# Why are calorimeters often made of materials with low specific heats?

When building calorimeters, a material with a low specific heat is often used (such as Styrofoam, aluminum, etc.). However, why would a material with a low specific heat be preferred? My thought process is below:

Scenario: An exothermic reaction occurs in a calorimeter.

Principle: In calorimetry, we are always trying to minimize the amount of heat transfer from the calorimeter to the surroundings. The amount of heat transferred from the reaction to the calorimeter doesn't really matter because we can calculate this quantity of heat using the calorimeter's calorimeter constant, which we can derive experimentally.

Case 1: Calorimeter made of material with low specific heat

If the calorimeter had a low specific heat, it would absorb less heat, but its temperature would increase more. Differences in temperature cause heat transfer, so a larger difference in temperature between the calorimeter and the surrounding air would mean more heat transfer from the calorimeter to the surroundings.

Case 2: Calorimeter made of material with high specific heat

If the calorimeter had a high specific heat, it would absorb more heat, but its temperature would not increase much. Differences in temperature cause heat transfer, so a smaller difference in temperature between the calorimeter and the surrounding air would mean less heat transfer from the calorimeter to the surroundings.

Therefore, a calorimeter with a high specific heat would be more effective because it would minimize heat transfer between the calorimeter and the surroundings.

Where is the flaw in my logic?

A good insulator has a higher specific heat capacity because it takes time to absorb more heat before it actually heats up (temperature rising) to transfer the heat.

Actually, a calorimeter with high specific heat would have less temperature change, which is harder to measure. Therefore, the error in the calculated enthalpy of the reaction would be higher. Specific heat does not mean heat conductivity, as described by Fourier's Law.

• Is the low thermal conductivity of Styrofoam and aluminum the main reason why they are used in calorimeters? Also, what is the relationship between specific heat and thermal conductivity? Commented Jun 4, 2018 at 2:30

I don't think designers care about the specific heat of the materials. Their primary considerations are going to be standard engineering considerations, e.g. strength, lightness, cost, ease of fabrication, resistance to corrosion and wear. Aluminum scores well on all of these, so it's a very common material for use in building small devices. Styrofoam can be used in a situation of extreme economy, because it's solid, has moderately low thermal conductivity, and is very easy to fashion into the right shape.

• Aluminum has a very high thermal conductivity, while Styrofoam has a very low thermal conductivity (en.wikipedia.org/wiki/Thermal_conductivity#Experimental_values). Does thermal conductivity matter when designing a calorimeter? Why or why not? Commented Jun 4, 2018 at 2:35
• It certainly matters when designing some parts of the calorimeter, e.g. its thermal insulation. It doesn't matter at all for designing other parts, like its case or inner lining. Commented Jun 4, 2018 at 15:48
• So, is aluminum often used to build the case or inner lining of a calorimeter instead of the thermal insulation of the calorimeter? Commented Jun 6, 2018 at 22:47

There is no flaw in your logic, but there is a bigger flaw behind it. All right, maybe high specific heat would be better. It is just that the said value is less important than heat conductivity. Heat escapes to the surroundings and leaves you with no way to evaluate it. You don't want that to happen. That's when styrofoam comes into play.

Aluminum is another story. You can't make a combustion chamber out of styrofoam. It has to be a metal. Well, then why not use aluminum. Then it is not a material with low specific heat. Compared to other metals, it is higher than many.

So it goes.

I'm not actually a chemist:), but I teach chemistry at a homeschool co op and do have a science background academically. I think what you might be missing in your thought process is clarifying between heat conductivity and change in energy. I was a bit confused on this too. As others have mentioned, heat conductivity is not correlated to specific heat. When you create a calorimeter, if you are trying to make the heat transfer to the calorimeter negligible, you would use something with a low specific heat: q (energy/heat)=(m)(c)(change in temperature) where m=mass of the calorimeter and c= specific heat. So therefore, the lower the specific heat, the lower the overall heat absorption from the item put in the water.