Viewing a single comment thread. View all comments

common_sensei t1_j6v33a6 wrote

Reply to comment by parrotwouldntvoom in What are the effects of adding rock salt to a cooler full of ice? by Ok_Kareem_7223

The phase change is endothermic, so ice near zero degrees will cool the surrounding ice down as it melts into colder water. You're right that the total energy won't change just by adding salt, but you will reduce thermal energy in the system to gain that potential energy in the liquid.

Your second point is dead on though, if anything, it should warm up faster because there's more temperature differential now.

18

Ihaveamodel3 t1_j6wkxp5 wrote

The rate of change will be faster, but will the total time to “warm” be faster? Since the colder one has a larger way to go to “warm”?

3

common_sensei t1_j6x18c0 wrote

It's still the same total energy. You'll lose ice getting down to minus whatever degrees, so while you're colder to start, you also have less ice.

Ignoring all the extra stuff that can happen (e.g. condensation on the outside of the colder cooler dumping extra energy into it, or freezing and making an insulating layer), a sealed ice+salt cooler should hit 1 degree Celcius before a sealed cooler with ice alone would.

0

parrotwouldntvoom t1_j6w1bu2 wrote

I’m not sure the endothermicity of Salt dissolution is enough to make a noticeable difference in this scenario outside of a lab, but I guess I could look it up.

0

Appaulingly t1_j6y9olg wrote

No the melting is endothermic.

>just adding salt can't take energy out of the system

The temperature decreasing does not mean that the total energy of the system has changed. There is an energy transfer between kinetic energy and potential within the system.

Only really in an ideal gas system does the temperature relate to the total energy.

2

parrotwouldntvoom t1_j6ygg72 wrote

Melting is endothermic in either the + salt case or the -salt case, so it should be a wash in the final consideration of temperature changes.

2

common_sensei t1_j6yu1tm wrote

You said it yourself in your first reply - it makes the ice melt earlier. The relevant concept is Gibbs free energy, where endo/exothermic is only part of the equation.

The only reason ice melts at 0 degrees in pure water is that that's the point where the gain in entropy from turning into a liquid balances out the increase in potential energy from turning into a liquid.

When you add salt to the water, you change the entropy part, making it more entropic to melt, which decreases the equilibrium temperature at which ice turns into water. The ice will melt faster when surrounded by salt, absorbing energy (and quite a bit of it! 334 J/g) until it hits the new depressed equilibrium temperature. Then it'll maintain that temperature by melting slowly, just like ice in pure water.

1