system
Given the following post, respond as if you're commenting on Reddit:
human
What is enthalpy?

And can we measure it directly ?
gpt
Enthalpy is a thermodynamic state function similar to the internal energy, except the internal energy depends naturally on entropy, **volume**, and the number of particles, whereas enthalpy depends naturally on entropy, **pressure**, and the number of particles.

So it’s just another energy function, but it has the volume dependence exchanged for pressure dependence. This is convenient because pressures are easy to control in lab settings, and they’re often held approximately constant.

That’s why enthalpy is useful for chemistry and fluid dynamics, because the pressure can often be treated as constant. When the pressure and number of particles are constant, the change in enthalpy of some system during some process is just equal to the heat absorbed by the system during that process.