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ZingPath: Applying Gas Laws

Vapor Pressure

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Applying Gas Laws

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Lesson Focus

Vapor Pressure


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You will identify the relationship between external pressure, temperature, and vapor pressure by observing water boiling under various conditions.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that the vapor pressure of a liquid increases as its temperature increases.
  • Explain that boiling point increases as external pressure increases.
  • Explain that vapor pressure is unaffected by changes in external pressure.
  • Define boiling point as the point at which vapor pressure equals external pressure.
  • Explain that boiling point declines with elevation due to a decrease in external pressure.

Everything You'll Have Covered

Most substances can exist in three states depending on the temperature and pressure of the substance. Pressure is the amount of force acting upon an area. Temperature is a measure of the average kinetic energy of particles in a sample of matter. Temperature is measured with an instrument such as a thermometer. A thermometer frequently contains a liquid that expands when heated and contracts when cooled. The tube that contains the liquid is narrow and has graduation marks, so small changes can be detected. One of the scales used to measure changes in temperature is called the Celsius scale. The Celsius scale defines the temperature at which water freezes as zero and the point at which water boils as 100.

Water in an open container will eventually move into the air. The conversion of a liquid to gas or vapor is called vaporization. When this conversion of liquid to gas takes place in a liquid that is not boiling, the process is called evaporation. During evaporation, only molecules with a certain amount of kinetic energy can escape from the surface of the liquid. Kinetic energy is the energy of an object because of its motion.

Water evaporates faster when heated. This occurs because heating the liquid increases the average kinetic energy of the particles. The increased energy allows more particles to overcome the attractive forces keeping them in the liquid state. In a closed container, the vapor above will exert pressure on the liquid below. The measure of the force of the vapor upon the liquid is called vapor pressure.

When a liquid is heated to a temperature at which the particles have enough kinetic energy to vaporize, the liquid boils. In a boiling liquid, bubbles of vapor form throughout the liquid, rise to the surface of the liquid, and escape into the air. The temperature at which the vapor pressure of a liquid is equal to the external pressure is called the boiling point.

External and atmospheric pressure refer to the pressure exerted by the collision of atoms and molecules in the air upon the surface of the Earth. As elevation increases, the pressure decreases. Liquids boil when the vapor pressure is equal to the external pressure. Therefore, liquids do not always boil at the same temperature. Atmospheric pressure is lower at higher altitudes, so boiling points decrease at higher altitudes.

A barometer is a device that is used to measure atmospheric pressure. In a mercury barometer, the height of the mercury is determined by the atmospheric pressure. One of the units used to measure pressure is the standard atmosphere (atm).

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Learners should be familiar with boiling, pressure, and temperature.
Course Chemistry
Type of Tutorial Concept Development
Key Vocabulary atmosphere, atmospheric pressure, barometer