The Science Behind Cold Packs - Part 1: What the heck is the Latent Heat of Fusion?
By the end of this brief article your old high school chemistry / physics teacher will be proud of you. We’re going to walk through a concept in material science that explains why cold packs, or just plain old ice, keep stuff cold.
Let’s start with a painfully obvious relationship between heat and temperature.
As shown above, as the amount of energy or heat you put into a substance increases, the temperature of the substance increases. Painfully obvious, we know, but bear with us. The relationship will hold true until you hit a phase change or a change in the substance’s state of matter (think solids transitioning into liquid as an example), then something odd starts to occur and the relationship starts to look like this:
Even though we have continued to add energy to the substance, temperature is not increasing. Interesting! We add heat, but no temperature increase! What’s going on here?
Said in a very basic way, energy is being absorbed during this phase change. The energy is being spent on overwhelming the bonds that hold together the molecules of the solid, thus converting the substance into a liquid. Not to get too Thermodynamic-y but when we say the energy is “spent” we do not mean lost, it is getting converted into potential energy that is contained in the liquid. Liquids, as a state of matter, have higher potential energies than their solid counterparts.
The amount of heat required to do this phase change is called the Latent Heat of Fusion, or the enthalpy difference between solid and liquid states. In physics articles you might see Latent Heat of Fusion abbreviated at LF, chemistry it’s usually ∆HF. Again, the energy needed to transition a substance from solid to liquid is the Latent Heat of Fusion. All else equal, the greater the Latent Heat of Fusion, the more heat that will be absorbed, the better the cold pack will perform. The cold pack is acting as the heat sink, the buffer that absorbs the ambient heat so that your food does not.
Once the phase change is complete we see our typical heat/temperature relationship resume:
But let’s focus on this dotted line representing the Latent Heat of Fusion and use an example to cement our understanding. Let’s use water.
Here we see our substance starting off as ice but when it gets to 0C /32F, it starts to melt, absorbing energy and maintaining a constant temperature. When all the ice is melted and we have completed the phase change to liquid, we once again see temperature rise as heat is added. Water has a latent heat of fusion equivalent to 334 J/g. That’s joules (a measurement of heat) per gram (a measurement of mass). It’s important to keep in mind that mass plays an important roll in the total heat of fusion: the more ice (more mass) we have the more heat we are able to absorb.
By the way, this same concept can be related to Dry Ice except that instead of the Latent Heat of Fusion, we would be talking about the Latent Heat of Sublimation as the solid form of carbon dioxide transitions directly to a gas form and absorbs a lot of energy along the way. Dry ice has a Latent Heat of Sublimation equivalent to 591 J/g, almost twice that of water’s Latent Heat of Fusion. Dry ice is not always the better refrigerant however as it is classified as a hazardous substance during shipping, it releases CO2 into the atmosphere, its expensive, and it may keep your perishables a lot colder than you want.
But back to Latent Heat of Fusion. Latent Heat of Fusion varies depending on the material and the nature of the bonds and structure of that material’s solid phase. Each material has its own Heat of Fusion. This emphasizes the importance of material selection and formulation when creating a gel for cold packs. We need to stretch our dotted line as long as possible so as to absorb the most heat as possible. At Minus Works we produce cold packs using a hydrogel material that optimizes for the Latent Heat of Fusion, allowing us to keep perishables colder, longer.
Want more science? Check out Part 2 of this Series: Designing a High Performance Cold Pack.Interested in Cold Packs for your application? Reach out to our sales team.