# Boiling point and vapor pressure

Does a liquid having higher vapor pressure have a lower boiling point? I think so because I read in wikipedia that, "A substance with a high vapor pressure at normal temperatures is often referred to as volatile." A volatile liquid has lower boiling point.

• Do you know the definition of "boiling point"? – SendersReagent Feb 26 '16 at 4:43
• Temperature at which vapour pressure is equal to the external pressure. – chem007 Feb 26 '16 at 5:56
• So would a compound with a higher bp or lower bp reach this first? – SendersReagent Feb 26 '16 at 13:17
• consider two substance A and B.If A has more vapour pressure than B then you can think it as,A has more particles in vapour phase than that B has in the vapour phase.Which means A will attain a vapour pressure which is equal to external pressure more quickly than B.Since A and B are same temperature (say 356K) and since A attains a vapour pressure equal to external pressure more quickly than B =>(implies) A boils quicker than B which implies A has lower boiling point than B. – Abhishek P G Feb 28 '16 at 17:10

## 2 Answers

Not necessarily. The vapor pressure of a liquid (or solid) is the pressure of the vapor in equilibrium with its condensed phase in a closed system. The boiling point of a liquid at a given pressure is the temperature at which the vapor pressure is equal to the given pressure. This means the vapor pressure is a function of the temperature, $P^* = f(T)$, and the boiling point is a function of the pressure, $T_{bp} = f(P)$.

So your question is a bit vague. In general, yes, liquids with higher vapor pressures will have lower boiling points. In any first year chemical engineering course, students are introduced to the Antoine Equation, an emperical relation between vapor pressure and temperature. Plotting the Antoine equation for a set of liquids lets one visualize the relationship between vapor pressure and temperature very quickly; these plots are called Cox charts. For example, examine the Cox chart below: Now look at the relationship between ethanol and benzene. At 50 F, benzene has a higher vapor pressure than ethanol. At pressures below ~10 psi, the general trend holds and benzene has a lower boiling point. However, the lines for benzene and ethanol cross at ~10 psi, so above ~10 psi benzene actually has a higher boiling point than ethanol.

So in general, yes, liquids with higher vapor pressures will have lower boiling points. However, the question is vague and exceptions do exist.

• Wait, so when I out toluene/water on high-vac, the toluene boils first?! Wow. – SendersReagent Feb 29 '16 at 17:35

Yes, the higher the vapor pressure the more volatile it is and the lower the normal boiling point.