Question

According to Joule’s law, when evaporation occurs, cooling occurs.
(i.e. latent heat is released). We have two containers with pure water, both at the same
room temperature. While using our IR camera, we add NaCl to one of the and observe
that the salty container gradually looks colder. If we know that salty
water is less prone to evaporation, hence less cooling [i.e. due to lowering the vapor
pressure - the same reason we add salt on icy roads] then, can you think what really
causes the cooling when NaCl is added?

Answer 1

The ice and water remain at equilibrium in the absence of salt.The state of matter changes when we add or remove heat from the ice.The freezing point of water gets further reduced when we add salt.Hence breaking the equilibrium as a result melting of ice starts.Heat is required to melt ice which is generally provided from outside in the form of latent heat of fusion which reduces temperature.Cooling occurs when we add salt to the ice.Reduction in temperature of salt takes place. Ice and water attain equilibrium which generally is attained below zero degree celcius.Adding more salt results in melting more ice and further temperature decreases.Due to redistribution of heat taking place among the latent heat of fusion of ice and specific heat of water cooling takes place.