It brings me no pleasure to dredge up this old chestnut again. The article,
“Which freezes faster, hot water or cold water?” http://www.straightdope.com/classics/a2_098b.html
was, I thought, well tackled by Cecil.
The article suggested that hot tap water takes longer to freeze than cold tap water. It also suggests that boiling water freezes more quickly, as per Scientific American column, aided by evaporation of the boiling water.
Since Cecils experiments would have used, presumably, approx. 60C and 20C water respectively, it is a wonder that he did not notice the effect.
And all this time I have been telling people that it is an old-wives tale. I have been lead astray.
Does anyone feel like designing an experiment in order to plot cooling curves down to freezing for water of a range of temperatures starting between 0C and 100C? I don’t know how thorough the original 1969 Mpepa paper was
The first thing I did was a search. All the ones I found agreed with Cec.
Looking at your search though is this thread, posted by Joe_Cool which covers the reasons why hot water freezes faster than cold. It has a good link too.
I know Cecil is limited to home improvised scientific apparatus and it may depend what two starting temperatures you choose for the experiment, but I fear the original column seriously needs updating. Otherwise many other people will be lead astray.
antechinus. Thanks for responding. I hadn’t read the link that Joe-Cool provided in that thread.
I think that the qualifiers which are used (wooden buckets, etc.) would make this a hard experiment to design. If the container acts as an insulator, then all bets are off.
There just haven’t been enough good actual experiments performed.
I think that HOW you word the question and qualify it can suggest that everyone is right.
Sorry to throw in a leftfield observation, but I distinctly recall seeing footage of some guy throwing a mug of boiling water straight up in the air outdoors on a cold day. Contrary to my expectations, the water appear to turn instantly into a cloud of icy powder and blew away.
Assuming it wasn’t some sleight of hand, I doubt that cold water would react the same way, though the temperature round my area doesn’t get low enough for me to try.
This is most likely to do with the huge and sudden amount of heat lost by the water to evaporation.
The surface area of the water is greatly increased by throwing it into the air. The viscosity of the water would be less as well, contributing to this effect. An increase in surface area leads to a geometric rate of heat loss.
If the air was cold enough, then it is not surprising this would occur.
This phenomenon is slightly different to the post topic, where the main reason for increase in heat transfer, I understand, is due to increased convection in the cooling (warm) water.
Somehing that may lead to confusion here is that some folks know that freezing warm water will make clearer, less cloudy ice cubes. (and ice sculptures at parties) I could see how another person may observe this act and draw the wrong conclusions.
There is a variable that I have not yet seen discussed in the original experiment or this thread. If you are trying to freeze water in an ice cube tray in the freezer compartment of a domestic refrigerator, the volume of the contents of the fridge in question plays a major role. If the freezer compartment in question is full of frozen food, the added heat will only cause the compressor and fan to run breifly if at all, regardless of the temperature of the water. The cold air will not circulate around the trays and across the surface of the water. Although heat is exchanged at a geometric rate as a function of the temperature differential, the overall freezing time for both will be longer, the warmer water will not catch up and the cooler water will freeze first. If the freezer compartment is nearly empty, the compressor and fan will circulate cold air considerably longer, and lower the temperature of both more rapidly. While it might seem that the colder water would still freeze faster, The warmer water will still be circulating in the tray and so losing heat uniformly through it’s entire volume at the point when ice begins to form on the surface, causing it to freeze a bit sooner. Of course, I could be completely wrong.
There are other factors to take into account concerning this topic. One of the most noted reasons hot water freezes before cold water is that the boiling process drives out any desolved gases in the liquid. I should also point out the fresh water (32 F) will freeze at a higher temp then salt water (28 F). Of course, now that raises the question; “Why do you throw some salt into a boiling pot?” Well, that has to be due to the fact that the disolved solid (salt) raises the boiling point of water a bit as does increased pressure. Which is why a pressure cooker cooks food so fast, because as the water boils, steam is released and pressure builds up. this is a combination of Charles and Boyles laws of pressure, temp and volume Sorry if I clouded the issue somewhat.
Originally posted by MintyClinch
Sorry to throw in a leftfield observation, but I distinctly recall seeing footage of some guy throwing a mug of boiling water straight up in the air outdoors on a cold day. Contrary to my expectations, the water appear to turn instantly into a cloud of icy powder and blew away.
