Heat storage in materials

Even when eating a simple bitterball, a burnt tongue can reveal that a large amount of heat is stored in the filling, while the crispy crust feels a lot cooler. Even though the entire bitterball has been subjected to the same heat treatment in the deep fryer or oven, there is more heat in the filling than at the outside. A painful – and perhaps recognisable – way to find out that different materials apparently can store – and release – different amounts of heat …

Heat capacity
If you supply heat to a (solid) material, for example in an oven, the material takes over the heat from the oven and the atoms of the (solid) material will vibrate harder and harder in their position, which manifests itself in a higher temperature. The absolute amount of material is important here: 2 kilograms of material can absorb twice as much heat as 1 kilogram of the same material. Moreover, different materials respond in different ways to this heat supply. If you supply the same amount of heat to different materials, you will observe that the temperature of these materials rise, but to a different extent for each material. The heat capacity is the ability or capacity of a material to store heat. The specific heat – also known as specific heat capacity – is a material property that indicates how much heat you have to supply to 1 kilogram in order for the material to rise 1 degree Celsius in temperature. The word ‘specific’ here refers to the kilogram of material to which the heat capacity relates. The fact that this specific heat capacity is material dependent has little to do with the difference between the types of atoms, but indeed with the amount of atoms. For example, 1 kilogram of the ‘light metal’ magnesium contains more than 8.5 times as many atoms as 1 kilogram of the ‘heavy’ lead. By way of comparison, the specific heat capacity of magnesium is 1023 and of lead 129 J/kg.C at room temperature. At the same temperature, 1 kilogram of magnesium can therefore retain almost 8 times as much heat as 1 kilogram of lead. But this factor of 8 in both cases is obviously not coincidental. Heavy metals – i.e. high-density metals – have a lower specific heat capacity than lighter metals. Because heavy atoms are associated with a low specific heat capacity, you can understand that normalised to the number of particles – so expressed per atom or per mole of atoms – the heat capacity has a constant value. And indeed, around and above room temperature many solids have a constant specific heat capacity of about 25 joules of absorbed heat energy per mole of atoms per degree Celsius temperature rise.

If you compare a pure metal with its alloy, then their specific heat capacities do not differ much. After all, the specific heat capacity depends on the amount of atoms per kilogram of metal, and that amount will not change significantly when a few percent of ‘foreign’ metals or other atoms are added. On the other hand, the thermal conductivity of a pure metal changes largely when you make an alloy out of it. Metals are good heat conductors if they have a regular lattice, and any distortion of this lattice – here due to the incorporation of foreign atoms – reduces the (heat) conductivity.