Relative Humidity Sponge Lab

Materials List

Each group needs:

• 1 dry sponge
• 1 plate, plastic or aluminum
• 1 syringe
• a cup
• water, 1 cup
• paper and pencil, to keep a tally
• Sponge Saturation Worksheet, one per student
• Relative Humidity Graph, as a handout for each group or each student, or as an overhead projector transparency to show the class
• (optional, if students are graphing data) graph paper, one sheet per student

Background

Under usual Earth conditions, water exists in any of three physical states: solid (ice, snow), liquid (which we typically call water) or gas (humidity, water vapor [steam]). According to meteorologists, humidity can be absolute or relative, giving us the terms absolute humidity and relative humidity. In common usage, the adjective is often dropped, and the speaker (or writer) just says "humidity," expecting us to know of which type they are speaking. Even some television and radio weathercasters, including those formally trained in meteorology, often do not clarify the difference when reporting the weather conditions.

Absolute humidity is defined as the ratio of the mass of water vapor contained per volume of moist air. Relative humidity, technically, is the ratio between the partial pressure of water in the air and the maximum possible vapor pressure of water at a particular temperature. It is dimensionless. Relative humidity is usually what the media announcers mean when they say "humidity," and it is useful in determining conditions for human comfort. But, it can be confusing because its value varies with air temperature. For example, the morning may have a relative humidity of 78%, which by afternoon drops to 53% as the air temperature rises. (Note: the absolute humidity for that day remained essentially constant, but the vapor pressure of water increased with the temperature). Similarly in winter, the outdoor relative humidity may be 63%, but when outdoor air permeates our warm homes and offices, the relative humidity level may drop to 35% or lower. (In this example, the absolute humidity is quite low in the outdoor air, but the vapor pressure of water at cold temperatures is also low, thus, the outdoor air is more humid, relatively speaking.) See Figure 1.

Figure 1. Idealized daily trend of humidity and temperature. The most comfortable weather conditions occur when absolute humidity remains constant and air temperature reaches its minimum as relative humidity reaches its maximum, or vice versa. copyright

Understanding the concept of saturation is important to understanding relative humidity. Saturation is defined as the condition in which air at a specific temperature contains all the water vapor it can hold; 100% relative humidity. At saturation, the partial pressure of water vapor in the atmosphere is at its maximum level for the existing ambient temperature and pressure. At saturation, equilibrium exists between water vapor and liquid water, and there is no net evaporation or condensation. Above saturation, some of the water in the air condenses to form droplets or clouds (collections of droplets). By better understanding humidity, one can better understand local weather. Given the temperature of a volume of air and its pressure, we can determine a saturation value. We can saturate a parcel of air by adding more water vapor to it (through evaporation or mixing with another parcel of more humid air), or by cooling the parcel down to its saturation temperature. Both processes are at work continually in the atmosphere, but the latter is more familiar to us as it forms fog or dew (or frost if cold enough). In this activity, we talk about the temperature of the air affecting the vapor pressure, but it is technically the temperature of the water molecules that determine vapor pressure. For a good discussion of this, see Professor Alistair Fraser's comments at http://www.ems.psu.edu/~fraser/Bad/BadClouds.html.

The saturation temperature of the ambient air is commonly called the dew point temperature or simply the dew point (or frost point if it is below freezing). You might hear a weathercaster or meteorologist discuss the dew point of a particular air mass. Dew point, like absolute humidity, varies little within an air mass. When the ambient air temperature equals the dew point, the relative humidity is 100% and the air is saturated. If the air temperature falls lower, water vapor begins to condense into very small liquid droplets to form clouds or fog (if it is near the ground). If the surface temperature of an object (vegetation, rooftop or car exterior) falls below the dew point while most of the surrounding air remains above it, dew forms through the condensation of water vapor onto that surface (or ice crystals, we call it frost, if it is below freezing).

Vocabulary/Definitions

absolute humidity: The ratio of the mass of water vapor contained per volume of moist air.

ambient: Surrounding or encircling, such as ambient air.

dew point: The saturation temperature of ambient air. Also called dew point temperature.

humidity: Dampness; the amount of water vapor in the air.

meteorologist: A scientist who studies meteorology (the atmosphere, weather and weather forecasting).

partial pressure: The pressure that one component of a mixture of gases would exert if it were alone in a container.

relative humidity: The ratio between the partial pressure of water in the air and the maximum possible vapor pressure of water at a particular temperature; it is dimensionless.

saturation: The condition in which air at a specific temperature contains all the water vapor it can hold; 100% relative humidity. The condition when the partial pressure of water vapor in the atmosphere is at its maximum level for the existing ambient temperature and pressure. At saturation, equilibrium exists between water vapor and liquid water, and there is no net evaporation or condensation.

