Abstract

This physics project seems like it should have an easy answer. Instead, it turns out to be a great illustration of why it is important to base scientific conclusions on the outcome of controlled experiments. Things don't always turn out as we expect!

Objective

The goal of this project is to investigate the question, "Can hot water freeze faster than cold water?" Thorough background research, a precise formulation of the hypothesis, and careful experimental design are especially important for the success of this experiment.

Introduction

It may seem counterintuitive, but folk wisdom and a body of published evidence agree that, under some conditions, warmer water can freeze faster than colder water (for an excellent review on the subject, see Jeng, 2005).

This phenomenon has been known for a long time, but was rediscovered by a Tanzanian high school student, Erasto Mpemba, in the 1960s. He and his classmates were making ice cream, using a recipe that included boiled milk. The students were supposed to wait for the mixture to cool before putting it in the freezer. The remaining space in the freezer was running out, and Mpemba noticed one of his classmates put his mixture in without boiling the milk. To save time and make sure that he got a spot in the freezer, Mpemba put his mixture in while it was still hot. He was surprised to find later that his ice cream froze first (Meng, 2005).

When Mpemba later asked his teacher for an explanation of how his hotter ice cream mixture could freeze before a cooler one, the teacher teased him, "Well all I can say is that is Mpemba physics and not the universal physics" (quote in Jeng, 2005). Mpemba followed his curiosity and did more experiments with both water and milk, which confirmed his initial findings. He sought out an explanation for his findings from a visiting university professor, Dr. Osborne. Work in Dr. Osborne's lab confirmed the results, and Mpemba and Osborne described their experiments in a published paper (Mpemba and Osborne, 1969).

Since Mpemba and Osborne's paper was published many scientists have tried to replicate their findings. In some cases people have seen the Mpemba effect, in other cases the hotter water does not freeze faster. Today most scientists consider the Mpemba effect to be a real phenomena but the variability in results has sparked a great deal of debate over what specific conditions are needed to see the effect and why it occurs. How can it be that hot water freezes faster than colder water? Somehow, the hot water must be able to lose its heat faster than the cold water. In order to understand how this could happen, you will need to do some background research on heat and heat transfer. Here is a quick summary, so that you can be familiar with the terms you will encounter. Heat is a measure of the average molecular motion of matter. Heat can be transferred from one piece of matter to another by four different methods:

• conduction,
• convection,
• evaporation, and
• radiation.

Conduction is heat transfer by direct molecular interactions, without mass movement of matter. For example, when you pour hot water into a cup, the cup soon feels warm. The water molecules colliding with the inside surface of the cup transfer energy to the cup, warming it up.

Convection is heat transfer by mass movement. You've probably heard the saying that "hot air rises." This happens because it is less dense than colder air. As the hot air rises, it creates currents of air flow. These circulating currents serve to transfer heat, and are an example of convection.

Evaporation is another method of heat transfer. When molecules of a liquid vaporize, they escape from the liquid into the atmosphere. This transition requires energy, since a molecule in the vapor phase has more energy than a molecule in the liquid phase. Thus, as molecules evaporate from a liquid, they take away energy from the liquid, cooling it.

Radiation is the final way to transfer heat. For most objects you encounter every day, this would be infrared radiation: light beyond the visible spectrum. Incandescent objects—like light bulb filaments, molten metal or the sun— radiate at visible wavelengths as well.

In addition to researching heat and heat transfer, you should also study previous experiments on this phenomenon. The review article by Monwhea Jeng (Jeng, 2005) is a great place to start. The Jeng article has an excellent discussion on formulating a testable hypothesis for this experiment.

Another excellent article, if you can find it at your local library, is by Jearl Walker, in the September, 1997 issue of Scientific American (Walker, 1977). Walker measured the time taken for various water samples to cool down to the freezing point (0°C), not the time for them to actually freeze. He measured the temperature of the water using a thermocouple, which could be placed at various depths in the beaker. Whether you use a thermocouple or a thermometer, it is important that the sensing portion of the device (thermocouple itself, or the bulb of the thermometer) be immersed in the water in order to get accurate readings. Walker used identical Pyrex beakers for his water samples, since they could go from the stove to the freezer without breaking. He used a metal plate over the stove burner to distribute the heat evenly to the beakers as they were heating. He heated the beakers slowly, and he also kept the beakers covered while heating, so that water that evaporated during heating would be returned to the beaker. Walker notes that "You cannot obtain accurate readings by first heating some water in a teakettle, pouring the water into a beaker already in the freezer and then taking a temperature reading. The water has cooled too much by then" (Walker, 1997, 246). Walker also reported that the air temperature in his freezer was between −8 and −15°C. He advises, "To maintain a consistent air temperature be sure to keep the freezer door shut as much as possible" (Walker, 1977, 246). For further details on his experimental procedure and findings, see the original Scientific American article.

The graph in Figure 1 shows some of Walker's data. The x-axis shows the time it took for the sample to reach 0°C (in minutes). The y-axis shows the initial temperature of the sample (in °C). The graph shows data from six separate experiments (a-f), each with a different symbol:

1. 50 ml water in small beaker, non-frost-free refrigerator (black squares),
2. 50 ml water in large beaker, non-frost-free refrigerator (red circles),
3. 50 ml water in large beaker, frost-free refrigerator (green triangles),
4. 100 ml water in large beaker, thermocouple near bottom (blue triangles),
5. 100 ml water in large beaker, covered with plastic wrap, thermocouple near bottom (light blue diamonds),
6. 100 ml in large beaker, thermocouple near top (magenta triangles).

Under some conditions (b, d, f), he found that samples that were initially hotter reached 0°C faster than samples that were initially cooler, confirming Mpemba's results. Under other conditions (a, e), hotter samples took as long or longer than cooler samples to reach 0°C. The results for experiment c are equivocal—it's difficult to say whether the time differences are significant or not. This kind of variability is one of the things that has intrigued people about the Mpemba effect and makes it so interesting to experiment with.

In this science project you will investigate the Mpemba effect for yourself. Will the conditions in your experiment lead to hot water or cold water freezing first? Make sure to thoroughly do your background research, formulate your hypothesis, and keep careful notes about your experimental design. This is also a great project to take time to systematically try different variables, like starting temperatures, tap water versus deionized water, humidity, and just about any other environmental factor you can think of! Who knows, maybe your data will help contribute to a greater understanding of the Mpemba effect.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

• Mpemba effect
• Heat transfer
• Conduction
• Convection
• Evaporation
• Radiation
• Phase change

More advanced students may also want to study:

• Supercooling
• Nucleation sites for initialization of crystal formation

Questions

• How does your freezer work to make things colder?
• What are some of the mechanisms that have been proposed to explain the Mpemba effect?
• How would you design an experiment to test one of the proposed explanations?
• Is the Mpemba effect always observed?

Materials and Equipment

• Identical Pyrex beakers for holding water; beakers should hold 100 mL or more
• Metal plate or stove burner to distribute heat evenly
• Cover for the beaker during heating
• Two thermometers
• Freezer (or other means for cooling water below freezing point)
• Stove (or other means of heating the water)
• Hot mitt
• Gram scale, such as the digital pocket scale from Amazon.com
• Clock or timer

Experimental Procedure

Variations

Frequently Asked Questions (FAQ)

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Careers

If you like this project, you might enjoy exploring these related careers:

Related Links

• Science Fair Project Guide
• Other Ideas Like This
• Physics Project Ideas
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