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. read more
There are no ideal gases. All of them somewhat deviate from ideal gas behavior. Water vapor is very good ideal gas at room temperatures. Its density is significantly lower than critical density. Critical density of water is about 0.3 g/cm³. read more
involved in treating water vapor as an ideal gas is calculated and plotted in this picture . It is clear from this figure that at pressures below 10 kPa, water vapor can be treated as an ideal gas, regardless of its temperature, with negligible error (less than 0.1 percent). read more