Does hot water freeze faster than 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

A pen and paper

1

Stir both water cups the same amount of time. Place both cups of water inside your freezer and start the timer.

2

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

3

4
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### 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.

How does it work you might ask? Several ideas have been put forward and no-one really is sure as to which effect plays the biggest role:

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.

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 got 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!

More on temperature and water rising or falling
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.

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.

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

### Variables to test

More on variables here

• What happens if you add salt to the water?
• Does it make a difference if you change the volume of water?

## 12 thoughts on “What Freezes first… Hot or Cold Water?”

1. Amal says:

Very helpful and useful for kids and everyone and it’s easy to understand.

Love it

1. Ben Newsome says:

Thanks Amal, we’re glad that you’ve been using these science activities!

2. Rose says:

What’s the independent, dependent & controlled variable of this experiment?
And what equipments have you used for this experiment?

1. Ben Newsome says:

Hi Rose!

The independent variable for this experiment was the temperature of the water (this was the variable that we could manipulate).
The dependent variable was the time it took for the water to change from liquid to solid (as this depends on the initial temperature of the water).
The controlled variables were the freezer temperature, the cup size, the volume of the water used in each test and the amount of stirring in each cup.
More details on variable testing here

You can find the details on the materials needed at the top of this page. Have fun with this experiment!

3. Zach says:

what are some risks that can take place when doing this experiment

1. Ben Newsome says:

Hi Zach!

If the water is close to boiling you really need to be careful. Also, water spills are a slipping hazard.

4. Is this an actual good experiment that will work in class

1. Ben Newsome says:

Hi Ben, give it go and see what happens! All the best.

1. Ben Newsome says:

Sure thing, just check with your teacher first and let them handle the hot water. Have fun!

5. i don’t have it yet,but it sounds like its super fun! : )

1. Ben Newsome says:

Its an interesting experiment! Please let us know what your results were… do they conform to the theory above? Have fun!

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