When I add salt to water, will its temperature drop? Or only when adding it to ice?

Question

I know that adding a solvent lowers the freezing point. But the temp will only lower adding it to ice as the ice will get to a lower temp before melting. Adding salt to water will do nothing to the temp if it is left at the temp it was the salt was put in at as it has already melted and cannot be lowered. Is this correct?

Also, salt does not need to dissolve does it? Only needs to be added for it to begin cooling?

10 points for best answer. Thanks in advance!

Answer 1

no,the only reason ice water gets colder when you add salt is that you have lowered the temperature that water can freeze,so instead of 32 d ice you have 32 d water. yes the salt needs to be in solution to do its work............tom

Answer 2

You are right that adding salt to ice lowers its temperature because it causes it to melt.  This phase change requires energy and it takes it from the available thermal energy, making an endothermic reaction, and the temperature change is predicted by how much energy is required to melt how much ice.  By the way, some of the salt must dissolve in the water for it to cause the ice to melt.   It is the intermolecular forces that cause the melting to occur, and these only happen when the chemicals mix on a molecular scale, i.e. dissolve.  

OK, now putting salt in water does not cause a phase change in the water, but it does cause a phase change in the solid salt going from solid to ionized solute.  Different salts have different energies associated with being dissolved in water versus being crystallized alone.  Unfortunately the answer is not simple, but adding table salt (sodium chloride) to water is slightly exothermic (releases heat) because there is some energy stored in the salt crystals and breaking the bond and associating with water molecules brings the salt ions to a lower energy state, releasing thermal energy.  Only a small amount though, as the water only rises a degree or so and is not readily noticeable unless you use a good thermometer (try it at home).  Other salts, however, have the opposite effect.  Take ammonium nitrate for example.  When you dissolve ammonium nitrate salt crystals in water it is endothermic and becomes very cold.  This is what they use in instant cold packs that you can buy in the pharmacy.  So it really depends on the type of salt and how stable its crystals are and how its ions associate with water molecules.  Not nearly as predictable as melting ice with salt.