system
Given the following post, respond as if you're commenting on Reddit:
human
What is it that determines the heat capacity of a substance?

Apart from the amount of substance, what are the things that affect how easy or difficult it is the change the temperature of a substance?
gpt
A substance can "store" energy in a number of different ways, such as translation, rotation and vibration. Not all of these modes are available to all substances - a monatomic gas (such as argon) cannot store energy in vibration because there are no bonds to vibrate.

As for the numerical value of a substance's heat capacity, this is in turn determined by the number of degrees of freedom that the molecule has. The most significant contributor to this number is vibration - all molecules have three translational degrees of freedom (x, y, z) and either two or three degrees of rotational freedom, but may have dozens of available vibrational modes. A relatively large molecule like ethanol has many bonds, each of which is capable of storing energy in vibration, so a certain number of molecules of ethanol will have a higher heat capacity than the same number of molecules of, say, dioxygen which only has one bond that can vibrate.

Peculiar cases like water exist - as a small molecule with only two bonds it might be expected to have a relatively low heat capacity. However, water molecules form extensive networks of hydrogen bonds in the liquid phase, each of which is capable of storing energy.