Getting water to zero degrees celsius
Discussion
Say you've just had a cook's temperature probe delivered and you want to find its accuracy. Using items normally available in the average (non-scientist's) kitchen, how would you check the accuracy at zero degrees celsius?
Initially, I thought of ice cubes from the freezer, but these could easily be several degrees cooler than zero.
If ice cubes were left in a glass, would the melt water be at zero just as the last of the ice disappeared.
I'm stumped . Over to the PH eggheads...
Initially, I thought of ice cubes from the freezer, but these could easily be several degrees cooler than zero.
If ice cubes were left in a glass, would the melt water be at zero just as the last of the ice disappeared.
I'm stumped . Over to the PH eggheads...
You will need a mixture of ice and water to get 0 degree's C.
http://www.canadacutlery.ca/download/calibrate-a-t...
http://www.canadacutlery.ca/download/calibrate-a-t...
cmsapms said:
The Nur said:
Put salt in it?
Salt, you say? How much?The Nur said:
eta: I may have demonstrated how retarded I am with this post.
I keep seeing the words "heat", "latent" and "fusion". But all that was 30 odd years ago.Now, bearing in mind I didn't read the question properly and only realised this after posting my reply..
I was thinking that you could put salt in some water, chuck it in the freezer until it is below the regular freezing point of water and then give it a little dip with the thermometer. Then I realised that you would have no reliable way (unless you have another thermometer that is a known quantity) of testing to see what the actual temperature of the salt water mix is.
I don't generally think before I speak.
Odie said:
You will need a mixture of ice and water to get 0 degree's C.
http://www.canadacutlery.ca/download/calibrate-a-t...
True but now much of each and after what time? You don't know the starting temps of each. To know this you'd need an accurate thermometer, and if you had one, you wouldn't need to do the test http://www.canadacutlery.ca/download/calibrate-a-t...
Or maybe the ice warms up until it gets to 0, and the water cools down until it gets to 0, and they sit like that at 0 and think for a bit while the rest of the ice thinks about melting. Yes, that must be it.
Some days I feel like Einstein, at others, Winnie-the-Pooh. This is a Winnie-the-Pooh day.
Agree with above.
An ice cube straight from the fridge might be below 0C.
But Physics is your friend!
1 calorie will change the temperature of one gram of water by one degree C. This is the Specific Heat of Water
But to freeze that gram of water, already at 0C, another 80 calories must removed. And adding 80 calories to a gram of ice (ignoring the reduced density of ice) will melt it, but not change its temperature. This is the Latent Heat of Crystalisation, the finite amount of heat that must be extracted to cause a liquid to become a solid.
So your cubes start to melt as soon as the temperature rises to 0C. Any more heat melts ice, and the ice/water mixture stays in temperature equilibrium with the added heat, until all the ice has gone.
Adding salt to ice will lower the freezing point of any water in there, and distort this picture, so that the melting takes place at a lower temeprature than 0C. So you want pure frozen water to achieve zero.
And the same physics makes the boiling point another fixed point to calibrate your gauge. At 100C any added heat goes into the Latent Heat of vapourisation, so that water boiling briskly will always be at 100C. BUT do this at sea level! If the air pressure is low, the water will boil at a lower temperature. If you live in Denver, Colorado, Mile-High City, or Kabul, Mexico City or Nairobi your water will boil at about 95C, but in the UK don't worry about weather highs and lows!
John
An ice cube straight from the fridge might be below 0C.
But Physics is your friend!
1 calorie will change the temperature of one gram of water by one degree C. This is the Specific Heat of Water
But to freeze that gram of water, already at 0C, another 80 calories must removed. And adding 80 calories to a gram of ice (ignoring the reduced density of ice) will melt it, but not change its temperature. This is the Latent Heat of Crystalisation, the finite amount of heat that must be extracted to cause a liquid to become a solid.
So your cubes start to melt as soon as the temperature rises to 0C. Any more heat melts ice, and the ice/water mixture stays in temperature equilibrium with the added heat, until all the ice has gone.
Adding salt to ice will lower the freezing point of any water in there, and distort this picture, so that the melting takes place at a lower temeprature than 0C. So you want pure frozen water to achieve zero.
And the same physics makes the boiling point another fixed point to calibrate your gauge. At 100C any added heat goes into the Latent Heat of vapourisation, so that water boiling briskly will always be at 100C. BUT do this at sea level! If the air pressure is low, the water will boil at a lower temperature. If you live in Denver, Colorado, Mile-High City, or Kabul, Mexico City or Nairobi your water will boil at about 95C, but in the UK don't worry about weather highs and lows!
John
Gassing Station | Science! | Top of Page | What's New | My Stuff