University of Virginia
Physics Department

## Trapped Inside

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.2

Objectives

Students will

• Investigate the properties of air.
• Define "matter."
• Make a rough measurement of the density of air.
• Conclude that air is matter.

Background Information

While we need air in order to breathe, many students do not realize that air is a form of matter. Because we live our entire lives in an environment in which we are always surrounded by air, we often forget that air is actually matter, and has a measurable mass. The mass/matter quality of air is what causes air pressure; this affects many things ranging from weather patterns to the inflation of sports balls, such as the soccer ball that will be used in this activity.

It is important to know that air pressure can be measured in several units. Among these are pounds per square inch, millimeters of mercury, and atmospheres. In this lab, we will use atmospheres. For convenience, scientists have set 1 atm = to standard air pressure.

### Student Activity

Materials

• Inflated soccer ball at standard playing pressure (this should be printed on the outside of the ball)
• Air pump
• Pump needle
• Balance
• Mason jar lid
• Ruler

Procedure

1. Collect all materials from the list.
2. Measure the mass of the jar lid, and record the mass on your data sheet.
3. Place the jar lid on the balance to hold the soccer ball in place while you are measuring its mass.
4. Measure the mass of the inflated soccer ball and the lid. Record the mass on your data sheet.
5. Measure the width (diameter) of the ball. One half of the diameter is the radius. Record your measurement of the radius (in cm) on the data sheet.
6. Place the needle in the soccer ball and allow air to escape from the ball. Do not squeeze the ball; wait until the air has stopped coming out on its own. This means the ball should now be at a pressure close the normal air pressure in the room.
7. Mass the ball and lid and record the mass on your data sheet. Find the mass of the air that escaped.
8. Complete the calculations and questions on the Data Sheet.

To print out a copy of the Data Sheet only, click here.

Data

Mass of jar lid: ______________ g

Mass of inflated ball and lid: ____________ g

Radius of the ball: _____________ g

Mass of partially deflated ball and lid: ___________ g

Calculations

Mass of the escaped air:
(Mass of inflated ball - Mass of partially deflated ball)

Volume of air inside the ball:
(V = (4/3)pi*R3)

Density of the air at normal pressure:
( Density = Mass of air/ Volume of air)

* You can find the density of air at normal pressure because a ball inflated to regular game pressure is at about twice the pressure of regular outside air (measured as one atmosphere). Thus, when you let out the "extra" air in letting the ball deflate to standard room air pressure, you are letting out about half of the air, or one atmosphere's worth.

Answer the following questions on a separate sheet of paper:

What is inside the ball? What does the mass measurement include?

Does the soccer ball maintain a constant volume as air is added to it?

As the ball was being inflated, what was "pushing" on the sides of the ball?

Did the mass of the ball change as it was inflated? What do you think caused the change in mass?

Define matter. Based on the properties of air you have observed in this experiment, what can you conclude about air in relation to the criteria for being matter?

The accepted value for the density of air is about 1.3 g/L, or .0013 g/cm3. How close was your measurement of the density? What may have caused the difference between your answer and the accepted value?

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.