Expt 031 -- Cartesian Diver-sions -- The Basics

A vast variety of squeezable-sinkable Cartesian divers can be made with the simplest of equipment, materials, and a little imagination. As the pressure changes, students can easily observe the effect on the volume of air within the divers.

Background

• There are really two appropriate ways to explain this phenomenon:
  1. If we consider the diver assembly to consist of the pipet bulb, the hex nut and the air and water inside, then as we squeeze the bottle, we force more water up into the assembly (because the air pocket inside the bulb is compressible). This adds to the mass of the assembly without changing the volume, thus increasing the assembly's density.
  2. On the other hand, consider the assembly to consist of the bulb, the hex nut and the air inside, but not the water -- it is part of the surrounding fluid. When we squeeze on the bottle, it compresses the air pocket and thus decreases the total volume of the assembly. Since the mass remains constant, the assembly's density increases.
• Either way, when the assembly's density increases, it soon surpasses that of the surrounding water, and the assembly sinks. When the pressure is released, the compressed air pocket inside the bulb pushes the extra water back out, and the assembly assumes its original density and floats back up to the surface.
• This assembly is formally known as a Cartesian diver -- after Rene Descartes, a 17th century French mathematician who used them to explain various principles of density and buoyancy.

Chemical Concepts

1. Whether an object floats or sinks in a fluid depends on whether that object's density is less than or greater than the density of the fluid.
2. D = m/V. Thus, if you add to the mass of an object without changing its volume, the object's density increases. You can also increase the density of an object by keeping the mass constant while you decrease the object's volume.
3. Boyle's Law: as the pressure on a gas sample is increased, it gets compressed into a proportionately smaller volume.
4. Whereas gases are easily compressible, liquids and solids are not.

Safety

• Use scissors to cut plastic pipets and assorted decorations.
• Use care with the hot glue gun in preparing closed-system divers. The glue gun may cause burns.

Procedure

1. Screw the hex nut securely onto the stem of a graduated plastic pipet. The nut makes its own threads as it goes. With a scissors, cut off the protruding stem of the pipet below the hex nut.

   !!!Click here to See Movie.

2. Use a beaker or the bottom half of a 2-liter bottle as a flotation testing tank. Place the pipet-nut assembly into the water and observe that it floats rather buoyantly in an upright position with the hex nut acting as ballast. Squeeze out some of the air and draw some water up into the pipet. Now check the buoyancy. If you draw up too much water, the assembly sinks. If this happens, simply lift it out of the water, squeeze out a few drops of water, release the squeeze to allow air back in to replace the water. Using these techniques, adjust the amount of water in the assembly so that it just barely floats (in other words: fine-tune the assembly's density to make it slightly less than that of the water).

   !!!Click here to See Movie.

   !!!Click here to See Picture.

Open diver

1. Place the assembly in a 2-L bottle, which is filled completely with water, and screw the cap on securely. Be careful not to squeeze out any water while transferring the open diver to the bottle. Observe the assembly diving to the bottom as you squeeze the bottle and rising back to the surface as you release the pressure.

   !!!Click here to See Movie.

Closed Diver

1. After adjusting the water level in the diver, seal the tip closed with a hot glue gun. Hold the diver gently, without displacing either air or water, because any distortion is sealed into the diver.

   !!!Click here to See Movie.

2. Place the assembly in a 2-L bottle, which is filled completely with water, and screw the cap on securely. Squeeze and release. Observe.

   !!!Click here to See Movie.

Compare

Compare the open diver to the closed diver. Note the differences in shape and volume.

!!!Click here to See Picture.

Questions

1. Why do some objects float in water and some objects sink?
2. What is density?
3. Describe two ways the density of an object can be increased?
4. What is pressure?
5. Describe the relationship between pressure and volume for a gas sample?
6. How are solids and liquids different than gases in terms of volume changes.
7. What caused the diver to dive when the bottle was squeezed.
8. Describe the differences between the open and closed divers.
9. Explain how submarines can dive and surface using the principles observed in your divers.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 031 Cartesian Diver-sions -- The Basics

Watch the movies and record your observations. Answer the questions.

Curriculum-

This experiment fits in with initial discussions of density, buoyancy, and or the compressibility of gases, liquids, and solids. Students may work with divers to explore the gas laws. Using one open-diver and one closed-diver is especially effective.

