Submitted by Ok_Kareem_7223 t3_10qviic in askscience
parrotwouldntvoom t1_j6uz3m3 wrote
There are a lot of goofy answers here. Adding ice will not make the system cooler, or the ice cooler, but it will make the water cooler because it will make the ice melt earlier. However, if your ice is at 0C already, adding salt will not make things cooler because just adding salt can't take energy out of the system. In reality, your ice is likely -10 to -20 C, and so adding salt helps.
I can't think of any reason that adding salt would make it stay cold longer. The duration of maintaining cold should be a function of starting temperature and the characteristics of the cooler.
common_sensei t1_j6v33a6 wrote
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.
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”?
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.
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.
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.
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.
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.
[deleted] t1_j71k3oa wrote
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PD711 t1_j6vk20o wrote
when i was a kid we got an ice cream maker one year for the 4th. it was a drum filled with salt and ice, and then a second container was put inside the first with the ice and salt surrounding it. and that container you put the milk, cream, sugar etc. and then it mixed the contents until it was ice cream.
[deleted] t1_j6wvkis wrote
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lupadim t1_j6wq40u wrote
1- Your pure ice is not at -10 to -20 C, this is impossible
2- Yes adding salt does cool it
eclectic_radish t1_j6wusft wrote
How is it impossible for ice to be between -10 and -20? I have my freezer set to -18°C, and everything that has been in there long enough is also -18°C
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