= Solution
{c}
No. Near the <liquid-gas critical point>, the <mass densities> of <liquid> and <vapor> approach each other, so the neglected condensed-phase specific volume becomes comparable with the <vapor> specific volume. The <vapor> also ceases to obey the dilute <ideal gas law>. Although the <latent heat> tends to zero, the specific-volume difference tends to zero too: the exact <Clausius-Clapeyron relation> contains their ratio. Setting only the numerator to zero therefore does not justify a vanishing physical <saturation pressure> slope. A realistic <equation of state> and coexistence data are required near the surface; the dilute colder upper <atmosphere> can still be described approximately by the earlier expression.
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