What freezes first ... hot or cold water? | Fizzics Education

# What freezes first … hot or cold water?

You will need:

One cup of cold water, 100mL in Volume

One cup of hot water, 100mL in Volume

One stopwatch

One stirrer

A freezer

Pen

Paper

1

Stir both cups of water for the same amount of time.

2

Place both cups of water inside the freezer and start the timer.

3

Keep checking at 5 minute intervals to see which freezes first.

4

### Why Does This Happen?

Hmmmm, you’ve completed an experiment whose results are quite tricky to explain.
Did you find that the hot water froze first? Under some conditions this can happen.

We started with two containers of water, which were identical in shape and held identical amounts of water. The only difference between the two was that the water in one was at a higher (uniform) temperature than the water in the other. Of course, if the hot water had started at 99.9° C, and the cold water at 0.01° C, then clearly under those circumstances, the initially cooler water would have frozen first. However, under some conditions the initially warmer water will freeze first – if that happens, you have seen the Mpemba effect which describes the phenomenon.

You might ask, ‘how does this work?’ Several ideas have been put forward and no-one is really sure as to which effect plays the biggest role:

Explanation 1. As the initially warmer water cools to the freezer temperature, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the warmer water to cool and freeze than the colder water.

Explanation 2. A convection current may have been setup in the warmer water. As the warmer water cooled it lost heat, primarily through the surface of the liquid, faster than the colder water. This is due to a great temperature difference between the cold freezer air and the warm water. The water from the bottom of the cup then rose to the water surface, bringing more heat energy to the cold freezer air. As the current is greater in the warmer water than the cold water, a greater amount of liquid, from the hotter cup, gets exposed to the cold freezer air. Think of a fan forced oven, circulating the hot air through the oven heats the oven faster than just allowing the air to sit still, bakers have known this over a thousand of years!

Explanation 3. The surrounding air around the cups may have more movement around the warmer cup, therefore drawing heat energy away from the warmer cup more effectively.

Explanation 4. Warm water holds less dissolved gas than cold water. There have been some suggestions that the presence of dissolved gases impede the production of convection currents in the colder water.

Explanation 5. The cold water may have supercooled, therefore not forming a solid as quick as the hot water.

Quote: I often put boiling water in the freezer. Then whenever I need boiling water, I simply defrost it. Gracie Allen.