does hot water really freeze faster than cold water? I fill ice trays with hot water because i was told that.?

Question

does hot water really freeze faster than cold water? I fill ice trays with hot water because i was told that.?

Answer 1

Hot water must first be cooled before it freezes. However, hot water will evaporate rapidly. The usual result is that ice-cube trays full of hot water freeze in about the same time as trays filled with cool water, but the hot-water cubes are smaller due to evaporation loss.



The "use hot water for ice cubes" urban legend may have originated in a practice from more refined days of boiling water before placing it in ice cube trays. Boiling drives most of the air bubbles out of the water and the result are ice cubes that are crystal clear. My grandmothers (both of them) did this whenever they entertained.

Answer 2

It can freeze faster-- the Mpemba effect it's called. It doesn't always happen.





http://physicsworld.com/cws/article/print/24493





Some things contribute. For example, evaporation means there is less to freeze. So when it gets to zero, there is still alot of energy to go to freeze it, so the one with less could freeze faster. I don't think this one is the exclusive, main, answer based on some websites that claim experimentation suggests something else is doing it too--it happens with closed containers I believe I read). I haven't tested this yet. (working on it)



Gas loss may also affect it (when you heat the water, there's less dissolved gas).



Also if you use hotter water, the tray cups hold less mass of hot water cause hot water takes up more space, has less density.



Other than gas loss, it probably has to do with convection currents, or insulation difference.

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EDIT: Some people say that it is because of the freezer sensing the heat, making it turn on, making the temperature colder. OF course this could contribute if you're testing them separately.



I plan to use outside next winter to eliminate that. Or in the freezer I assume I can put them in at the same time and that would work?

Answer 3

No, hot water does not freeze faster. Try it yourself. Fill one tray with hot water and another with cold.



The hot water initially loses it's heat faster because of the larger temperature difference between the water and the freezer. However, at some point it will be at the same temperature the cold water (in the other tray) was when you put it in the freezer. By that time, the cold water will be even colder.

Answer 4

Hot water COOLS faster than cold water in the freezer due to the temperature differential and cooling rate.



Once the hot water gets to the cold water temp., it will cool in the same amount of time.



It will take TIME for the hot water to get to the temp. of the cold water.



Thus this is a myth.

Answer 5

Let's use a simple thermodynamics equation to put this issue to rest.



-Q (heat removed from water to make it freeze) = m (mass of water being frozen) * C (specific heat of water) * -ΔT (change in temperature to make the water freeze)



Comparing hot water being frozen with room temp water being frozen, we see that the change in temperature is greater for hot water being frozen. Intuitively (and primarily looking at the formula) this means that more heat is being removed from hot water being frozen than room temp water being frozen. We're taking more heat energy out of the hotter water, right?



That said, your freezer has a fixed rate at which it removes X amount of heat per second from your water to make it freeze. If the heat to be removed is greater for hot water, then consequently more time will be needed to remove that heat compared with room temp water.



Hope this gives you a more concrete idea. I simplified some aspects of heat radiation since it's beyond the scope of what you're looking for!

Answer 6

I am happy that people are F I N A L L Y understanding this!