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TE Activity: Dripping Wet or Dry as a Bone?

Students use a sponge and water model to explore the concept of relative humidity and create a percent scale.

Understanding the science of humidity is important to many engineers. Environmental engineers consider the specific weather and atmospheric conditions of an area when researching air pollutant transport and concentrations. Industrial engineers find that the amount of water in the air affects how difficult or easy it is to remove pollutants from the air. Civil engineers carefully control indoor humidity levels, as needed; high humidity can affect electronics equipment, paper (starts to curl), food storage and lab experiments (mold growth), in addition to creating an uncomfortable work environment and moist conditions that cultivate poor air quality.

Learning Objectives
Materials
Introduction/Motivation
Vocabulary
Procedure
Attachments
Safety Issues
Troubleshooting Tips
Assessment
Extensions
Activity Scaling
References

Grade Level: 5 (4-6)	Group Size: 4
Time Required: 45 minutes	Activity Dependency : None
Expendable Cost Per Group : US$ 3
Keywords: air pollution, air, pressure, environment, humidity, weather

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Colorado Math
- 1. represent, describe, and analyze patterns and relationships using tables, graphs, verbal rules, and standard algebraic notation; (Grades 5 - 8) [2005]
- 1. read and construct displays of data using appropriate techniques (for example, line graphs, circle graphs, scatter plots, box plots, stem-and-leaf plots) and appropriate technology; (Grades 5 - 8) [2005]
- 1. estimate, use, and describe measures of distance, perimeter, area, volume, capacity, weight, mass, and angle comparison; (Grades 5 - 8) [2005]
Colorado Science
- Standard 1: Students understand the processes of scientific investigation and design, conduct, communicate about, and evaluate such investigations. (Grades 0 - 12) [1995]
- Standard 5: Students know and understand interrelationships among science, technology, and human activity and how they can affect the world. (Grades 0 - 12) [1995]
- Standard 6: Students understand that science involves a particular way of knowing and understand common connections among scientific disciplines. (Grades 0 - 12) [1995]
- 4.2 Students know and understand the general characteristics of the atmosphere and fundamental processes of weather. (Grades 0 - 12) [1995]
- 2.2 Students know that energy appears in different forms, and can move (be transferred) and change (be transformed). (Grades 0 - 12) [1995]

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.

A graph showing both temperature and humidity on the y-axis vs. time (midnight to midnight) across the x-axis.

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 it maximum, or visa versa.
click for 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. 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).

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 or dew point temperature:	The saturation temperature of ambient air.
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.

Before the Activity

Gather materials and make copies of the Sponge Saturation Worksheet.
Find the day's humidity (sources: newspaper weather page, television or radio weather broadcast, or Internet weather website)

With the Students

Ask students: What is humidity? How does it affect the weather? Have them brainstorm answers and write them on the board.
Divide the class into groups of four students each. Distribute supplies (1 dry sponge on a plate, a cup of water, 1 plastic spoon and a copy of the worksheet; see Figure 2).
Have the students squeeze the sponges so they can see there is no water in them. Ask the students how much water is in the sponge? (Answer: None)
One spoonful at a time, have the students slowly and carefully pour water onto the sponges. Have the students count how many spoonfuls of water are being added and have one group member keep a tally (see Figure 1).

Two photographs show the activity setup before and after the experiment. The "after" sponge that has water in it is much darker than the dry one.

Figure 2. Sponge and water model activity, before and after.
click for copyright

After a few spoonfuls, stop the students and ask them the following questions: What is happening to the water? Where is it going? What do you think is going to happen as you keep filling the sponge with water? Can you put water into this sponge forever? Will we be counting forever?
Have the students resume adding water to the sponge by the spoonful until the sponge starts to drip water (reaches saturation). Ask the students to explain what has happened to the sponge? Why is water dripping from it?
Explain to the students that the sponge is like the air. Ask the students: How does the water dripping from the sponge act like rain or a cloud? Explain that 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. Write the following statement on the board:

Sponge full (100% saturated) = ___ spoonfuls of water

Ask the students to share how many spoonfuls it took to reach 100% saturation of their team's sponge. (There may be differences among the group data; this is a good time to have a discussion about why the differences exist. Does it have to do with experimental apparatus, experimental procedure, etc.?) Have the students record the number of spoonfuls to reach 100% saturation on their worksheet.
Ask the students if they know how many tablespoons of water had been poured into the sponge when it was halfway full of water? Write the following statement on the board:

Sponge ½ full (50% saturated) = ___ spoonfuls of water

Ask the students how many spoonfuls of water were in the sponge when the sponge was empty? Write the following statement on the board:

Sponge empty (0% saturated) = 0 spoonfuls of water

Have students begin to complete their worksheet data table with 0 spoonfuls of water and 0% saturation.
Ask them to write the number of spoonfuls (up to their 100% saturation number) and calculate each of the corresponding % saturations. (If you want the students to graph the data, have them do it now).
Ask the students to use their worksheet data tables (and/or graphs) to answer the following questions:

How many tablespoons of water would be in the sponge if the sponge were nearly ready to drip, but not actually dripping?

How many tablespoons of water would be in the sponge if it were nearly empty, but not totally empty?

Ask the students to make other predictions from their data/graph (such as, how many spoonfuls are required to reach 40% saturation? What would happen if you added more spoonfuls than are required to reach 100% saturation? etc.)

Explain to the students that they just used a scale to measure/predict how much water is in their sponge and how close it is to dripping. Meteorologists can measure how close is it to raining by using a humidity scale for water present in the air.
Display the attached Relative Humidity Graph (either via the overhead projector or hand out student copies).
Have students make comparisons to the data (and graph) just collected. (The Relative Humidity Graph is much more complex and is dependent on temperature.)
In conclusion, ask the students:

If it is almost ready to rain, what is the humidity? (They should 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 were 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 the air quality in buildings.)

Pre-Activity Assessment

Brainstorming: As a class, have the students engage in open discussion. Remind students 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?

Activity Embedded Assessment

Worksheets: Use the attached worksheet and graph, as directed in the Procedure section, to help students follow along with the activity.

Post-Activity Assessment

Question/Answer: Ask the students the questions at the end of the Procedure section, and discuss as a class.

Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder Amy Kolenbrander, Daria Kotys-Schwartz, Janet Yowell, Natalie Mach, Malinda Schaefer Zarske, Denise Carlson © 2004 by Regents of the University of Colorado.
The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0226322. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Last Modified: July 26, 2007