Materials Needed: Empty plastic soft drink bottle.

Procedure:   Make sure the bottle is clean.  Place your mouth firmly around the mouth of the bottle and blow into the bottle. Squeeze the side of the bottle. What happens?  Pull your mouth away from the bottle?  What happens now?  Again, place your mouth over the mouth of the bottle, but this time, draw as much air from the bottle as you can.  What happens?

What Happened: When you blew, you forced air into the bottle.  This compressed the air inside the bottle which increased the air pressure inside.  You were able to feel the increased pressure because the bottle resisted your squeeze.

When you drew air out of the bottle, the bottle began to collapse.  Air was taken out of the bottle, and the pressure inside dropped.  Since the air pressure on the inside was less than the air pressure on the outside, the outside air pressure caused the bottle to collapse.

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Bernoulli’s Principle

Removing air is not the only way to lower air pressure.  A fast moving current of air will do the same thing.

Materials Needed: Strip of paper, 3 cm (1 in) by 23 cm (9 in).  (Notebook paper is fine.)

Procedure:   Hold the narrow side of the strip of paper just underneath your lower lip and blow across the paper.  What happens?

What Happened: The paper strip rose. 

A fast moving stream of air has a lower air pressure than a slower air stream.  As the stream of air moved over the top of the paper, the air pressure over the paper dropped. The air pressure underneath the paper stayed the same.  The greater air pressure underneath lifted the paper strip and it rose. The idea that a moving air stream has lower air pressure than air that is not moving is called “Bernoulli’s Principle”.

Materials Needed: A piece of stiff paper (such as construction paper), 3 cm (1 in) by 20 cm (8 in).

Procedure: Fold the paper 3 cm (1 in) from each end into a “U” shape.  Place this paper upside down on a table top near the edge as shown.

Blow gently underneath the paper.  Gradually blow harder.  What happens?

What To Look For: Does the paper blow away?  Can you blow hard enough to blow it away?

What Happened: When you blew underneath the paper, you created a moving stream of air.  We know that Bernoulli’s Principle says that air in this moving stream is lower than the still air above it.  The higher air pressure above the paper pressed the paper down, and the harder you blew, the more the paper was forced down.

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Using Bernoulli’s Principle to Levitate a Balloon

CAUTION!  This experiment should be done only with adult supervision.  You should always be very careful when you use an electric fan.  Keep your fingers out!

Materials Needed: Balloon; paper clips; small electric fan; several books.

Procedure: Use the books to position the electric fan so that it is blowing straight up.  You should also make sure that air can get to the back side of the fan. You may need to use several books to support  the fan in order to do this.

Inflate the balloon and tie it off.  Slip one paper clip onto the neck of the balloon.  Turn the fan on low and hold the balloon in the middle of the air stream. Let it go.

What To Look For: The balloon should be suspended above the fan.  If it falls toward the fan, try turning the fan up higher, or removing the paper clip.  If the balloon blows away, add a second paper clip.

What Happened: The force of the moving air underneath the balloon was enough to hold it up.  The weight added by the paper prevents the balloon from going too high.  But that is only part of the story.  The balloon stays inside the moving stream of air because the pressure inside is the air stream is lower than the still air around it. As the balloon moves toward the still air outside of the air stream, the higher pressure of the still air forces the balloon back into the lower pressure of the air stream.  Bernoulli’s Principle at work again!

Going Further: Try this with a beach ball if you have one.  The beach ball is probably heavy enough so that you won’t have to use anything to weigh it down.  If you do need extra weight, try taping one or more paper clips to the ball.

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Using Bernoulli’s Principle to Fool Your Friends

CAUTION!  Take care when using the straight pin in this experiment.  Make sure that the pin cannot poke you in the eye by not holding the spool straight up!

Materials Needed: Small piece of cardboard; sewing thread spool; straight pin.

Procedure:   Cut about a 5 cm (2 in) square from a small piece of cardboard such as a “3 x 5" card.  (The size is not critical.)  Place a straight pin through the center of the cardboard, and place the cardboard and pin in one end of spool.  The straight pin must be shorter than the spool! Tip the spool up just enough to keep the cardboard and pin from falling out.  (See CAUTION!) Blow through the bottom of the spool.  Try to blow the cardboard off of the spool.