Assuming it wasn’t some sleight of hand, I doubt that cold water would react the same way, though the temperature round my area doesn’t get low enough for me to try.
What gives?
This is due in part to a combination of what is discussed in the last couple of postings,:
Increased surface area quickens freezing process
Disolved gases have been driven out.
Slight decrease in pressure as the water leaves the container.
Note: this last condition is what’s called the “Venturi Effect” the Venturi effect states that liquids and gases volocity increases, their pressure decreases. thereby the boiling point drops and the surface area increases and the smaller droplets freeze faster then larger droplets.
This explaination is also valid in helping to understand why hot water pipes freeze before cold water pipes (less desolved gases).
There are other factors to take into account concerning this topic. One of the most noted reasons hot water freezes before cold water is that the boiling process drives out any desolved gases in the liquid. I should also point out the fresh water (32 F) will freeze at a higher temp then salt water (28 F). Of course, now that raises the question; “Why do you throw some salt into a boiling pot?” Well, that has to be due to the fact that the disolved solid (salt) raises the boiling point of water a bit as does increased pressure. Which is why a pressure cooker cooks food so fast, because as the water boils, steam is released and pressure builds up. this is a combination of Charles and Boyles laws of pressure, temp and volume Sorry if I clouded the issue somewhat.
Actually, the reason you throw a pinch of salt in your boiling water is to make whatever you’re cooking taste salty. Adding salt does increase the boiling point, but in the amounts typically used in cooking, the effect is negligible.
I did a brief scan of the articles and sources listed to try and avoid restating a point, but I wanted to share a thought experiment that helped me settle this issue with myself some time ago.
Assume: For some set of circumstances, hot water freezes more quickly than cold water.
Postulate: Since the temperature of the water that began hot reaches the freezing point before the cooler water, at some time before that the two quantities of water must have been at the same temperature.
Conclusion: Since the variable of concentration, temperature, becomes equal at that point, it seems obvious that it is some other factor that actually results in a differing cooling rate. Whether it be convection, contact surface properties, or whatever, there are means besides initial temperature control to adjust those.
Comparative Example: Harry states that it takes longer to clean dog hair off his rug by vacuuming it than it does by using a pair of tweezers to pick up the individual hairs. Harry doesn’t own his own vacuum, nor easy transportation, and so therefore vacuuming his rug requires walking 100 miles to a friend’s house and back to borrow a vacuum, plus the same journy to return the vacuum. While his original statement can be shown to be true given certain factors, it is not a direct result.
I recall my own version of this experiment. The babysitter made the claim that hot water froze faster than cold. I disagreed, but rather than argue proposed we simply test it. We each took identical pitchers, filled them with our chosen temperature of water, and placed them side-by-side on a shelf in the basement freezer. When mine was a solid block of ice, hers was just beginning to form some ice crystals along the top. I was kind and didn’t even say anything when we looked, I knew it couldn’t be comfortable being proved wrong by a young child
I cannot see why this might be called ‘the Mpemba effect’.
As far as I can see, Aristotle has considerable priority in the identification of this unusual claim. But the whole point about this issue, and one that Cecil and the rest seem to have missed, is that it is famous as an unusual claim, and one best examined by experimentation rather than accepted because someone quotes an apparent authority - be they Aristotle, Cecil, or anyone in between.
The reason why this ‘old chestnut’ comes up in discussion groups (and has for the last 650 years) is that it was an example cited by Roger Bacon (NOT Francis Bacon!) in his staggeringly foresighted treatise on knowledge written in the 1260s. (as an aside, the man is a hero of mine - it is not an exaggeration to say he invented science, in an age when defining where knowledge came from could prove very dangerous)
The point of Bacon’s piece is that while deference to authority is common (and essential for a 13th century monk!), experience and experiment ‘brings quiet to the mind’. Here is a translation of a relevent part of his work - it may be nearly 3/4 of a millennium old, but it is as immediate and important a piece of writing as any you will read today, and could well be taken as a central guidance document for contributors to the ‘Straight Dope’. http://www.fordham.edu/halsall/source/bacon2.html
If anyone is thinking of doing an experiment, I would suggest looking at Bacon’s description - he would certainly have done the work meticulously.
