Why doesn't water vapor behave like an ideal gas?

Answer 1

Water does not behave exactly like an ideal gas because water is a polar molecule. And, the polar molecules of water, if properly oriented in space, have strong attraction to one another, and may form a hydrogen bond and condense back into liquid form. Also, when a gaseous water molecule has a collision with liquid water, if it is oriented in the right way (i.e. an oxygen atom comes into contact with a hydrogen atom), it will move back into the liquid phase. So, because water molecules move back into the liquid phase much more easily than ideal gas molecules, water vapor does not behave exactly as an ideal gas would.

Answer 2

Water vapor deviates from ideal gas behavior at high pressures and low temperatures due to intermolecular forces like hydrogen bonding. These forces cause water vapor to have higher than expected compressibility and heat capacity compared to an ideal gas. Additionally, water vapor can condense into liquid water or freeze into ice, further complicating its behavior.