Temperature of ice

[bobclean]

As part of my job I have to check water temperatures, and before I start I 'calibrate' my temperature probe with ice (0c) and boiling water (100c). I get the ice from the ice tray in a fridge/freezer. Here is my question: why is the ice constantly at 0c? I know that freezers are capable of going down to approx -20c, so why does the temperature of the ice never change? Not even by one degree?! Any ideas?

[shaun_04]

you have a weird freezer?

the fact of the matter is that ice IS NOT always at 0c.

[shaun_04]

"I know that freezers are capable of going down to approx -20c"
then obviously your freezer isn't set at -20, it is set at 0. Sensors detect the temperature and cool the freezer accordingly to acheive the set temperature.

[bobclean]

Thanks Shaun, I think I will try to obtain ice from another freezer to see if that makes any difference.

[Richard1966]

Surely, 0c is the temperature at which liquid water turns to ice, in the same way that at 100c the water becomes steam. Water can exist at temperature above 100 and below 0 so using the method described to calibrate a thermometer is highly inaccurate

[bobclean]

It may seem highly inaccurate, Richard, but if you have a temperature probe, just try it. I get the same results consistently. I dip the end of the probe into a kettle, and when the water boils, my readout is always exactly 100c, no matter how long the kettle boils. I know that water can exist at higher temperatures, but using this method, I have never observed even a one degree fluctuation. Likewise with the ice, 0c, no fluctuation.

[boogieboogie]

Is this not because ice is water at 0c as snow is water at another degree c
water only needs to be 0c to be frozen, the air around the ice may be -20c
possibly

[Rollo]

Water will only freeze at 0c if it is pure water and the atmospheric pressure is 760 mb of mercury.

Any impurities in the water and variations of atmospheric pressure will change the freezing point.

Try dissolving some salt in the water before you freeze it. Wat temperature reading do you get?

[ChuckFickens]

temperature probes should be calibrated using 3 temperature controlled baths that should be calibrated themselves on a regular basis. one bath at a low temp, one at a high temp and a middle one to check the calibration has worked, I used to use a -20c, a +140c and a 60c bath.


Try taking your kettle up a mountain and check the temperature when it's boiling, it will be several degrees lower.

If you are using the method you describe to calibrate your probe then of course the water boiling in your kettle will always be 100c and the ice from your freezer will always be 0c, because that's what you are setting the probe to read when it is placed in these mediums.

[bobclean]

Chuck, thanks for your answer, but I think I may have inadvertantly mislead you. The temperature probe I use is not adjustable. I use the term 'calibrate' only to mean that I check my probe is correct when placed in boiling tap water (in Rochdale, Lancs), not up a mountain, but near the Pennines! The ice is taken from the ice tray in a fridge freezer.

[boogieboogie]

Ok here's your answer:
If you boil water it can't go above 100 degrees C unless all the water evaporates and then the steam can rise in temperature. The same applies to ice. An ice water mix is 0 degrees C but pure ice can be cooler than 0.

[Whickerman]

and just to completely screw you over, 0�C is not so much the freezing point of water, it's the melting point of ice. Or so i was taught in school! (It's also the old definition for triple point - where water can exist as liquid, solid and gas).

[bobclean]

boogie, brilliant answer, thank you. I am still a little confused though, because my probe is in direct contact with the solid ice as well as the melt water, so it still raises the question, why no temperature fluctuation? Is the ice at 0c? Is it lower at the core but 0c at the edge? Still can't figure it out!

[boogieboogie]

Because the ice is not 'pure' ice it will always be 0 degree c. I don't know where 'pure' ice is sourced from but put your probe in that and the temp will vary. I think ice skating rinks use 'pure' ice.
As wickerman states o degree c is not just the freezing point but the 'triple point' which is why you get no fluctuation.

[ChuckFickens]

it's really hard to measure the temp of ice because the act of inserting the probe will warm the ice where it touches the probe and melt it so you are actually measuring the temp of the thin layer of ice cold water that surrounds the probe.

[mibn2cweus]

I have a bag of ice 'cubes' in my freezer which are cylindrical and hollow at one end. I took one out of my freezer and placed it over my temperature probe. The reading dropped to several degrees below zero and the ice 'cube' had frozen to the tip of the probe. After a few seconds the probe broke loose just as the reading climbed to zero. Soon after water began to drip down the probe. Unless your probe is frozen to the ice it is likely measuring the temperature of water between your probe and the ice which is melting at the point of contact, as ChuckFickens pointed out a simple and common case of the measuring process corrupting the measurement.

[gen2]

Where are you placing your probe?

I suspect you are not measuring the temperature of either ice or steam. The end of your probe is in LIQUID water.

If liquid water is in equilibrium with its vapour, then the highest temperature it can reach (at STP) is 100 degrees Celsius.

Similarly, if solid water is in equilibrium with its liquid phase, the lowest temperature it can reach (at STP) is 0 degrees Celsius.


If you place your probe into a compartment of an ice-tray and place the whole lot in a deep-freeze, then as long as the end of the probe remains surrounded by ice, you will register temperatures below zero.

Caveat:
All the above assumes we are using pure water. the presence of solutes will lower the freezing/melting point and elevate the boiling point.

[bobclean]

Excellent answers, thanks all. I think I agree with the view that my temperature probe is in contact with water, rather than the actual solid ice itself, (the melt water being at 0c). Thanks again, everyone.

[Bert]

The triple point of water is defined as 0.01 degrees Celsius. With this definition, the freezing point of water (under standard pressure) is 0 degrees Celsius. I may not understand Whickerman's distinction between the freezing point of water and the melting point of ice. They are, in theory at least, the same thing. However, water may be cooled many degrees below 0 before it will freeze, so long as there are no particles of dust etc in it and the vessel it is in is very smooth.