored by nature. Energy has to be removed from the system (temperature lowered) to allow it to default to the ordered state. When you add a solute to the liquid you increase the entropy (it is more disordered). Thus you have to remove more energy (lower the temperature even more) to restore the ordered state.

Another way of thinking of it is with the vapor pressure. At the freezing point the vapor pressure of the liquid and the solid is equal. Adding solute molecules reduces the vapor pressure of the liquid by reducing its concentration at the surface. The vapor pressure of the solid is not effected, or less effected because the solute is not soluble or less soluble in the solid. The system then has to be brought to a lower temperature to have equal vapor pressures for solid and liquid.