Intro Lesson: Collecting Gas Over Water

The total pressure of a mixture of gases is equal to the sum of all of the gases combined

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Dalton's Law of Partial Pressure

• When a liquid, such as water, is sitting in a closed container some of the liquid evaporates into a gas.

• The gas above the liquid exerts a "pressure" on the container.

• This pressure is called "vapor pressure".

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What is Vapor Pressure (Pvap)

• Notice in the chart to the left that as the temperature of the water increases the vapor pressure of the water increases.

• This makes sense because at higher temperatures more water evaporates)

The of Vapor Pressure of water is dependent upon the temperature of the water

• What you see to the far left is a picture of an inverted graduated cylinder full of water.

• Notice, as you move from left to right, the volume of the water is decreasing.

• This is due to a gas being pumped into the system through the brown tube

• The new gas displaces the water and pushes it down.

• The area on top of the water now contains the gas produced AND some water vapor (a mixture of gases!!!)

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​Collecting Gas Over Water

• Atmospheric pressure is caused by the atmosphere pushing down on

  you or a container.

• ​The total pressure inside a gas collected over water is EQUAL to the atmospheric pressure.

• The pressure of the gas collected is equal to the atmospheric pressure minus the vapor pressure.

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Atmospheric Pressure
and collecting a gas
over water