A constant volume gas thermometer is calibrated in dry ice (which is carbon dioxide in the solid state and has a temperature of -80°C) and in boiling ethyl alcohol. What is the purpose of calibrating the thermometer in these two substances?

Calibrating a constant volume gas thermometer with dry ice and boiling ethyl alcohol is essential for creating a precise and consistent standard for temperature measurement, relying on the ideal behavior of gases, especially at low pressure, to establish an equal-step Celsius scale.

Explanation:

The purpose of calibrating a constant volume gas thermometer in substances like dry ice and boiling ethyl alcohol is to ensure accuracy across a temperature scale. Such calibration is necessary because different thermometers tend to give varying readings for the same temperature.

By using substances with known boiling or freezing points, like dry ice (-80°C) and boiling ethyl alcohol, a standard thermometer is established, making temperature measurements more consistent and reliable. Gases, especially at low pressures, tend to behave more ideally and consistently compared to liquids and solids, which is why they are chosen as the working fluid in ideal gas thermometers.

Furthermore, gases like the noble gases exhibit relatively consistent thermal expansion ideal for establishing a temperature scale, where every degree represents an equal step in temperature. This ensures that the Celsius scale, defined by the expansion of an ideal gas at low pressure, is not merely comparative but also additive, allowing for consistent and accurate temperature measurements.