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Exploring Freezing Point Depression with an Image of a Thermometer

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This post was written by Lesley Anderson, 2021-2022 Albert Einstein Distinguished Educator Fellow at the Library of Congress.

Phase changes can be one of the first chemistry concepts that students learn. Even in elementary school, students explain how solids become liquid above 0 degrees Celsius. For example, solid ice melts into liquid water when it is removed from a freezer. But before students can dive into phase change diagrams, they need to be able read measurements from a thermometer and identify key temperatures at which phase changes will occur.

Image of a thermometer documenting specifc temperature reading such as the boiling point of water at normal pressure
Page 30 of the book Measurements for the Household. Massachusetts. Commissioner of Standards, 1916

Students can conduct a primary source analysis with this image of a thermometer published in a catalog from the Commissioner of Standards, Massachusetts, 1916. Ask students to observe the similarities and differences between the left and the right side of the thermometer. Ask them to reflect on what the difference in these two scales might be. Record what students wonder about this image. Students may ask questions like:

  • This thermometer uses “centigrade” instead of Celsius. What’s the difference?
  • Is there a place on a thermometer where the temperature measurement for Fahrenheit and centigrade is the same?
  • Why are the temperature of pasteurizing milk, the freezing point of mercury, and the melting point of solder listed as important measurements for the household?
  • Why is the temperature of a room without exercising people recorded as a fixed temperature? Wouldn’t this be more likely to be a range?

This primary source could be used to introduce students to measurement tools, understanding scale, calculating measurements through dimensional analysis, or phase changes in chemistry.

Consider this primary source. At a first glance, students may observe the box in front of the man and reflect that he is playing chess. Upon closer investigation, and by providing information like the title of the source from the bibliographic record, students may begin to wonder what experiment Dr. Dorsey was conducting.

Ask students to graph the measurement of Dr. Dorsey’s sample that remained liquid at -6 degrees Fahrenheit onto the first image of the thermometer. Students should be sure that they have plotted the point on the correct side of the scale. This will enable students to visualize just how cold liquid water remained in Dr. Dorsey’s supercooling experiment.

This primary source analysis can be extended by diving into the process of understanding supercooling, or even getting students to understand how adding a solute to water can lower the freezing point. For example, adding salt to roads during icy weather causes water to re-freeze at a lower temperature, making the roads safer for drivers. For a concrete experience, students can make ice cream using salt to lower the freezing point. Students can record the temperature of the ice before and after adding the salt and then they can plot the measurements from their ice cream experiment on the thermometer primary source to see how it compares to the supercooling and other common household measurements.

How might you use these primary sources with your students?

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  1. Freezing point depression is the phenomenon where the freezing point of a solvent is lowered when a solute is added to it. The amount of depression depends on the concentration of the solute. This concept can be explained using the image of a thermometer.

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