Does hot water freeze faster than cold?

[Huntindave]

Originally Posted by KPKyllo

I don't think phase change explains the formation of ice when the temperature is above freezing. In my 4 years of college chemistry and 12 years as a laboratory chemist I don't ever recall reading about or witnessing water freezing above 32 degrees Fahrenheit. The explanation may be that it was warmer where you read a thermometer than where your car was parked.

Surely you jest.

Originally Posted by KPKyllo

I don't think phase change explains the formation of ice when the temperature is above freezing. In my 4 years of college chemistry and 12 years as a laboratory chemist I don't ever recall reading about or witnessing water freezing above 32 degrees Fahrenheit. The explanation may be that it was warmer where you read a thermometer than where your car was parked.

In the same way that water will evaporate (change phase) from your skin even though it doesn't get to boiling point.

And the way that if you dip a digital meat thermometer at room temp. say 70 degrees, into a cup of water at 70 degrees, then remove the thermometer from the water and wave it around until the water evaporates. All of the sudden your thermometer is reading 60 degrees. How can that be? The room is 70, the water is 70, but now the probe is reading 60. Magic? No. Phase change removing heat energy from the temp probe lower the temp reading below ambient.

The evaporation of moisture from your car (moisture pulling heat from the metal or glass) can lower the surface temp of the car to level below the air temp.

It happens mostly on cold, clear nights when the humidity is low allowing for easier evaporation.

[Huntindave]

Originally Posted by mk cant log in

And the way that if you dip a digital meat thermometer at room temp. say 70 degrees, into a cup of water at 70 degrees, then remove the thermometer from the water and wave it around until the water evaporates. All of the sudden your thermometer is reading 60 degrees. How can that be? The room is 70, the water is 70, but now the probe is reading 60. Magic? No.

A ten degree change, due to the couple drops of water left on a meat thermometer probe ?

Originally Posted by Huntindave

A ten degree change, due to the couple drops of water left on a meat thermometer probe ?

Try it, I just did. The temp drop was 11.2 degrees. Digital thermometer, not a mechanical dial model.

[kzoofisher]

This time of year with the RH in your house around 40% you'll have a dew point in the 40s. That'll cool a thermometer pretty quickly. It will also cool the layer of air right at the ground quickly. Water evaporating from the soil absorbs a lot of heat. In low humidity conditions you will have a dew point near freezing and as the temp drops below that all the moisture in the air suddenly condenses and absorbs much more energy from the rapidly cooling air. You can have 38 degrees 6' in the air and much cooler air right on the ground. Ice forms and will stay until until enough heat builds up around it to make it change state again.

But back to the Mpemba effect. The thing I found most interesting was that they can't explain why it sometimes happens and sometimes doesn't. Evaporation and the huge loss of energy associated with it (970BtU/#) was my go to explanation. I always thought that cooled the entire mass of water faster and reduced the volume slightly. Guess not. I hope they figure it out one of these days.