EXPERIMENT: DEW POINT
The atmosphere is usually not saturated with water vapor. The comparison of the actual amount of water vapor in the air to the maximum amount it could hold expressed as percent is called the relative humidity. When air cannot hold any more water, it is said to be saturated or have 100% relative humidity.
Relative humidity is affected by temperature. As air is heated, the molecules of various gases move further apart as the air expands. This permits extra room for water vapor molecules. The figure below shows a set number of water molecules in the air. In cold temperature, the water molecules are crowded by air molecules but in warm temperatures, more space is available between molecules.
Warm air can hold more moisture than cool air. As air is heated with a fixed amount of water vapor molecules in it, the relative humidity decreases. For example, if air is saturated (100% relative humidity) and is heated, it would no longer be saturated because the air molecules would move further apart, and more water vapor could be added. In this case, the relative humidity might decrease from 100% to 90%. (The air is now only 90% saturated). If warm air with a relative humidity of 60% is cooled enough, the relative humidity may reach 100%, or saturation. This has occurred because the air molecules have moved closer together upon cooling. In this case, there is now no more room for any additional water vapor molecules. So the air has reached a point of saturation again at the cooler temperatures. The temperature at which the air becomes saturated with water vapor is called the dew point.
In this investigation, you will attempt to determine the dew point.
OBJECTIVES
Define dew point.
Determine the dew point of the surrounding air.
Collect data and summarize findings.
Supplies Needed
shiny metal can
ice cubes
water
thermometer
stir rod or spoon
Procedure
Record the temperature of the air.
Fill a shiny metal can with water at room temperature.
Slowly add ice to the water and stir gently with stir rod or spoon. Continue to add ice until a film of water appears on the surface of the metal can.
Record the temperature when the dew develops on the can.
Repeat this experiment to confirm the data.
Analysis and Conclusions
Write a scientific summary of your findings. Include your hypothesis, data, observations, and conclusion. Be sure to answers the questions below in your report as well as explaining your method and results.
Why must the metal can be shiny?
Why must the ice be added slowly?
What advantages would be in repeating the experiment several times?
What is the average dew point?
If possible, compare your findings with someone else who did the experiment. How are your results similar or different, and why?
What did you find out from this investigation? Be thoughtful in your answer.
