[henry321]

We did an experiment at school where salt was poured over ice to make the overall temperature of the ice/water mixture drop. I was wondering, if you poured salt over ice but left it in an open container in a warm room, would you still get a colder solution? Thanks.

[Donaldson Tan]

I doubt the solution become colder. Solvation of salt (NaCl) is an exothermic process. In fact, adding salt lowers the melting point of ice, hence more ice is formed, thus it seemed as if the temperature of the solution drops.

[Yggdrasil]

I doubt the solution become colder. Solvation of salt (NaCl) is an exothermic process. In fact, adding salt lowers the melting point of ice, hence more ice is formed, thus it seemed as if the temperature of the solution drops.

Salt lowers the melting point of ice because it increases the entropy of the aqueous phase so the (Delta)S from aqueous to solid becomes higher.  Since for ice melting at its melting point (Delta)G = 0 and therefore, T = - (Delta)H / (Delta)S, a larger (Delta)S results in a smaller T.

Therefore, since the temperature of an ice-water mixture at thermal equilibrium will be the melting temperature of ice, adding salt to an ice-water mixture will lower the temperature of an ice-water mixture.  So therefore, in the sittuation henry321 describes, the temperature of the ice-water mixture which forms when ice melts will be colder than an ice-water mixture without salt.

[henry321]

will the thermal equilibrium temperature be the melting point of the ice even if the ice/water mixture is exposed and is in a hot room?

[Donaldson Tan]

What talking you?

The melting point of ice is pressure dependent. Even if you expose the ice-water mixture to heat, the melting point remains at 273K as long as the pressure stays at 1atm.

[Yggdrasil]

will the thermal equilibrium temperature be the melting point of the ice even if the ice/water mixture is exposed and is in a hot room?

Ideally you would have the ice/water mixture in an insulated container, but even in a non-insulated container, the temperature of the ice/salt water mixture should "stabilize" at a temperature lower than 0^oC (273K).  However, unlike the temperature v. time curve of pure water/pure ice system, the temperature will not completely level off during the melting.  Mixtures melt over a range of temperatures (see the figure in http://en.wikipedia.org/wiki/Solid_solution , and note that at compositions other than 100% a and 100% b, the liquid + solid phase exists over a range of temperatures).