AP Chemistry Vapor Pressure

Why some liquids evaporate faster, why liquids boil, and how to read the curves, in four worked examples.

Vapor pressure questions are intermolecular forces questions in disguise. If you can name the forces in two liquids, you can rank their vapor pressures, boiling points, and how quickly they evaporate, all from one comparison. The Unit 3 review mentions vapor pressure in passing, so this page gives it the practice it deserves, including a chart you can read numbers from. It stays qualitative, in line with how the course treats the topic, and uses no vapor pressure equations.

What Vapor Pressure Is and What Controls It

In a sealed container with a liquid, molecules keep escaping from the surface while others return from the vapor. When the two rates match, the system is in dynamic equilibrium, and the pressure of the vapor above the liquid is its vapor pressure. The idea of an equilibrium with balanced opposing rates is the one developed in Unit 7. Only two things set the value:

The intermolecular forces. Stronger forces keep molecules in the liquid, giving a lower vapor pressure and a higher boiling point. Weaker forces do the opposite.
The temperature. A higher temperature puts more molecules in the high-energy tail of the energy distribution, so vapor pressure rises, and steeply, not in a straight line. The same distribution is shown on the kinetic molecular theory page.

A liquid boils when its vapor pressure equals the external pressure. The normal boiling point is the temperature at which the vapor pressure reaches 1 atm, 760 torr.

Worked Example 1: Ranking Two Liquids With the Same Formula by Vapor Pressure

Ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃) both have the formula C₂H₆O. Which has the higher vapor pressure at the same temperature, and which has the higher boiling point?

Ethanol has an O–H bond, so its molecules form hydrogen bonds, plus dipole-dipole and dispersion forces.
Dimethyl ether has no O–H bond, so it cannot hydrogen bond with itself. It has dipole-dipole forces and dispersion forces of similar size.
Ethanol's stronger forces hold its molecules in the liquid, so dimethyl ether has the higher vapor pressure and ethanol has the higher boiling point (about 78°C against −25°C).

Same formula, different structures: the comparison turns on which forces each molecule can form, the skill practiced in London dispersion forces and molecular polarity.

Worked Example 2: Reading Vapor Pressure Curves for Water and Ethanol

Use the graph to compare the two liquids at 25°C, read each normal boiling point, and explain the order.

At 25°C: ethanol's vapor pressure is about 59 torr and water's is about 24 torr, so ethanol is higher.
Normal boiling points: where each curve reaches 760 torr, ethanol at about 78°C and water at 100°C.
Explanation: both hold their molecules with hydrogen bonds, but each water molecule has two O–H bonds and two lone pairs, so it can form more hydrogen bonds than ethanol, whose molecule has one O–H and a nonpolar ethyl group. Stronger overall attractions give water the lower vapor pressure and higher boiling point.

Worked Example 3: Finding the Boiling Point at a Lower External Pressure

In a city where the atmospheric pressure is 630 torr, at what temperature does water boil, and why is it not 100°C?

A liquid boils when its vapor pressure equals the external pressure. On the water curve, the vapor pressure reaches 630 torr at about 95°C, so water boils there. At 100°C the vapor pressure is 760 torr, which is needed only when the surrounding pressure is 760 torr.

The effect runs both ways. A pressure cooker raises the pressure above the liquid, so the water must reach a temperature above 100°C before it boils, and the hotter water cooks food faster. Water at its boiling point stays at a constant temperature while it vaporizes, the plateau seen on a heating curve.

Worked Example 4: Judging Claims About Amount, Volume, and Temperature

A student states that (a) adding more liquid to a sealed flask at constant temperature raises the vapor pressure, and (b) heating the flask raises it. Evaluate each claim.

(a) False. At a fixed temperature the equilibrium vapor pressure depends only on the substance and the temperature. More liquid, or a larger flask with some liquid still present, does not change it.
(b) True. Heating raises the fraction of molecules with enough energy to escape, so the vapor pressure increases, and by a large factor for even a modest temperature rise.

A full-credit explanation names the particle behavior: more molecules reach the energy needed to leave the liquid, until the rates of evaporation and condensation balance again at a higher vapor pressure.

Five Vapor Pressure Practice Questions With Answers

Which has the higher vapor pressure at 25°C, hexane (C₆H₁₄) or water? Explain briefly.
Show answer
Hexane. It is nonpolar with only dispersion forces, which are weaker overall than the hydrogen bonding in water, so more hexane molecules escape the liquid.
Which has the higher boiling point, propane (CH₃CH₂CH₃) or ethanol? Explain.
Show answer
Ethanol. It forms hydrogen bonds, while propane has only dispersion forces, so ethanol needs a higher temperature before its vapor pressure reaches the external pressure.
Why does food cook faster in a pressure cooker?
Show answer
The higher pressure raises the boiling point of water, so the water and food reach a temperature above 100°C before boiling.
What happens to the vapor pressure of a liquid when the temperature increases, and is the change linear?
Show answer
It increases, and not linearly: the curve rises more and more steeply as the temperature goes up.
A liquid has a vapor pressure of 760 torr at 80°C. What is its normal boiling point?
Show answer
80°C, because the normal boiling point is the temperature at which the vapor pressure equals 1 atm (760 torr).

Common Vapor Pressure Mistakes

Saying a high vapor pressure means a high boiling point. It is the opposite: a high vapor pressure means weaker forces and a lower boiling point.
Thinking the amount of liquid changes the vapor pressure. At a given temperature it does not.
Treating boiling and evaporation as the same thing. Evaporation happens at the surface at any temperature; boiling happens throughout the liquid when vapor pressure equals the external pressure.
Describing the temperature dependence as linear. The curve gets steeper as temperature rises.
Forgetting that external pressure sets the boiling point. Water does not boil at 100°C everywhere; it does so only at 760 torr.

Vapor pressure differences are also the working principle behind distillation, one of the separation methods in Unit 3. To see how your preparation for the unit might translate into a score, the AP Chem Score Calculator is a quick way to check.

Frequently Asked Questions

What is vapor pressure?

Vapor pressure is the pressure exerted by the vapor above a liquid when evaporation and condensation are happening at the same rate in a closed container. It depends on the substance and the temperature.

How are vapor pressure and intermolecular forces related?

Stronger intermolecular forces hold molecules in the liquid more tightly, so fewer escape into the vapor and the vapor pressure is lower. Weaker forces give a higher vapor pressure.

How does temperature affect vapor pressure?

Vapor pressure rises with temperature, and it rises steeply rather than in a straight line. A higher temperature puts a larger fraction of molecules at or above the energy needed to escape the liquid.

Why does a liquid boil when its vapor pressure equals the external pressure?

At that point vapor can form bubbles inside the liquid without being crushed by the surrounding pressure, so vaporization occurs throughout the liquid instead of only at the surface.

Does adding more liquid raise the vapor pressure?

No. At a fixed temperature the equilibrium vapor pressure depends only on the substance and the temperature, not on the amount of liquid or the container volume, as long as some liquid remains.

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