Difference between revisions of "Script:Measuring the melting/freezing point"

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Revision as of 07:00, 16 January 2020

Hi! Good to have you here.

This video is about how to measure the melting and freezing points of a substance.

Now I'm going to put in a disclaimer which will mean nothing to normal people, but will hopefully keep the internet trolls at bay. This series of videos is for people who are interested in chemistry, not metrology, so I'm using appropriate terms and precision for that purpose. Yes, there are triple points and Vienna Standard Ocean Water, and the 2019 redefinition of kelvin, and that may fascinate you, but those values were chosen carefully so that people who are not metrologists can make comprehensible generalizations such as I will use here. Temperatures in this video are plus or minus one degree centigrade. We aim no higher.

While the freezing and melting points of a material are intuitively the same, practically they can differ by measurable amounts. In 1742, a particularly clever Swede named Anders Celsius (1701–1744) showed that while the freezing point of water is dependent on atmospheric pressure, the melting point is essentially unaffected. This is indirectly because liquids are slightly compressible, while solids are much less so. That's why people chose the melting point of ice, rather than the freezing point of water as the zero for the scale - measuring the melting point gave more consistent results.

Nevertheless, this video is at least partially about measuring freezing points, so let's consider that.

Finding the freezing point of a material is more complicated than just putting a thermometer in the material and checking the temperature once it's frozen. If you were to check the temperature on the inside of an ice cube in your freezer, it would be substantially below 0C or 32F. Chances are your household refrigerator's freezer compartment is about -18&degC; or zero Fahrenheit, and the ice in your freezer will be about the same.

We'll start with distilled water. The science books tell us that the freezing point of water is 0°C. And indeed if we maintain a water sample at that temperature for long enough, it will indeed freeze. But how do we show that it wouldn't freeze at 0.001 degrees as well? Or 0.0001 degrees? We could do a large series of tests where we keep the temperature extremely stable at precise temperatures for long periods of time and wait to see if water freezes at temperatures closer and closer to zero, but that would be awkward and always result in a range, rather than a specific value.

Happily, there is a better way, due to a kind of weird physical property of most liquids: the "latent heat of fusion". The latent heat of fusion is an amount of energy that must be shed by the (still liquid) material in order to become solid. If that energy isn't released, the material will not freeze. This means that a liquid can go below its freezing point while still being liquid due to the energy in the liquid itself that hasn't been lost yet. Liquids which are still liquid but below their freezing point are called "supercooled".

So when a small portion of the liquid loses enough energy for a brief period of time, it will solidify and shed the heat of fusion into the remaining liquid, raising the temperature

Inside the bag is a solution of sodium acetate that is supercooled below its rather "warm" freezing point of 54°C. Snapping the disc causes the liquid to become a solid and brings the sodium acetate up to its freezing temperature. Latent heat of fusion: 264–289 kJ/kg.