11

Cooling and heating water in a can

Hot water is poured into a thin, shiny aluminum can. The water is stirred, and its temperature is recorded (Fig. 1, left). The temperature drops quickly at first, and more slowly later on. Finally, it reaches a constant value; interestingly this value is above the temperature of the environment.
If very cold water in a drinking can is left standing in a warm room, its temperature rises quickly at first, and then more slowly (Fig. 1, right). In the case presented here, the drinking bottle is inside an insulating jacket, and the water is stirred during the observation.

Left: Temperature of hot water left standing in a thin-walled aluminum can. The final temperature is above the temperature of the environment (here about 20°C). Right: Temperature of cold water left standing in an insulated drinking bottle. The final temperature is above the temperature of the environment (the curve that starts at about 23°C).

Interpretation

Hot water loses heat through the wall of its container to the environment. The difference of temperatures between the water and the environment serves as the driving force for the flow of heat. The magnetic stirrer used to stir the water produces some heat. That is why the temperature of the water settles above the temperature of the environment: In the end, the heat produced by the stirrer must leave the container, so we still need a temperature difference.

If the water is colder than the environment, heat will flow from the environment into the cold water. Again, if the temperature difference is high, the flow will be strong, the temperature of the water will change fast. Since heat is generated by the stirrer, the final temperature of the water will be above that of the environment (Fig. 1).