3
$\begingroup$

This question already has an answer here:

I've read that all items in a freezer are at the temperature at which the freezer are set to (obviously items have to be in the freezer long enough). So, if I put normal tap water in a freezer which is set to -26C (which is what mine is set to) and it freezes, and I leave that ice long enough, it will eventually reach -26C.

What really bothers me is, if I put a +/- 20% saltwater solution in my freezer, and it eventually freezes (by which time regular water have long time been frozen, and according to the statement above should be at the same temperature as the saltwater ice), it feels much more colder than the regular tap water which was left to freeze.

If the statement is true that the regular ice and 20% saltwater ice is both at -26C, why are the saltwater ice feeling so much colder then. Are the online sources I've checked out lying?

I have no physics background, so this makes no sense to me that both ice are at the same temperature

$\endgroup$

marked as duplicate by Gert, Buzz, Jon Custer, Kyle Kanos, ZeroTheHero Feb 1 at 22:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ When we feel the temperature, the rate of energy transfered is more important the temperature. Meaning your salt water probably has a higher heat conductivity $\endgroup$ – B. Brekke Jan 31 at 18:49
  • $\begingroup$ What happens when you hold both ice cubes? Does the salty ice melt more quickly in your hand? $\endgroup$ – noah Jan 31 at 19:18
  • 1
    $\begingroup$ How quickly it melts wouldn't give a definite answer for the heat conductivity, because it also depends on the heat capacity $\endgroup$ – B. Brekke Jan 31 at 19:27
  • $\begingroup$ your question is basically answered here physics.stackexchange.com/questions/119688/… $\endgroup$ – Noldig Jan 31 at 21:22
3
$\begingroup$

If the statement is true that the regular ice and 20% saltwater ice is both at -26C, why are the saltwater ice feeling so much colder then.

Ice is actually a fairly good insulator. If you were to put a large mass of copper in your freezer and pick it up, it would pull a lot of heat from your hand and feel much colder.

But with the ice, your hand puts heat into only the outer layer which warms quickly. It continues warming until it reaches the melting point. So for the most part, you are touching ice/water at the melting point.

If there's enough salt in your ice, then it doesn't warm up as much before melting. So you're holding ice/water that is much colder (on the outside) than the pure water item. Since the outside is colder, the heat transfer from your hand is more effective.

The inside of both ice cubes is as cold as your freezer, but you're not touching the inside.

$\endgroup$
0
$\begingroup$

Adding salt to water lowers its freezing temperature (and elevates its boiling temperature). It's called Freezing Point Depression (FPD). So liquid salt water near its freezing temperature will feel colder than the liquid fresh water near its freezing temperature.

But I can see be no reason for the frozen salt water at -26 C to feel colder than the frozen fresh water at -26 C, unless the thermal conductivity of the frozen salt water is greater than the thermal conductivity of frozen fresh water. Because how hot or cold something "feels" depends not only on the temperature of that something, but also on the rate of heat transfer from that something to or from your skin, which is a function of the thermal conductivity of that something in addition to its temperature.

I looked up the thermal conductivity of salt vs fresh water. I could only find data for the liquids and it indicates they are about same. So you would have to find data on frozen salt vs fresh water.

Hope this helps.

$\endgroup$
  • $\begingroup$ Melting salt water ice doesn't just feel colder than melting fresh water ice, it is colder because of exactly what you said above---freezing point depression. The "freezing point" and the "melting point" are the same thing, so if it freezes at a lower temperature, then it also melts at a lower temperature. $\endgroup$ – Solomon Slow Jan 31 at 23:58
  • $\begingroup$ @SolomonSlow My answer was based on very briefly touching the frozen sea and fresh water and not long enough to bring the surface up to the melting point $\endgroup$ – Bob D Feb 1 at 22:25
  • $\begingroup$ Touching dry sea ice or wet sea ice? If the surface of the ice was wet, then it was at the melting point. Any time you have an interface between solid phase and liquid phase of the same substance, the temperature at the interface must be the melting/freezing point. $\endgroup$ – Solomon Slow Feb 1 at 22:57
  • $\begingroup$ @SolomonSlow I am talking about briefly touching the frozen salt water at -26C and fresh water at -26 C. Neither one is nearly at the melting point at -26C upon touching it. Frankly, I'm not sure what you are talking about. $\endgroup$ – Bob D Feb 1 at 23:21

Not the answer you're looking for? Browse other questions tagged or ask your own question.