What Happened: This is Bernoulli’s principle at work again.  The force of the moving air underneath the cardboard created an area of lower pressure.  The higher pressure from the still air above the cardboard was greater than the moving air beneath it. The greater pressure from above pushed down on the cardboard and prevented you from blowing it away from the spool. 

Try this on your friends!

When a car or truck moves down the highway, it moves through a stream of air.  Depending on how the car or truck is shaped, the air can make the ride smooth or rough.  If the air stream cannot move smoothly, it will create drag on the vehicle and will require much more fuel to move.  These next experiments show how air streams move around different shaped objects.

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Movements of Air Around a Flat Surface

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: A piece of stiff cardboard cut to about 10 by 16 cm (4 by 6 in); modeling clay; alcohol lamp or short candle (about 8 cm or 3 in); a friend.

Procedure:   Make a stand for the cardboard from modeling clay as shown.  Place the cardboard about three to four inches in front of the lighted candle or alcohol lamp.  Press down on the clay to hold the cardboard firmly on the table top.  Have a friend blow gently onto the cardboard while you observe the flame on the other side. Have your friend to blow harder until the candle is blown out as you watch.  What do you see?

Now turn the cardboard so that the edge faces the candle.  Relight the candle and again have your friend to blow until the candle goes out.  Is anything different?

What Happened: When the air stream hits the flat surface of the cardboard, it is forced to spread across the front of the cardboard before it can go around.  As it goes around, it swirls near the ends of the cardboard creating “turbulence”.  This swirling turbulence causes the flame to be pulled toward the cardboard. It also causes the flame to be pulled in several different directions which may cause the candle to “sputter” before going out.

When the thin edge is in the airstream, air can move much more efficiently, and you should see much less turbulence.  The candle is much easier to blow out as well.

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Movements of Air Around a Round Surface

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: Round glass bottle, such as a soft drink bottle; candle or alcohol lamp.

Procedure: Place the glass bottle about three to four inches in front of the alcohol lamp or candle.  Have a friend to try blowing out the flame by blowing on the bottle from the side opposite of the flame.  What do you see?

What To Look For: How much force is needed to blow out the flame?

What Happened: The flame was much easier to blow out than when blowing on a wide flat surface as in the first part of the last experiment.  We say that the bottle is much more “streamlined” than the flat cardboard, because it’s round shape allows a stream of air to flow around it much more smoothly than a flat surface.  Cars, planes, boats and other fast moving objects are designed to be streamlined to allow air to move around them as smoothly as possible.  When vehicles are streamlined, they are much more fuel efficient, because energy is not wasted by fighting unnecessary drag of the air.

So far you have seen some of the effects of air pressure.  These next experiments will illustrate another very important characteristic of air - how it behaves when its temperature changes.

Materials Needed: Two or three liter plastic soft drink bottle; quarter (or other coin that will cover the mouth of the bottle); refrigerator.

Procedure: Remove the cap from the bottle and place the bottle in the freezer portion of the refrigerator for about a half hour.  When the bottle is ready, wet a quarter and remove the bottle from the freezer.  Place the quarter on top of the mouth of the bottle and observe what happens.  Save the cap and bottle for use in the next experiment.

What Happened: After a moment or two, the quarter begins to move.  Do you know why?  See if the next experiment helps you to understand?

Materials Needed: Two or three liter plastic soft drink bottle; refrigerator.

Procedure:   If you are using the bottle from the last experiment, allow it to warm back to room temperature.  Screw the cap on the bottle and squeeze the bottle.  Note how easy or hard it is to squeeze.  Remove the cap and place the bottle into the freezer portion of the refrigerator for about a half hour.

Remove the bottle from the refrigerator and immediately put the cap back onto the bottle. Allow the bottle to warm back up to room temperature.  When the bottle has warmed, squeeze the bottle again.  How easy or hard is it to squeeze now? Remove the cap.  What happens?

What Happened: The bottle was much easier to squeeze before you placed it into the refrigerator than after it was removed.  You should have also heard the sound of rushing air when you removed the cap.  Do you know why?  The next experiment should give you more information.

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How Does Air Temperature Affect A Balloon

Materials Needed: Balloon; refrigerator; ruler; string.

Procedure:   Inflate the balloon and tie it off.  Place the string all the way around the balloon at its longest point.  Remove the string, stretch it out, and measure its length.  Write this number down.  Do the same thing for the widest point around the balloon. 

