What is the Coldest Possible Temperature? Estimating Absolute Zero.

The Idea

What is the coldest thing possible? Some people might say it is giving a physics test on a Friday afternoon before winter break. (Now that's cold!) But, the coldest temperature possible is absolute zero. This experiment is a nice, simple way to estimate this fundamental property of nature. With some extra care, a more accurate value can be obtained. We know that matter contracts as it gets cold. The basic idea here is to determine what would be the temperature if the volume were to contract to the point where it approached zero. We can't get to that point. In fact, we can't even get close in an ordinary lab. But, we can measure how much the volume changes for a given change in temperature and make a graph to determine at what temperature the volume would be zero. That temperature is absolute zero.

What You Need

To estimate absolute zero

• 250 mL Pyrex flask
• pair of tongs suitable for safely handling a hot flask
• beaker (large enough to fully immerse the flask)
• hotplate
• thermometer
• bucket
• graduated cylinder

To measure absolute zero with greater precision

• A temperature volume or pressure-volume apparatus, such as shown in Figure 91-1.

Method

1. Put the beaker on the hotplate.
2. Fill the beaker with water to a level that will allow the flask to be immersed without causing the water to overflow.
3. Turn on the hotplate.
4. Place the empty flask in the beaker, so it is heated from outside, but without having the heated water spill into the flask.
5. After the air in the flask has had a chance to reach equilibrium with the heated water (about 5–10 minutes at a constant temperature), measure the temperature of the water. The boiling point is a good stable measurement point, but a temperature less than this can be used if it is stable.
6. Fill the bucket with cold water. You can use ice to bring the temperature down.
7. Remove the flask and immediately place it neck side down in the bucket. Hold the neck of the flask underwater as it cools. You may find it helpful to lightly insert a rubber stopper while you are transferring the flask. You can also try to use a one-hole stopper temporarily plugged with a short section of a stirring rod.
8. Once (in your judgment) it reaches equilibrium, measure the temperature of the water in the bucket. This should take less than one minute.
9. The air has contracted and some water has entered the flask. Measure the volume of the water in the flask. (The most accurate reading occurs when the air pressure about the water is in equilibrium with the outside air. This can be established by raising the bottom of the flask so that the liquid level in the flask is at the same height as the liquid level in the bucket.) See Figure 91-2.
10. Subtract the volume of water from the total capacity of the flask.
11. Plot the two points you measured on a graph with volume on the y-axis and temperature on the x-axis. Leave enough room on both axes so that the point where the line connecting the points extrapolates to zero volume fits on the graph. Draw that line and determine the temperature where the volume would be zero.

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