Activity-

Laboratory or Demonstration or Home Experiment

• Divers may be used as a demonstration tool or as an experiment for students.
• Variations: Although the standard diver described above is amusing and educational, the real fun comes in trying some creative variations, such as those in the following four experiments - 032 to 035. See CoopLearn below.

Safety-

• Use scissors for cutting the plastic materials. Open diver projects may be done by students with little supervision.
• Use care with the hot glue gun in preparing closed-system divers. The glue gun may cause burns.
• Supervise use of the hot glue gun.

Time-

• Demonstration:
  • Teacher Preparation: 5 -10 minutes to prepare divers. Divers may be reused indefinitely.
  • Class Time: 10 minutes
• Student Experiment:
  • Teacher Preparation: 5 minutes to collect materials.
  • Class Time: 20 to 30 minutes to build divers, observe and answer questions.

Materials-

• 1 2-L plastic soda bottle
• 1 large beaker or cup or the bottom half of a 2-L soda bottle
• 2 disposable graduated plastic pipets
• 2 stainless steel hex nuts (1/4" or of a size that can screw securely onto the stem of the pipet available)
• scissors
• hot glue gun and glue

Disposal-

Retain divers for future demonstrations. Dispose of plastic with normal trash.

Lab Hints-

• Stainless steel hex nuts are strongly recommended. Both galvanized and brass hex nuts end up oxidizing over time.
• It is considerably more convenient to adjust the density and to test for flotation in a large beaker or tub of water, rather than in the bottle itself! A cut-off 2-L bottle works well as a testing tank!
• It is also advisable to fill the 2-L bottle completely with water. That way, when you squeeze, all of your work goes into compressing the air pocket in the diver and not into compressing a large air space at the top of the bottle.
• Makeup students can easily do the open diver experiments at home after viewing the movies. Print a handout with the questions.

Answers-

Q1. Why do some objects float in water and some objects sink?

A1. An object floats in a fluid (be it a liquid or a gas) if the object is less dense than that fluid. The object sinks if it is more dense than the fluid

Q2. What is density?

A2. Density is the ratio of a substance's mass to its volume. It can be considered an indication of how tightly concentrated the matter is in a given substance.

Q3. Describe two ways the density of an object can be increased?

A3. Since density is a ratio or fraction, it can be increased by increasing the numerator (the mass) while holding the denominator (the volume) constant. It can also be increased by holding the numerator (mass) constant while decreasing the denominator (volume).

Q4. What is pressure?

A4. Pressure is defined as the force exerted on a surface per unit area. Gas samples tend to exert pressure on a surface by the action of their particles colliding with that surface.

Q5. Describe the relationship between pressure and volume for a gas sample?

A5. Pressure and volume vary inversely for a gas sample. In other words, as the pressure of a gas sample increases, the sample's volume decreases proportionately, and vise-versa.

Q6. How are solids and liquids different than gases in terms of volume changes.

A6. In solids and liquids, the particles are so close together that the volume of a sample is due almost entirely to the volume of the particles themselves, not the space between them. Since compressing the volume of the particles requires very high pressures, solids and liquids are generally considered incompressible. In gases, however, the particles are so far apart that the volume of a sample is due almost entirely to the empty space between the particles, and not to the volume of the particles themselves. Increasing pressure has a profound influence on the volume of a gas sample, for it pushes the particles closer together; thus gases are considered compressible.

Q7. What caused the diver to dive when the bottle was squeezed.

A7. Consider answer A6 above. Since liquids are not compressible and gases are, as the bottle was squeezed, the pressure is transferred through the water and compresses the air pocket inside. As more water is pushed in through the mouth of the diver, the density increases. When the density of the diver surpasses that of the water, the diver dives to the bottom.

Q8. Describe the differences between the open and closed divers.

A8. The closed diver actually changes shape and collapses as pressure increases. The volume of the diver decreases significantly. The open diver volume remains the same, but it fills with water as the air volume decreases in response to the increase in pressure.

Q9. Explain how submarines can dive and surface using the principles observed in your divers.

A9. Pumps compress air and allow water into special tanks to submerge the submarines. To surface, the submarines allow the air to expand back to its original volume and expel the water.

CoopLearn-

The each of the following four experiments describes a variation of a Cartesian diver. Encourage each group of students to choose a different variation. The ones shown are only suggestions to get started. Many variations are possible. The first two are suitable for open divers. The last two require closed divers.

Key Words 1-

gases, submarine, pressure, volume, Cartesian divers, buoyancy, gases, liquids, compressibility