Place the balloon in the freezer for 30 minutes.  Remove the balloon and very quickly take the same measurements.  (You may even want to do this while the balloon is still in the freezer.)  Are the numbers the same or different?

Allow the balloon to warm back to room temperature and take these measurements one more time.  What are they now?
 
What Happened: You should have seen that the measurements decreased slightly when the balloon was chilled, but as the balloon warmed back up, they increased.  In fact, they were probably about what they were at the beginning.  Hopefully, you’re beginning to see what’s going on.  If so, you should be able to predict what will happen in the next experiment.

Materials Needed:  Two or three liter plastic soft drink bottle with cap; refrigerator.

Procedure:  Place the cap on the soft drink bottle tightly and put the bottle in the freezer portion of the refrigerator.  Remove the bottle.  What do you see?  With the cap still on, allow the bottle to return to room temperature.  Are there any changes?

What Happened:  When you removed the bottle from the freezer, it should have partially collapsed.  However, as bottle rewarmed, the bottle should have expanded back to it’s original size.

In each of the experiments in this series, air was cooled and allowed to rewarm.  In every case, when the air was cooled, it contracted.  When it warmed back up, it expanded. 

In the first experiment, when you cooled the bottle without the cap, you started with a bottle of cold air.  When you placed the wet coin over the top, you formed a weak seal.  As the air inside the bottle warmed, it expanded, and the pressure inside began to increase.  The expanding air broke the seal and escaped, which caused the coin to jump.

In the next experiment, you used the bottle cap to seal the bottle of cold air.  As the air warmed, it again expanded, but there was no way for it to escape.  The pressure inside increased, which made the bottle more difficult to squeeze.  When you removed the cap, the air rushed out and relieved the pressure inside.

When the balloon was cooled, it shrank, and when it was rewarmed, it expanded again.  Likewise, when the sealed bottle was cooled, it collapsed because the cooled air inside contracted, but when the air warmed back up, it expanded again, causing the bottle to return to its proper size.

In all of these experiments, you observed the same thing.  When air was heated, it expanded.  When air was cooled, it contracted.  You have probably also figured out that if this happens in a sealed container, the pressure inside is affected by the expansion or contraction.  

The next two experiments will help us better understand how pressure and temperature are related.

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Using Pressure and Temperature Changes to Crush a Soft Drink Can

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: Aluminum soft drink can; pair of kitchen tongs; bowl of water; stove, hotplate or alcohol lamp and burner stand.

Procedure:  Pour about 1 cm (½ in) of water into the bottom of the drink can.  Using the tongs, place the can over a heat source and allow the water inside the can to boil.  (The steam helps to raise the temperature inside of the can.)  With the tongs, very quickly remove the can from the heat and turn it upside down into the bowl of water.  Be careful not to spill hot water on yourself, or on anyone around you.  

What Happened: When the air inside the can was heated, it expanded.  The opening in the top of the can allowed the expanding air, along with the steam, to escape as it was heated.  When you removed the can and put it into the water upside down, two things happened.  First, the surrounding water very quickly cooled the air inside of the can.  As a result, the air inside contracted.  Second, the water sealed the top of the can so that air from the outside could not get in. Because the can was sealed as it was cooled, the pressure inside of the can dropped very quickly, and the air pressure outside of the can caused the can to collapse.

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Another Can Crushed By Pressure

You should now be able to explain what happens in this experiment.

CAUTION!  Always be careful to follow all safety precautions when using a stove, and use with adult supervision only!

Materials Needed:  One gallon metal can with a screw top; hotplate or stove top; oven mitts; metal sink or insulated potholders; water. 

Procedure:   Place about an 3 cm (1 in) of water in the bottom of the can.  With the top off, heat the can over the hotplate or stove until it boils.  Using the oven mitts, remove the can from the heat and place the can in a metal sink or on potholders.  Then, quickly screw the top back on and observe what happens.

What Happened: Once again, the air inside the can was heated.  The heat caused the air to expand, so that some of the air was forced out of the top along with the steam.  When the can was removed from the stove, the heating stopped.  By placing the cap on the can, air from the outside could not get in.  As the air inside cooled, it contracted, and the pressure inside the can dropped.  This allowed the greater pressure outside to crush the can.

