system
Given the following post, respond as if you're commenting on Reddit:
human
If you have a single molecule of a substance, what state of matter is it in?

If you had say 1 molecule of water is it a solid, liquid, or gas and how do you tell? What is the minimum number of molecules required for something to be classified as a solid liquid or a gas?
gpt
There seems to be a lot of dicussion about how to define temperature and pressure for a single molecule. Though the simple answer is that it would be in a gas phase. Why? Well consider an ideal gas is defined as the particles having *no* intermolecular interactions, it's not that big of a stretch to realise that when you have only a single molecule you actually have an ideal gas.

The temperature in such a single molecule in a box would be defined based on entropy and energy. The entropy (S) is proportional to the logerithm of number of available quantum states at a fixed energy (U), and the temperature is then given by T=1/(dS/dU). The pressure could be found through similar paths, but the easiest is just to assume an ideal gas and take p=n R T/V, where we know both n and T.