## Interpret Pressuretemperature Relationships

J Background: As you go up a mountain, both tem-1 perature and air pressure decrease. Temperature 1 decreases as you get farther away from the atmosphere's heat source—Earth's surface. Pressure decreases as you ascend the mountain because there are fewer particles in the air above you. Pressure and temperature are also related through the expansion and compression of air, regardless of height.

Question: How does the expansion and compression of air affect temperature?

Materials clean, clear, plastic 2-L bottle with cap plastic straws scissors thin, liquid-crystal temperature strip tape watch or timer

Safety Precautions ^ El tC Ejt Procedure

1. Read and complete the lab safety form.

2. Working with a partner, cut two pieces of straw, each the length of the temperature strip. Then cut two 2-cm pieces of straw.

3. Lay the two long pieces on a table. Place the two shorter pieces within the space created by the longer pieces so that the four pieces form a supportive structure for the temperature strip as shown in the figure.

4. Tape the four pieces of straw together. Place the temperature strip lengthwise on the straws. Tape the strip to the straws.

5. Slide the temperature-strip-straw assembly into the clean, dry bottle. Screw the cap on tightly.

6. Place the sealed bottle on the table so that the temperature strip faces you and is easy to read. Do not handle the bottle any more than is necessary so that the temperature will not be affected by your hands.

7. Record the temperature of the air inside the bottle as indicated by the temperature strip.

8. Position the bottle so that half its length extends beyond the edge of the table. Placing one hand on each end of the bottle, push down on both ends so that the bottle bends in the middle. Hold the bottle this way for 2 min while your partner records the temperature every 15 s.

9. Release the pressure on the bottle. Observe and record the temperature every 15 s for the next 2 min.

### Analyze and Conclude

1. Interpret Data What was the average temperature of the air inside the bottle as you applied pressure? How did this differ from the average temperature of the bottled air when you released the pressure?

2. Graph the temperature changes you recorded throughout the experiment.

3. Explain how these temperature changes are related to changes in pressure.

4. Predict how the experiment would change if you took the cap off the bottle.

5. Infer Given your observations and what you know about the behavior of warm air, would you expect the air over an equatorial desert at midday to be characterized by high or low pressure?

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