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13.11: Vapor Pressure Curves

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  • One of the first lessons in cooking is how to boil water. Yes, it sounds simple, but there are a couple of hints that speed things up. One hint is to put a lid on the pot. The picture above has water boiling uncovered with the steam escaping to the atmosphere. If the lid is on the pot, less water will be boiled off and the water will boil faster. The buildup of pressure inside the pot helps speed up the boiling process.

    Vapor Pressure Curves

    The boiling points of various liquids can be illustrated in a vapor pressure curve (figure below). A vapor pressure curve is a graph of vapor pressure as a function of temperature. To find the normal boiling point of liquid, a horizontal line is drawn from the left at a pressure equal to standard pressure. At whatever temperature that line intersects the vapor pressure curve of a liquid is the boiling point of that liquid.

    Figure 13.11.1: Vapor pressure curves.

    The boiling points of liquid also correlate to the strength of the intermolecular forces. Recall that diethyl ether has weak dispersion forces, which meant that the liquid has a high vapor pressure. The weak forces also mean that it does not require a large input of energy to make diethyl ether boil and so it has a relatively low normal boiling point of \(34.6^\text{o} \text{C}\). Water, with its much stronger hydrogen bonding, has a low vapor pressure and a higher normal boiling point of \(100^\text{o} \text{C}\).

    As stated earlier, boiling points are affected by external pressure. At higher altitudes, the atmospheric pressure is lower. With less pressure pushing down on the surface of the liquid, it boils at a lower temperature. This can also be seen from the vapor pressure curves. If one draws a horizontal line at a lower vapor pressure, it intersects each curve at a lower temperature. The boiling point of water is \(100^\text{o} \text{C}\) at sea level, where the atmospheric pressure is standard. In Denver, Colorado at \(1600 \: \text{m}\) above sea level, the atmospheric pressure is about \(640 \: \text{mm} \: \ce{Hg}\) and water boils at about \(95^\text{o} \text{C}\). On the summit of Mt. Everest the atmospheric pressure is about \(255 \: \text{mm} \: \ce{Hg}\) and water boils at only \(70^\text{o} \text{C}\). On the other hand, water boils at greater than \(100^\text{o} \text{C}\) if the external pressure is higher than standard. Pressure cookers do not allow the vapor to escape and the vapor pressure increases. Since water now boils at a temperature above \(100^\text{o} \text{C}\), the food cooks more quickly.

    Figure 13.11.2: Pressure cooker.

    The effect of decreased air pressure can be demonstrated by placing a beaker of water in a vacuum chamber. At a low enough pressure, about \(20 \: \text{mm} \: \ce{Hg}\), water will boil at room temperature.

    Summary

    • A vapor pressure curve is a graph of vapor pressure as a function of temperature.
    • Boiling points are affected by external pressure.

    Contributors

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.