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CHAPTER 2 CHAPTER INVESTIGATION A
Relative Humidity
MATERIALS
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2 thermometers
cotton or felt cloth
3 rubber bands
plastic bowl
water at room temperature
scissors
pint milk carton
ruler
Relative Humidity Chart
PURPOSE
In this lab, you will
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make a psychrometer (sy-KRAHM-ih-tuhr)
measure relative humidity in two places
PROBLEM
Which place will have higher relative humidity?
HYPOTHESIZE
After 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 _____________________________________.
PROCEDURE
Check 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 &quot;psychrometer.&quot;
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.
a. Pour water into the carton, b. Pour all the water out of
just below the hole. The
the bowl.
cloth should be wet.
c. Place the whole
psychrometer in the empty
bowl.
5. Record temperatures at Location 1, &quot;Science Room.&quot;
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Wait 10 minutes. Read the temperature (&deg;C ) on the dry-bulb thermometer. Write
it in Data Table 2.1 under &quot;science room.&quot;
Read the wet-bulb thermometer (&deg;C). Write it in the following table.
Data Table 2.1: Relative Humidity at Various Locations
Location 1
Name of place
Location 2
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.
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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.
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Write the answers in the &quot;Subtract&quot; line of Data Table 2.1.
8. Use the &quot;Relative Humidity Chart&quot; to find the relative humidity.
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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 ANALYZE
1. 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
4. Analyze If the difference between the readings on the two thermometers is large, the
relative humidity is (circle one) high/low because
CONCLUDE
5. Interpret Which place had the higher relative humidity?
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
Hypothesize
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.
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 bulb
Conclude
5. Answers will vary depending on observations and results in the investigation.
Answers will vary based on the hypothesis and the lab results.
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
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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
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. 133-142
SCIENCE TOOLKIT, F14
Lab Management
Divide students into groups of three or four. Assign different groups to take two readings