What freezes faster, hot water or cold water?

You tell me.

Freezing occurs when fresh water is about T0 = 273 deg K and there is a small bit of impurity to begin the crystallization.

The rate of change in heat content varies as dQ/dt = k (T1 - T0) where the T's are temperatures of the water in your case. k is a constant of proportionality that is material specific; we can assume it is the same for the hot and cold water cases.

We assume the same quantity of water, but the hot water has Q > q amount of heat where q is the cold water amount. Let the hot water temperature be T2 > T1 the cold water temperature and T0 = 273 deg Kelving the freezing point of fresh water.

Then compare freezing rates dQ/dt//dq/dt = k (T2 - T0)/k(T1 - T0) so that dQ/dt = dq/dt (T2 - T0)/(T1 - T0) and because (T2 - T0) > (T1 - T0) we see that dQ/dt > dq/dt and the rate of change is faster for the hot water. So the hot water does approach the freezing point faster.

But, and this is a big BUT, the hot water Q > q of the cold water; so the hot water started farther away from the freezing point. In fact, once the hot water temperature cools down from T2 ---> T1, its rate of heat loss will equal that of the cold water starting point. That is dQ/dt = dq/dt once T2 = T1.

And that's the answer. Hot water does indeed cool faster initially, but once it reaches the cold water temperature, it cools at the same rate the cold water does. And given that the hot water started with a higher heat content Q > q it will take longer to reach freezing point because of that extra time between T2 and T1 that the cold water does not have.

My Mom used to tell me that hot water froze faster and I believed it at the time. Then, when I was studying Chemistry and Physics (heat transfer), I learned that hot water loses heat faster, but only down to the temperature of the cold water. Starting from that point, it would still take the amount of time the cold water takes. So, for hot water, you have the time it takes to cool down plus the time it takes to freeze from cold. That means hot water takes longer to become frozen.

Edit: I have been looking up info on the Mpemba effect and found that it is true under some circumstances and experimental conditions.

Per John Baez: "In short, hot water does freeze sooner than cold water under a wide range of circumstances. It is not impossible, and has been seen to occur in a number of experiments. However, despite claims often made by one source or another, there is no well-agreed explanation for how this phenomenon occurs. Different mechanisms have been proposed, but the experimental evidence is inconclusive."

So, the answer is both yes and no, depending on the conditions. There is a good discussion of this at the referenced site.

whoever says this is true is an idiot, because cold water simply freezes faster. However, they are not entirely wrong so to speak. If you were to take two like amounts of water, boil one and chill the other and then let them both sit out until both have reached the same room temperature and THEN throw them in the freezer, you will see that the one which was boiled freezes first.

Boiling changes the 'stuff' in the water just enough so that it cools faster! Remember, they both have to be brought to room temperature before placing in the freezer.

A lot of people like to say that hot water freeze faster right because of Mpemba effect. But I think this is 50 50 true(this is what I think) I have ask my friend once that hot or cold water will freeze faster. My friend immediately said that hot one because of Mpemba effect, but I agree with him. Because I have read about Mpemba effect before. But I think that cold water will freeze faster because a molecules of hot water will move faster right because hot water molecules will have more energy and more energy that mean more kinetic energy. But for cold water a molecules of cold water it has less energy than hot water. So it will move slower than hot water. And if you give a cold temperature to hot water it need to take quite a long time to make it freeze because it need to wait for a molecules of hot water to move slower you know to make it has less kinetic energy and freeze. But for cold water it molecules already move slow because it not has much energy so it will not take quite a long time to make a cold water freeze right --.

Experiment at my school and at my home with my friends (Mpemba experiment and cold water and hot experiment).

Hot water does, It is called the Mpemba effect. It has been observed that, in certain specific circumstances, warmer water freezes faster than colder water. New Scientist observed that starting the experiment with containers at 35 °C (95 °F) and 5 °C (41 °F) to maximize the effect.

It can but only under certain conditions - I dont think anyone know the full reasons.

Let me give you an analogy

Suppose you are on a beach - 10m from the sea and a friend is already in the water - you both race to run to a buoy 20m out to sea. Your initial speed entering the water may allow you to overtake your friend before he can reach the buoy.

Hot water.

It's counter intuitive, but true...hot water freezes faster.

Here is a link to an Edu article that explains it very well.

http://math.ucr.edu/home/baez/physics/General/hot_...

hot water will freeze faster.

hot water freezes faster I've done the test, you should try it your self..........

Hot of course................