Let’s review what you’ve learned about air and air pressure:

1.   Air has weight and takes up space.
2.   Air exerts pressure on everything around it.  This pressure is about 14.7 pounds per square inch at sea level.
3.   Air may be compressed by forcing more air into a closed space.  Liquids such as water cannot be compressed.
4.   Bernoulli’s Principle says that air pressure is lower in a moving air stream.
5.   Air expands when warmed and contracts when cooled.
6.  When air expands in a closed container, pressure increases.  When air contracts in a closed container, pressure decreases.

This is quite a lot to know about air, but there is more!

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Air Is Required For Burning

CAUTION!  Always be careful to follow all safety precautions when using fire, and use  with adult supervision only!  Keep your candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: Glass jar or tumbler; candle with safety holder.

Procedure:   Light the candle.  Carefully cover the candle with the glass container.  Observe what happens.

What Happened: The candle flame gradually died out.  You probably already know why.  One of the gases in air is oxygen, and oxygen is required for burning.  The candle flame will burn so long as there is oxygen inside of the jar.  However, when all of the available oxygen is used up, the flame goes out.

Burning is one type of a process known as “combustion”.  Combustion is a chemical reaction that breaks down an element or compound, the fuel, by adding oxygen.  This reaction releases energy in the form of heat.

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A Closer Look at the Candle Flame

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

In the last experiment, you saw that oxygen is required for burning.  In this experiment you will see two things that are produced by burning.

Materials Needed: Candle in a safety holder; glass jar or tumbler.

Procedure: Light the candle.  Carefully hold the outside bottom of the glass container in the flame.  What do you see?

What To Look For:   Do you see any droplets of water on the glass?  What else do you see?

What Happened: You should have seen droplets of water forming on the outside of the glass near the flame.  Water vapor is produced when many fuels are burned.  As the candle burned, water vapor was formed and condensed on the cooler surface of the glass.

You should have also seen some soot on the inside of the bottom.  A candle burns because the melting wax produces a flammable gas, the fuel.  As the fuel burns, heat is produced. When many fuels burn, they don’t burn completely.  This soot from the candle flame is made mostly of carbon that did not completely burn.

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your candle flame at least three feet away from anything that can burn.

This experiment has been around for a long time.  In fact, you may have already seen it at school, or may have done it yourself.  But this experiment doesn’t show what most people think it shows!  Let’s see why.

Materials Needed: Glass bowl; glass jar or tumbler; modeling clay; candle.

Procedure: Use a little modeling clay to make a holder for the candle in the bottom of the glass bowl. Make three of four clay balls about 3 cm (1 in) in diameter.  Press these balls onto the bottom of the bowl so that the jar or tumbler can rest on them when placed upside down over the candle.  Make sure to leave some space between the three balls to allow water to move between them.

Fill the bowl with about 8 cm (3 in) of water.  Light the candle and place the jar or tumbler upside down over the flame and allow the mouth to rest on the clay balls.  Observe what happens as the candle goes out.

What To Look For:   Watch the water level inside the jar.  Do you see any air bubbling out from under the jar or tumbler as it is placed under the water?  If you don’t see this at first, repeat the experiment and watch for it.

What Happened: You should have seen the water level in the glass rise up about one fifth of the way up the side of the container.  This is usually explained by saying that air is about one fifth oxygen, and that as the burning candle uses the oxygen, water is drawn up into the container to replace the oxygen.  However, this isn’t exactly correct.

It is true that air is about one fifth oxygen, and that the candle cannot burn without using oxygen.  However, as the oxygen is being used up, another gas is being produced.  As you saw in the last experiment, anything that is being burned produces water and may also produce carbon in the form of soot or ash.  But the burning candle is also producing a gas called carbon dioxide. In fact, about as much carbon dioxide is being formed as oxygen is being used up.

Combining what we have learned so far, we cans say that burning can be represented by saying. “fuel combined with oxygen produces heat, carbon dioxide gas,  water vapor and sometimes carbon”.  Burning can also be represented by the following equation:

Going Further:  If carbon dioxide, a gas,  is being produced by burning at the same time the oxygen is being used up, why does the water get drawn up into the container as the candle goes out?   Think about this before you try the next experiment.  Also, think about why some of the gasses bubbled out when you first placed the jar over the candle.

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your candle flame at least three feet away from anything that can burn.

This experiment takes a little while to set up, but it is well worth the time required to do it, and it solves the mystery of the missing gas from the last experiment!

Materials Needed: Glass bowl; glass jar or tumbler; modeling clay; candle; two used model rocket engine igniters (unused igniters will also work); paper match; tape; wire; 60 cm (2 ft) of insulated wire; 9 volt battery; plastic tubing or two flexible soda straws.

