Engagement

• Small changes in Earth's global average temperature can produce dramatic changes in climate. Scientists are trying to determine how much Earth's mean temperature is changing. One way to do this is to monitor sea level. Warming results in higher sea levels, which may strongly affect the population who live near the ocean. There is geologic evidence for very large variations in Earth's sea level in the past. There are two main causes of these sea level changes: (1) thermal expansion or contraction of the existing ocean water, and (2) changing volume of water due to the melting or freezing of polar ice caps and glaciers. Changes in the climate affect both of these.
• Over the past hundred years, sea level averages have risen 15 cm (6 in) for about 0.5°Celsius degree (0.9°F) of average global temperature increase. Some computer models predict that a doubling of the present carbon dioxide level in the atmosphere of 0.032% over the next hundred years would increase ocean waters by 3°C (5.4°F). Using the 15 cm/0.5°C above, this would cause a sea level rise of about one meter (3.3 ft).

Activity

1. Mark the point on the bottle that is 7 cm from the top. Optionally, measure the inside and outside diameters of the bottle at the top and and at 7 cm below the top to determine the dimensions. For example, for a sample 750 ml wine bottle, the measured top and 7 cm position outside diameters were 2.4 cm and 3.0 cm respectively. The corresponding inside diameters were 1.8 cm and 2.4 cm (assuming constant thickness).
2. Fill the 750-ml bottle with water to just below the bottom of the neck. Ice can be used to bring the temperature down to 20°C if desired.
3. Support the thermometer in the bottle so that the bulb is well below the neck in the main body of water. Use either string or something similar to an emory board placed horizontally on the bottle top with the thermometer suspended through it.
4. Add or pour out water so that with the thermometer in the bottle, the water level is 7 cm below the top, as measured with the ruler held vertically.
5. Fill the cooking pan about two-thirds full of water and place on the heat source.
6. Center the bottle in the pan. The water level in the pan should be at least three cm below the top.
7. Heat the water to at least 80°C while recording the water level every 5 or 10°C in Table 1. To prevent the water from spilling, turn off the heat source if the water in the bottle reaches to within one cm of the top. Graph height versus temperature and the change in height versus temperature. Does the height change more or less rapidly at higher water temperatures?

Explanation

• Most materials expand on heating, although some contract over limited ranges of temperature. On average, liquids expand about ten times as much as solids, and gases expand much, much more than liquids. The correction for the expansion of the solid bottle was not included here because it will be very small compared to the expansion of the water. Note that the expansion for water is much greater near its boiling point than near room temperature.
• When oceans heat up, they expand as well, causing a rise in sea level. This, along with melting and freezing of water in glaciers and polar caps, has caused large variations in sea level in Earth's past as the average global temperature changed. Such changes could occur in the future as well. What you constructed in this activity was basically a thermometer. If you had calibrated the expansion versus temperature in this experiment, then you could determine temperature by measuring the height of the water. However, water is not very suitable for use in a thermometer for many reasons: its amount of expansion changes greatly with temperature range, it evaporates easily, and it does not expand as much as some other liquids. Most thermometers use mercury because it expands quite a lot, and does so uniformly over a very large range of temperatures. (Interestingly, the first thermometer constructed did, in fact, use water.)