Scuba diving is more than a sport. It's a science that operationally integrates physics, chemistry, physiology, and oceanography. It's also pretty cool.
 
 

Begin the lesson by showing students a can of compressed air. "Air blasters" are often available as commercial dusting sprays at photographic supply stores. Explain that the can contains a large volume of air that has been compressed into a small space. Within this space, the gas is stored under considerable pressure. When the nozzle is pressed, some of the pressurized gas escapes from the can.

 Attach a length of plastic tubing or a nozzle extension to the can. Press the nozzle to demonstrate the directional flow of air. Fill a fish bowl with water. Position the free end of the tubing beneath the water's surface. Press the nozzle and have students observe the rush of rising air bubbles.

 Challenge the class to critically analyze their observations. What causes the rush of bubbles? What do the bubbles contain? Predict how much air is stored in the can. Can compressed air support living things that require oxygen?
 

Scuba-from the phrase "self-contained underwater breathing apparatus"-refers to a type of diving in which an individual carries his or her own supply of air. This air supply is stored within a steel or aluminum cylinder called a scuba tank. A device called a regulator "taps" the pressurized air and adjusts its flow for breathing.
 
 

Prior to the dive, a mechanical compressor fills the tank with a large volume of air. The pressure produced by this compressed air can exceed 200 times the standard atmospheric pressure! As a diver breathes, flow-adjusting devices called stages drop the flow pressure. Air that is exhaled does not return to the scuba tank. Instead, it is released and observed as the rush of rising bubbles. Sport divers can safely dive to a depth of about 39 meters (130 feet) or five atmospheres of pressure.
 
 

Although the first crude scuba apparatus was invented over 150 years ago, it was Jacques-Yves Cousteau and Emile Gagnan who perfected the modern day Aqua-Lung. Unlike the "hard-hat" divers that relied on a surface air hose, Cousteau (in the early 1940s) had attained untethered freedom.
 
 
 

1. Suppose a balloon filled with air was released from the sea bottom. How does the surrounding pressure change as the balloon rises? What is likely to happen to the balloon before it reaches the surface? Explain.

 2. The bends is a life-threatening illness that results from too rapid a return to surface air pressure. During ascent, nitrogen dissolved in the blood stream comes out of solution as lung bubbles that can block the flow of blood to critical body organs. When construction of underwater foundations for the Brooklyn Bridge was underway, laborers worked in pressurized chambers. Upon a rapid return to the surface, many suffered from the bends. If you were in charge of this construction project, how might you protect these workers from decompression sickness? 

 Imagine entering a freshwater pond or lake. Take a deep breath and you're likely to float. Exhale, and you'll probably find yourself sinking. These "ups and downs" depend upon the amount of air in your lungs. As the volume of this gas increases, you become more buoyant. As the gas volume drops, you lose buoyancy and begin to sink.

In this main activity, you'll construct a device called a Cartesian diver. Like a floating person, this diver has a buoyancy that depends upon its volume of trapped air. As you explore its behavior, you'll uncover the relationship between pressure and volume. 

Materials
 
 

1. Fill the container with tap water.

 2. Lower the medicine dropper into the container. Squeeze the bulb slightly so that the glass tube becomes partially filled with water.

 3. Set the dropper floating within the container. Add more water to the container so that the level of water rises to the brim.

 4. Screw on the container lid. The seal should be tight enough to prevent the leakage of water.

 5. Squeeze the center of the plastic container. What happens to the medicine dropper? Release your pressure. What happens now? Note: If the dropper remains afloat, you'll need to open the container and fill the dropper tube with more water.

 6. Take a close look at the air bubble trapped within the medicine dropper. What happens to the bubble's volume as you squeeze the container? Can you explain the connection between this change in volume and the behavior of the medicine dropper? What happens to the bubble's volume when you release your grip? How does a change in volume relate to the movement of the medicine dropper?

Extend the activity

 Can you modify the design of your medicine dropper so that it can recover items that are scattered at the bottom of the container? First, design a diver that can retrieve paper clips and other objects attracted to magnets. Then, redesign your diver to "recover" targets that have eyelet-like handles. 

Is it possible to make a Cartesian diver out of other materials, such as the plastic cap to a pen, weighted with a bit of clay? See what objects will work.

Questions

 1. Does squeezing the bottle force more water into the air or compress the air, making the diver heavier and causing it to sink?

 2. How do these demonstrations relate to scuba diving equipment? How do they explain free divers' use of stones for weight as they dive?