Results

The experiment was run with the two samples being frozen at different times. This enabled us to use a large container and keep the the container, freezer and volume of liquid a constant.

mpemba experiment results

Figure 1: the freezing curve of water at room temperature

mpemba experiment results

Figure 2: the freezing curve of water at 40 °C

To properly compare the results from the two traces we exported the data to a spreadsheet program. This allowed us to overlay the two sets of data on the one graph. If you measured the data from two samples at the same time you would be able to see this result instantly in PicoLog.

mpemba results

Figure 3: both sets of results

The results from the graph above shows that hot water sample reaches the 0 °C freezing point sooner that the water sample at room temperature: this is known as the Mpemba Effect.

Discussion of results

The Mpemba Effect was first discovered by a Tanzanian student while making ice cream in 1969. The Mpemba Effect is the phenomenon whereby hot water can, under certain conditions, freeze faster than cold water.

Scientists have identified a number of factors to explain the Mpemba effect (conduction, evaporation and convection) but so far all the theories are only speculative.

Conduction — this effect is more suitable when water samples are cooled in a freezer. When the sample containing the hotter water is placed on a shelf, in the freezer, the heat will melt the ice under the container. When this re-freezes there will be a better conductor of the heat away from the container than the sample of cold water just sitting on the shelf.

Evaporation — this is important if the temperature of the water sample is high and the container has a large exposed surface area. Evaporation has the effect of removing mass from a high temperature sample, which will then compensate for the greater temperature range.

Convection — water is at its most dense at 4 °C. If a water sample starting at 4 °C is placed in a freezer the water at the surface will cool rapidly. Because the water at the surface is less dense that the water beneath the frozen layer will stay at the surface and form a insulating layer for the water beneath.

The hotter sample will behave differently. The water at the surface will cool rapidly due to evaporation, and will become denser that the hot water below. The denser surface water will then sink to the bottom of the container and push the hotter water to the surface. The process is then repeated and this circulation of water causes a rapid drop in the water temperature.

Teacher's notes

Safety issues:
Students should be careful to avoid burns from both the hot and frozen water.

Time:
This experiment requires several hours to complete.

General notes:
This experiment demonstrates the Mpemba Effect. The Mpemba Effect is where the same quantities of hot and cold water are cooled together and it is found that the hot water reaches zero Celsius first. An interesting investigation in which the reasons for it are still debated.

Target age groups:
Ages 14-19

National curriculum:
Science Key Stages: 4 and 5