Getting a Bang Out of Breath Spray: Studying the Chemistry and Physics of a Small Explosion
AbstractHold onto your hats! In this science fair project, you will make a device that sends a film canister across the room with a small chemical explosion. The energy for the explosion is derived from the combustion of ethanol. You will determine the launch velocity of the canister, as well as devise ways to study changes in gas pressure and volume due to the explosion. This science fair project is sure to take your breath away!
David Whyte, PhD, Science Buddies
This science fair project is based on information on following website:
- Field, SQ. (2008). Thermodynamics: The film can cannon. Retrieved November 14, 2008.
Shoot a film canister into the air by igniting trapped ethanol with a spark. You will study the chemistry and physics of the explosion.
Three things are usually required for a chemical explosion: a chemical reaction that occurs very rapidly, a large increase in gas pressure, and a confined-reaction vessel in which the pressure of the gaseous products can increase to a point that the gases break violently out of the container. In this chemistry and physics science fair project, you will use the combustion of ethanol to provide energy for a small explosion. The chemical equation that describes the combustion of ethanol is shown below. (Note: Hover over the equations in this Introduction with your cursor to view enlarged formulas.)
The chemical equation states that ethanol (C2H6O) combines with oxygen (O2) to form water (H2O), carbon dioxide (CO2), and energy in the form of heat. It also forms energy in the form of light, but this is small compared to the heat energy.
Ethanol and oxygen are the reactants, and water and carbon dioxide are the chemical products of the equation. Note that the equation is balanced, so that the number of atoms of each element is the same on both sides; for example, there are two carbon atoms on both sides.
Oxygen, which forms about 20 percent of our atmosphere, is a very reactive chemical. When it reacts with ethanol, the reaction proceeds very quickly and produces a substantial amount of heat. But a mixture of oxygen and ethanol is stable unless you provide a "spark" or other source of energy to get the reaction started. Once the two chemicals begin to react, the reaction itself produces enough energy to sustain further combustion. The reaction stops when the reactants are used up.
One way to create an explosion is to increase the number of molecules that are in the form of a gas. If all of the molecules in Equation 1 were gas molecules, there would be a net increase from four molecules (one ethanol and three oxygen) to five molecules (three water and two carbon dioxide). Creating more gas molecules in a fixed volume will increase the pressure, just as pumping air into a tire or basketball increases its pressure. If the pressure is greater than the container can handle, an explosion can occur.
For this science fair project, you will ignite a small amount of ethanol to launch a film canister. The volume of the canister is small enough that there is little danger of getting injured, but it will make quite a pop! Just how big a pop? Watch the video below for a preview. Some of the ethanol will be in the form of a liquid. Also, some of the water that is a reaction product will be liquid. Thus, some of molecules will not be in the form of a gas because the temperature will be low enough that they will condense to the liquid form.
Increasing the amount of gas molecules is not the only way to increase pressure. Another way to increase pressure in a container with a fixed volume is to increase the temperature. An increase in temperature causes the molecules in the gas to move faster. The collisions of these more energetic molecules against the sides of the container results in a higher pressure.
In summary, the pressure in a closed container will increase if more molecules of gas are added, or if the temperature increases. This relationship is captured in the ideal gas equation:
The gas equation states that the pressure times the volume equals the product of the number of moles, the gas constant, and the temperature. The key point about this equation, for the purposes of this science fair project, is that it clearly shows the mathematical relationship between pressure, amount of gas, and temperature. The pressure, P, is directly proportional to n, and T. If you double P in the equation, keeping V, n, and R the same, then T will also double. The same goes for n and P. The equation is precise only for "ideal" gases in equilibrium with their surroundings, which is not the case in an explosion. But the ideal gas equation is a useful approximation of this real-world experiment.
A version of the experimental setup is shown in Figure 1. The apparatus consists of a piece of wood with the top of a film canister glued onto it. The metal ends of a grill spark igniter are passed through a hole in the wood and through the film canister top. The metal ends will deliver a spark when the red button is pushed. When you add a fuel to the film canister and attach it to the plastic top that is glued to the board, a spark will ignite the fuel and send the canister flying. The fuel used to launch the canister is Binaca breath spray (ethanol and isopropane). A tire pressure gauge is used to measure the maximum pressure created inside the canister when the ethanol gas is ignited.
The lid of a film canister is glued to a wood board with a hole drilled through both the lid and board. Two leads from an ignitor are placed into the film canister through the hole in the wood board. Another hole is drilled through the wood and canister lid and a pressure gauge is inserted into the hole in the wood.
Figure 1. Picture of a film canister projectile. The top of a film canister is glued to a piece of wood. A spark generator is attached, through a hole in the wood, to deliver a spark when the red button is pushed. A pressure gauge is also attached and is connected to the space inside the canister by another hole in the wood. Binaca breath spray is used as fuel to launch the canister. Note the electrical tape around the body of the spark unit to prevent a shock.