Procedure:   Use modeling clay to make a candle holder and three small balls in the bottom of the bowl just as you did in the last experiment.

You will need two 30 cm (12 in) pieces of insulated wire.  You can use wire from the Christmas light set used in the electricity experiments, or if you can find a small two wire electrical cord, you can have an adult to help you cut off a 30 cm (1 ft) piece of this cord.  Carefully trim about 2 cm (½ in) of insulation from each end of both wires. Twist one end of each wire to each end of the igniter.  Put a piece of tape over these twists to hold them securely.

Tape the igniter to the candle so that the center of the igniter touches the candle wick.  Next, place the head of a paper match between the wick and the igniter.  Make sure that the igniter, match head, and wick are touching firmly, and that the bare parts of the two wires leading to the igniter don’t touch.

At this point, you should test your setup to see if you have done everything properly.  Touch the other ends of the two wires to the terminals of the battery.  The thin wire of the igniter should heat up enough to light the match, and the match should light the candle.  If it does not, check your setup carefully.  If needed, you may use small bits of modeling clay or tape to hold everything in place.  If you use an unused igniter, the thin wire inside will heat the chemical spot on the igniter causing it to burn.  This will also light the match.

Once your setup is working, rewire the candle for the rest of the experiment. Your igniter is probably good for two or three uses, but if the small wire is broken, replace the igniter with another one.

Fill the bowl with about 8 cm (3 in) of water.  Place the jar upside down over the candle.  Next, stick the rubber tubing up inside the jar above the water.  If you don’t have rubber tubing, use two flexible drinking straws taped together. Draw some of the air out of the jar to raise the water level inside about to about 1/4 the volume of the jar.  Place your finger over the end of the tubing and pull it out.  Stick a piece of tape on the side of the jar to mark the water level.

Use the battery to light the candle as before.  Observe what happens.

What To Look For:  Carefully notice what happens to the water level from the time the  candle lights until the time it goes out.

What Happened: As the candle began to burn, you should have seen the water level go down.  However, when the candle burned out, the water level again rose, and returned to about the same level as at the beginning.  Here’s why.

When the candle was lit, the air inside the jar was heated.  Oxygen was being consumed by the burning candle, but about the same volume of carbon dioxide was being formed at the same time, so the total volume of gases stayed about the same. However, remember that heated gases expand.  So, the gases inside the jar expanded due to the heat, and pushed the level of the water down.  By raising the level of water in the jar before you started the experiment, you were able to see the gases expand without letting them escape.

When the candle flame went out, the gases inside the jar began to cool.  As gases cool they contract.  The contracting gases caused the pressure to drop inside the jar and water was drawn back inside by the lowered pressure.

Remember that carbon dioxide is being formed as oxygen is being used up.   Since about the same volume of carbon dioxide is being produced as oxygen is being used, and since you sealed all of the gases inside the jar, once the gases cooled, the water level inside of the jar didn’t change all that much from when you started.

This experiment doesn’t change the fact that oxygen is being used up when fire burns.  Nor does it change the fact that air is about one fifth oxygen.  However, it does show that the water drawn up into the jar in the “classic” candle experiment is drawn up because of expanding and contracting of air, and not because oxygen is being used up.  It is something of a coincidence that the water level rises up about one fifth of the way up the jar.

CAUTION!  Always be careful to follow all safety precautions when using fire, and use  with adult supervision only!  Keep your candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: Steel wool or soap pad made from steel wool; scissors; candle with safety holder; pair of pliers or tongs; oven mitt.

Procedure:   If you are using a soap pad, wash all of the soap out of the pad and allow it to dry.  Cut a 3 cm (1 in) square from the steel wool or soap pad and spread out the steel strands.  Next, light the candle.  Using the oven mitt, grasp the steel wool with the pliers and hold the steel wool at arms length in the candle flame.  Observe what happens.

What Happened: The steel wool burned in a shower of sparks.  If iron is heated hot enough, and is cut into pieces small enough that are completely surrounded by oxygen, it will burn. This is another example of burning.  You should have noticed some brown or black products produced by burning.  This is called “iron oxide”, but you know it better as rust.
 
You may have seen similar sparks while burning a sparkler.  The sparks from a burning sparkler are also produced by bits of burning metal.