Question

what is critical constants?

Answer 1

the physical constants that express the properties of a substance in its critical state

Answer 2

Critical constants are values specific to a substance, for example it's volume, temperature, density, etc. Within these categories, though, are additional meanings. For example, critical temperature of a particular gas means that above that temperature, the gas can't be liquefied no matter what the pressure is.

The critical density is the density of a substance when it is at its critical temperature and pressue.

Hope this helps!