Pre-Activity Assessment

Brainstorming: As a class, have students engage in open discussion. Remind them that no idea or suggestion is "silly." All ideas should be respectfully heard. Write their ideas on the board. Ask the students:

• What is humidity?
• How does it affect the weather?
  • How are clouds formed? (Answer: Warm air near the land (or water) surface rises. This rising, and subsequent pressure drop, results in the air expanding, thereby using some energy of the molecules, and cooling the air. At these lower temperatures, some of the water vapor in the air condenses to form clouds or rain. See Figure 3 for more information.)

• Figure 3. The process of cloud formation.

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Before the Activity

• Gather materials and make copies of the Sponge Saturation Worksheet.
• Find the day's humidity measurement.

Procedure

1. What is humidity? How does it affect the weather?
2. In groups of four students gather supplies (1 dry sponge on a plate, a cup of water, 1 plastic spoon and a copy of the worksheet; see Figure 2).
3. Squeeze the sponges to verify that they contain no water. How much water is in the sponge? (Answer: None)
4. 5 CC or mL at a time, slowly and carefully pour water onto the sponges. Count how many milliliters of water are being added; it helps to have one group member keep a tally (see Figure 1).

• How does the water dripping from the sponge act like rain or a cloud? Air can "hold" water, too, similar to the sponge. Ask if anyone knows another name that means something is full (of water, like the air or the sponge). Share the word "saturated." Explain that when something (air or the sponge) is full (of water), we say that it is saturated.

sponge full (100% saturated) = ___ mL of water

1. How many milliliters did it take to reach 100% saturation of your team's sponge, is it similar or different from other groups? If so, why? (If differences among the group data; lead a discussion about why the differences exist. Does it have to do with experimental apparatus, experimental procedure, etc.?)
2. Record in your data table the number of milliliters to reach 100% saturation.

# of milliliters ÷ total # of milliliters for saturation X 100% = percent saturation (%)

Example calculation:

If the sponge was fully saturated with 15 milliliters of water, for 10 milliliters, the calculation is

10 ÷ 15 X 100% = 66% saturation

1. How many milliliters of water had been poured into the sponge when it was halfway full of water?
   1. sponge half full (50% saturated) = ___ milliliters of water
2. How many milliliters of water were in the sponge when the sponge was empty?

sponge empty (0% saturated) = 0 milliliters of water

1. Complete your data tables with 0 milliliters of water and 0% saturation.
2. Write the number of milliliters (up to the 100% saturation number) and calculate each of the corresponding % saturation.
3. Refer to your data tables (and/or graphs) to answer the following questions:

• How many milliliters of water would be in the sponge if the sponge were nearly ready to drip, but not dripping yet?
• How many milliliters of water would be in the sponge if it were nearly empty, but not totally empty?
• Make other predictions from their data/graphs (such as, how many milliliters are required to reach 40% saturation? What would happen if you added more milliliters than are required to reach 100% saturation? etc.)

1. You just used a scale to measure/predict how much water is in your sponge and how close they are to dripping. Meteorologists can measure how close is it to raining by using a humidity scale for water present in the air.
2. Refer to Relative Humidity Graph
3. Make comparisons to the data (and graph) just collected. (Note that the graph handout is much more complex and is dependent on temperature.)
4. Conclusion Questions:

• If it is almost ready to rain, what is the humidity? (Expect them to inquire: Are you talking about relative or absolute humidity? Answer: Close to 100% relative humidity.)
• If the air is nearly, but not completely dry, what is the humidity? (Answer: Lower, but it depends on the temperature.)
• If the air was halfway full of water or halfway saturated, what is the humidity? (Answer: Encourage them to look along the 50% relative humidity line and see how it changes with temperature. Notice the dew points at each of these temperatures.)
• Why is humidity important to engineers? (Answer: Because it helps to predict the weather, which affects pollutant transport and concentrations, and the amount of water in the air affects how difficult/easy it is to remove pollutants from the air. Engineers are also concerned with humidity levels indoors. Buildings with high humidity are not good for storing food or electronics equipment; even copier paper starts to curl. Laboratories also control humidity levels so that the moisture does not affect their experiments. High humidity levels also cause bacteria and molds to grow, which can affect indoor air quality.)

Adapted from: https://www.teachengineering.org/activities/view/cub_air_lesson04_activity3