By timing how long the canister is in the air, you can calculate the launch velocity. Once you have the launch velocity, you can calculate the kinetic energy of the canister. The kinetic energy of the canister is the energy due to its mass and velocity. The formula for calculating the kinetic energy is given below.
You can also calculate the maximum potential energy, which is determined by how high the canister flies; the higher it goes, the higher the potential energy. The equation for potential energy is given below. If you shoot the canister straight up, it will have zero velocity for a very brief time at the highest point as it stops going up and starts coming down. At this moment, the kinetic energy is at its lowest and the potential energy is at its highest.
Several basic physics formulas for ballistic projectiles are shown below. The projectiles are assumed to be shot 90 degrees (for best height) or at a 45-degree angle (for best range). The formulas are approximations because they assume there is no air resistance. "Time of flight" is the time it takes the canister to return to the same level from which it was launched.
For the equations below:
Equations to use when launching the canister straight up:
The velocity of the canister at launch equals one-half of the product of the acceleration due to gravity (9.8 m/s2) and the time of flight.
The maximum height of the canister is one-half of the launch velocity squared, divided by the acceleration due to gravity (9.8 m/s2).
Equations to use when the canister is shot at a 45-degree angle:
The launch velocity (45 degrees) equals 0.71 times the acceleration due to gravity, times the time of flight.
The maximum height (45 degrees) equals the launch velocity squared, divided by 4 times the acceleration due to gravity. (You can get launch velocity from Equation 5).
The range (45 degrees) equals the launch velocity squared, divided by the acceleration due to gravity.
Equations for the energy of the canister. Assume the mass of the canister is 4 grams (g).
The kinetic energy of the canister at the time of launch is equal to one-half its mass, times the launch velocity squared.
The potential energy of the canister at its maximum height equals its mass times g, times the height.
Note from Equation 3 that you can determine launch velocity if you know the flight time. Time of flight is easy to measure; just use the stopwatch to time the interval from launch to landing. Once you have calculated the launch velocity, you can calculate the maximum height the canister flies using Equation 4. Remember that the equations are approximations and do not take into account the air resistance. You will use the trajectory equations to determine the launch velocity of the canister, its maximum height, and its kinetic and potential energy.
The kinetic energy of the canister gives you a minimum value for the chemical energy of the combustion reaction. It is a minimum value because energy is lost as heat, friction, and in other ways. As you work through the procedure, consider where the energy is flowing. The overall flow of energy when the canister is launched straight up follows this sequence:
- Chemical energy: Energy released as heat causes an abrupt pressure rise in the canister.
- Kinetic energy: The canister is launched upward.
- Potential energy: This reaches a maximum at the maximum height. It returns to zero when the canister lands.
- Kinetic energy: The potential energy is converted to kinetic energy as the canister falls back to Earth.
When the fuel is ignited, the combustion of the alcohol will create a sharp rise in the pressure inside of the canister. One goal of this science fair project is to measure the pressure in the canister. You can then estimate the temperature. The situation inside the canister is far from ideal, so the estimates will be very rough. Another goal is to measure the volume change over the course of the explosion, using a balloon attached to the canister. Now you're ready to have a blast!
Terms and Concepts
- Chemical explosion
- Chemical reaction
- Gas pressure
- Chemical equation
- Balanced equation
- Ideal gas equation
- Mole (mol)
- Kelvin (K)
- Launch velocity
- Kinetic energy
- Potential energy
- How does the combustion of ethanol in the film canister compare to the combustion of gasoline in a car engine?
- How does the combustion of ethanol in the film canister compare to the metabolism of ethanol by yeast?
- Will any concentration of ethanol gas in air form an explosive mixture, or are there upper and lower limits in its concentration?
- The alcohol used in most breath sprays is "specially denatured." What does it mean to denature the alcohol?
- What would a graph of pressure in the canister vs. time after spark ignition look like?
- What would a graph of temperature in the canister vs. time after spark ignition look like?
- Where does the energy from the combustion of ethanol end up?
- Is it true that when the kinetic energy of a canister launched straight up is at its maximum, the potential energy is near zero, and vice versa?
Materials and Equipment
- Empty film canisters with tops (4; you only need 2, but have a couple extra on hand in case you mess up while drilling.)
- Piece of wood, 4- × 6- × 1/2-inch
- Sand paper, medium grit (1 sheet)
- Epoxy cement, clear. Gorilla® epoxy is available online at Amazon.com
- Disposable paper plate (4)
- Craft sticks (1 bag)
- Electric drill and drill bits. The drilled holes need to be big enough to just allow the igniter wires to pass through, so start with a 5/32-inch drill bit and increase in size, if needed.
