Name Period Date

CHAPTER 2 CHAPTER INVESTIGATION A

Relative Humidity

MATERIALS

2 thermometers cotton or felt cloth 3 rubber bands plastic bowl water at room temperature scissors pint milk carton ruler Relative Humidity Chart

PURPOSEIn this lab, you will

make a psychrometer (sy-KRAHM-ih-tuhr) measure relative humidity in two places

PROBLEMWhich place will have higher relative humidity?

HYPOTHESIZEAfter you do step 9, complete the sentence below:If I measure humidity in a place that is damp, then the relative humidity will be (circle one) higher/lower because _____________________________________.

PROCEDURECheck off each step as you do it.

1. Read the thermometers. They should be the same.

2. Make the wet-bulb thermometer.

a. Wrap a piece of cloth around the bulb.

b. Put a rubber band tightly around it.

c. Dip the cloth in water. Be sure it gets wet.

3. Attach both thermometers to your "psychrometer."

a. Poke a small hole in one side near the bottom.

b. Wrap 2 rubber bands around the carton. Use them to hold the thermometer against the side with the hole.

c. Push the end of the cloth through the hole. Then slide the other thermometer onto the other side.

4. Add water to your psychrometer.

a. Pour water into the carton, just below the hole. The cloth should be wet.

b. Pour all the water out of the bowl.

c. Place the whole psychrometer in the empty bowl.

5. Record temperatures at Location 1, "Science Room." Wait 10 minutes. Read the temperature (°C ) on the dry-bulb thermometer. Write

it in Data Table 2.1 under "science room." Read the wet-bulb thermometer (°C). Write it in the following table.

Data Table 2.1: Relative Humidity at Various Locations

  Location 1 Location 2

Name of place science room 

Dry-bulb temperature   

Wet-bulb temperature   

Subtract:wet from dry. (dry - wet = ?)

   

Relative humidity   

6. Pick a new place to test. Write its name in Data Table 2.1. Let your psychrometer stand in the new spot for 10 minutes. Then read the

temperatures. Write them in the table.

7. Subtract the wet-bulb reading from the dry-bulb reading for each place. Write the answers in the "Subtract" line of Data Table 2.1.

8. Use the "Relative Humidity Chart" to find the relative humidity. Find the dry-bulb temperature for Location 1 in the left column of the Relative

Humidity Chart. Put a ruler under that row. In the top row of the chart, find the number that matches the difference for

Location 1. Move your finger down that column to the line with the ruler under it. This is the relative humidity at Location 1.

Record the relative humidity in the bottom row of Data Table 2.1.

9. Repeat step 8 for Location 2.

OBSERVE AND ANALYZE1. Record Observations Draw your psychrometer on a separate sheet of paper.2. Identify Variables

List the experiment's variables-things that changed.List the experiment's constants-things that stayed the same.

Data Table 2.2

Variables Constants

1. 1.

2. 2.

3. Compare List the wet-bulb and dry-bulb readings at both locations. 

Location 1 Location 2

Wet-bulb reading   

Dry-bulb reading   

Are wet-bulb readings or dry-bulb readings usually higher?

4. Analyze If the difference between the readings on the two thermometers is large, the relative humidity is (circle one) high/low because

CONCLUDE5. Interpret Which place had the higher relative humidity?

Compare your results with your hypothesis. Was your hypothesis correct?

6. Identify Limits What mistakes could someone make when doing this experiment?

7. Apply Was the relative humidity different in the two places?

Why do you think it changed or stayed the same?

Relative Humidity Chart

ANSWER KEYHypothesize higher; sample answer: there is more water to evaporate into the air Observe and Analyze 1. Students should provide accurate drawings of two thermometers attached to a milk carton with rubber bands. 2. 1. location of readings     1. psychrometer setup     2. humidity     2. Relative Humidity Chart 3. dry-bulb readings (may be equal if humidity is 100 percent) 4. low; it shows that water is evaporating quickly from the wet bulbConclude 5. Answers will vary depending on observations and results in the investigation.Answers will vary based on the hypothesis and the lab results. 6. Sample answer: Temperature readings may have been inaccurate. 7. Sample answer: Relative humidity was different at the two places chosen. Trees, windows, the heating system, and other factors may affect the humidity recorded at a particular location.

TEACHER NOTES Lab Preparation

Review how to use the Relative Humidity Table before beginning the investigation. Tell students to use a ruler as a guide by placing it below the line for the dry-bulb reading.

Prior to the investigation, have students read through the investigation and prepare their data tables. Or you may wish to copy and distribute datasheets and rubrics.

UNIT RESOURCE BOOK, pp. 133-142SCIENCE TOOLKIT, F14Lab ManagementDivide students into groups of three or four. Assign different groups to take two readings inside the school, two readings outside, and one reading inside and one reading outside. Then compare results and discuss patterns.SAFETY Tell students to notify you immediately if a thermometer should break -- they should not try to clean up the glass themselves.INCLUSION Some students may have difficulty with the Relative Humidity Table. Review how to subtract measurements before beginning the investigation.