While this provides justification of the linear trend in fig. , it is misleading and can cause confusion to the undergraduate student learning the material for the first time. At low pressures well removed from the critical point, it is reasonable to assume ∆Z vap ≈ 1. However, at the critical point where the two phases cease to exist, ∆Z vap → 0. Therefore, ∆Z vap decreases with increasing temperature. Likewise, ∆H vap is not constant as evident by the Watson equation typically introduced in the undergraduate curriculum . Below the critical point ∆H vap > 0, and at the critical point ∆H vap → 0. We therefore find also that ∆H vap decreases with increasing temperature. Knowing this, the previous approach used to arrive at eq. ( ) is less than satisfying, even though it appears to agree with fig. .