- Safety goggles
- Igniter. Grill igniters are available at most hardware stores. The spark mechanism from electronic lighters will also work.
- Electrical tape
- Binaca® breath spray or other sprays with flammable components, including hairspray, perfume, deodorants, or bug repellent
- Pressure gauge, the kind used for automobile tires, 120-psi maximum
- Balloons, medium size (1 package; you will need 3 for the experiment, but should have extras on hand)
- Metal file
- Lab notebook
- Optional: camcorder
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Making the Canister Launching Device
Attach the lid of the film canister to the 4- × 6- × 1/2-inch board, near the
middle of the board, with the epoxy cement.
- Roughen the topside of the canister's lid with a piece of sand paper and the spot on the board where you plan to attach the lid.
- Completely mix the epoxy in the ratio recommended on the package. Usually, the ratio is one part resin to one part hardener. If the epoxy is not mixed in the proper ratio, it will not act as an adhesive or harden.
- Use a craft stick and a disposable paper plate to mix the epoxy together.
- Apply a thin layer of epoxy on the topside of the canister's lid and the board. Firmly press the lid and the board together for 5 minutes.
- Follow the directions on package, and make sure to dry and cure the epoxy for the recommended time.
- Put on your safety goggles, then slowly drill two holes in the canister lid, near the middle of the top. The holes should be about 1/4 inch apart. Continue to slowly drill all the way through the board. The holes should be big enough to just allow the wires from the igniter to pass through. Start with a 5/32-inch drill bit and increase the size if you need to. See Figure 2.
Figure 2. The top of a film canister is glued to the 4- × 6- × 1/2-inch board. The two spark igniter leads are glued into place. The hole leading to the pressure gauge allows measurement of the pressure inside the canister.
Drill a third hole for the pressure gauge in the canister top across from the holes that you drilled in step 2.
- Slowly drill the hole through the canister lid and through the board so that there is room on the opposite side to attach the pressure gauge.
- Turn the board over. Drill a wide, shallow hole into the board, with a larger drill bit, about ¼ inch deep and the same diameter as the pressure gauge, to provide a firmer point of attachment. See Figure 3.
Figure 3. The pressure gauge is attached to the side opposite the film canister. The small hole is drilled through to the inside of the film canister. The larger hole provides a firm setting for attaching the gauge.
Glue the ends of the igniter leads onto the inside of the canister lid using the epoxy. Try to fill in the holes with the epoxy as much as possible without covering the leads with epoxy. See Figure 2.
- The ends of the wires should be spaced about ¼ inch apart so that a spark can pass between them.
- Push the leads into the holes firmly. This may take some effort. Make sure that the thicker or ceramic lead sits a little bit higher than the metal lead. However, if the ceramic lead is too high, and the leads are too far apart in height, the igniter will not spark. You will have to experiment with the positioning.
- Attach the other ends of the wires to the igniter body.
- Wrap the igniter body with electrical tape to cover the bare wires. Caution: The igniter will deliver a shock if you touch the unprotected wires attached to the body of the mechanism while pressing the button, so be careful.
Test that a spark is formed when you push the igniter button.
- If you don't see a spark, use the metal file to clean the tips of the spark wires and then push the igniter button to see if a spark forms.
The back of the board will have the pressure gauge and both of the leads attached through it. Attach the end of the pressure gauge over the hole, using epoxy cement as shown in Figure 4.
- Mix the epoxy together as directed by the package using a craft stick and a disposable plate.
- With a craft stick, dab epoxy around the hole in the wood without covering the hole with epoxy.
- Spread out a thin film of epoxy on the plate and carefully dip the rim of the pressure gauge in the epoxy. Avoid covering the opening of the pressure gauge with epoxy.
- Attach the pressure gauge to the back of the board over the drilled hole. Hold the pressure gauge firmly for 5 minutes.
- Follow the directions on the package and allow the epoxy to dry and cure for the recommended time.
Figure 4. The pressure gauge and the two leads for the spark igniter attached to the side of the wood board opposite the film canister.
Launching the Canister
- Put on your safety goggles.
- Spray your selected fuel into the canister; for example, a brief spray of Binaca®.
- Snap the canister onto the lid.
- Point the canister toward an open area. Caution: Make sure there are no people or breakable objects in the way.
- Click the sparker.
- The canister should take off with a loud Pop!
If there is no explosion, first make sure that you can see a spark when you press the button.
- If you have a spark but it is not working, you may have added too much fuel. Just take the canister off briefly to add some more air (oxygen), let some fuel out, and re-attach it to the base.
- Once it is working, measure the time of flight. Hold the base near the ground and launch the canister straight up. Using the stopwatch, time how long it takes for the canister to return to the ground.
