Script:Measuring the melting/freezing point

Intro

Hi! Good to have you here.

This video is about how the melting and freezing points of a substance are measured.

My initial tests will involve water, since it was the basis of temperature measurements for several centuries. Later I'll show a practical application of something called supercooling. Paradoxically, we'll use a supercooled material to produce heat on demand. Stay tuned.

Troll Repellant

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 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 here. Temperatures in this video are plus or minus one degree centigrade. I aim no higher.

Overview

Most materials we come in direct contact with have three primary physical states, which correspond to specific temperature ranges: Below their freezing points they are solid, between their melting and boiling points they're solid, and above their boiling points they are gaseous. If you want to know about plasmas and einstein-bose condensates, you're watching the wrong channel. Likewise if you want to discuss glassy amorphous solids and things that sublimate rather than melt. Such subjects are beyond the scope of this video.

While the freezing and melting points of a material are intuitively the same, in practice 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.

Freezing Points

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°C 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".

Supercooling

The latent heat of fusion is an amount of energy that must be shed by the (still liquid) material in order to become solid. It kind of makes sense. It's going from an energetic and disordered state to a less energetic and highly ordered state. This displeases entropy, who charges a heat tax for materials which do so. If that energy isn't released, the material will not freeze. Practically 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 of the liquid a bit, so that it is precisely at the freezing point. Then the liquid sheds some heat to the environment, and a bit more of the liquid joins the (low energy, high order) state, which and heats the rest, keeping the temperature precisely at the freezing point during the entire crystallization process. Once the entire mass is solid, the temperature will begin to reach equilibrium with its environment.

This long stay at precisely the freezing point of the material is what we're going to look for.

The Freezing Point of Water

This is a small vial of distilled water, taken as a sample from another video. We're going to put a thermocouple in it to track the temperature, chuck it in the freezer, and map the temperature over time.

• Insert relevantvideo and graph

Here, we see the temperature drop well below the freeing point of water, but if we were to look at it, it would still be a liquid.

Here, we see the temperature rises to almost exactly zero degrees, and it stays flat at that temperature for X minutes. If we peaked at it now, we'd see that some of the water was frozen, but there was still some liquid water in the vial.

Lastly, we see the temperature drop once again, and keep falling until it reaches the ambient temperature of the freezer. At this point it is completely solid.

Since this plateau is so clear and so flat for so long, we have an excellent measurement of the exact freezing point of our material: zero degrees, just as we expected. That's a relief.

Melting points

Unsuprisingly, we see a similar plateau in reverse as we melt a solid. Essentially the process happens in reverse: the temperature climbs to the melting point, and holds there while the crystal dissolves, absorbing energy from the liquid all the while maintaining the temperature at the melting point

The Melting Point of Water

First we take a temperature reading of the ambient temperature of the fridge. We find it's at X degrees. Now we take the same small vial of water we used to measure the freezing point of water, and pop it in the fridge. As it slowly warms up, we'll measure the temperature over time and see what comes of it.

• Video

And again we see the plateau, this time in reverse. The temperature rises from below 10 degrees up as far as zero, then flattens out. During this time, the crystals of ice are absorbing energy from the surrounding environment, and beginning to break the crystalline form up into a liquid. The temperature stays there for the entire dissolution of the ice, then resumes its rise to the ambient temperature of the fridge. And once again, we see that the plateau temperature is at zero degrees, giving us a precise melting point.

Heat pack

Inside the bag is a solution of sodium acetate trihydrate 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.