- Repeat steps 1-8 of this section at least two more times and average the results. Record the data in your lab notebook.
Determining the Canister's Launch Velocity, Kinetic Energy, and Potential Energy
Use the equations in the Introduction to determine the canister's launch velocity, kinetic energy, and potential energy.
- Calculate the launch velocity based on the time of flight.
- Calculate the kinetic energy. Assume the canister has a mass of 4 g.
- Calculate the maximum height, based on the time of flight.
- Calculate the maximum potential energy based on the maximum height. Assume the canister has a mass of 4 g.
- Graph the kinetic energy and potential energy as a function of time from launch until the canister hits the ground.
- How does the kinetic energy relate to the chemical energy of the combustion reaction?
- As an option, devise a way to measure the maximum height and compare this to the calculated value.
Measuring the Pressure Maximum in the Canister
In this first part, the canister should be held against the ground so that it remains on the base. The idea is to determine whether keeping the canister attached to the base results in a higher pressure than when the canister is free to fly off.
- Put on the safety goggles.
- Fill the canister with fuel and attach it to the launcher.
- Hold the launcher upside down and press the top of the canister against the ground.
- Ignite the fuel and watch the pressure gauge.
- Record the pressure in your lab notebook.
- Reset the gauge.
- Repeat steps 1-6 of this section at least two more times and average the results.
Assuming that "n" stays the same, use the pressure
(the value of "P" in the ideal gas equation; Equation 2, [PV=nRT])
to calculate the maximum temperature inside the canister.
- Assume that atmospheric pressure is 14 pounds per square inch (psi). This is the "baseline" pressure. If the pressure on the gauge reads 14 psi after the explosion, then the pressure inside the canister doubled due to the explosion. In general, if the gauge reads X psi after the explosion, then the maximum pressure in the canister is (14 + X) psi.
In this part, the canister should be allowed to fly freely. Compare the results of this test with the test above.
- Put on the safety goggles.
- Fill the canister with fuel and attach it to the launcher.
- Hold the launcher base against the ground making sure that the pressure gauge doesn't touch the ground.
- Ignite the fuel and launch the canister straight up.
Check the pressure gauge.
- Depending on how tightly the canister is attached to the base, you may or may not get a pressure reading when you launch the canister into the air.
- Record the results.
- Repeat steps 1-5, of Part 2, at least two more times and average the results.
- Estimate the maximum temperature inside the canister. Use Equation 2 (PV=nRT) and assume that only P and T are changing.
Measuring the Volume Change in the Canister
- Put on your safety goggles.
- Drill several holes in the bottom of another film canister as shown in Figure 5.
A deflated balloon lays next to a film canister that has holes drilled into the bottom face. These will attach to a film canister lid that has been glued to a wooden board and has two holes drilled that hold the leads of an ignitor and a pressure gauge.
Figure 5. A canister with holes drilled in the top and a balloon can be used to determine how the volume of gases changes. The balloon is stretched over the canister, fuel is added, and the canister is attached to the base. If there is an increase in the number of gas molecules, the balloon will stay inflated even after the gases cool.
- Place a balloon over the bottom of the film canister, covering the holes you drilled.
- Fill the canister with fuel and ignite it.
- Watch as the balloon expands and estimate the diameter, in centimeters. Record this number in your lab notebook. Hint: You will have to be quick.
- Try recording the balloon with a camcorder, with a size marker behind the balloon, to estimate the diameter.
- Repeat steps 1-3 at least two more times.
Calculate the average volume increase you observed.
- The volume of a sphere is 4/3 π R3, where R = radius. Remember, the radius is 1/2 the diameter.
- The volume of a cylinder equals the height times the area of the base.
- Calculate the ratio of the volume of the balloon to the volume of the canister.
- Compare this volume increase with the pressure increase you measured earlier.
- Note how the volume changes over time from immediately after ignition to the time that the system (balloon, canister, etc.) has returned to room temperature.
- Is the increase in pressure after ignition due to an increase in n or T, or both? Hint: If there is an increase in n, then the volume will be higher after the temperature has returned to normal.
Ask an Expert
- Estimate the mass of ethanol used in each launch. For Binaca, look at the label for total mass and the approximate number of sprays per bottle. Calculate the energy generated per milligram (mg) of fuel.
- Try other fuels besides whichever one you selected, such as hairspray, perfume, deodorants, or bug repellents.
- Devise a way to measure the energy generated by the explosion using a calorimeter. A StyrofoamTM cup with some water might be a simple calorimeter.
- Devise a way to measure how high the canister travels. Compare this with the calculated value.
- Launch the canister at 45 degrees and measure the time of flight and the range. How does the range you measured compare with the calculated value?
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