Hands-on Science Carnival 2010 Activity Stations: Pressure & Buoyancy

Cartesian Divers, Dancing Raisins, Bladder Fish and Diet Coke vs Coke)

[Shopping List: soda bottles w/ caps; BB's; water bomb balloons; ketchup, honey packets; disposable pipettes; hex nuts and/or lock washers; Fizz-Saver pumps (Steve Spangler); raisins (soak some overnight in water); tall clear plastic cups; 7-Up (and a couple other types of carbonated beverages such as ginger ale, diet 7-Up, sparkling water, plain water, etc.); rice, spaghetti and other pasta]

Cartesian Divers (Fish)

1. Place 4 BB's into a small water bomb balloon (fish guts)
2. Dangle gently to allow air to fill balloon fish, then tie closed.
3. Test for buoyancy in a large cup or bowl- balloon fish should float.
4. Place fish into any size plastic soda bottle, fill completely with water, then screw on top tightly.
5. When bottle is squeezed, balloon fish will sink; stop squeezing and it will float to the surface again.

Tips: It may take a little practice to get the right amount of air into the balloon fish. Try to make them just slightly less dense than water, so that little kids can make this work without too much effort.

Cartesian Divers (Pipettes):

1. Cut most of the tip off a plastic disposable pipette (how much to cut depends on how much weight you will use in the next step, so you will need to experiment some).
2. Slide a hex nut or one or two lock washers into the neck of the pipette. You can also squeeze a small amount of water into the pipette for weight, but be sure to leave plenty of air.
3. Place pipette into any size plastic soda bottle, fill completely with water, then screw on top tightly.
4. When bottle is squeezed, the pipette will sink; stop squeezing and it will float to the surface again.

Tips: As before, it may take a little practice to get the right amount of weight on the pipette. Try to make them just slightly less dense than water, so that little kids can make this work without too much effort.

What's Happening: Gravity pulls everything towards the Earth's center (down), including water (or air) as well as anything in it, such as our fish. The effect of gravity on the water (or air) creates water (or air) pressure, which increases with depth. When any object is placed into water (or air), it's volume must displace or push some of the water (air) out of the way, which creates a force called buoyancy, equal to the weight of the water displaced, which pushes back against gravity. The size of the upward buoyant force is equal to the weight of the water (or air) displaced, while the size of the downward gravity force is just the weight of the object. Thus if the weight is greater than the buoyant force, the object will sink, while if it is less, it will float. Since density is equal to weight divided by volume, we can also say that if the density of the object is greater than the density of water, then it will sink, and vice-versa.
Initially the balloon fish or pipette floats, so its weight must be less than the weight of the volume of water it displaces, or we can also say that the density of the balloon or pipette is less than the density of water. When we squeeze the bottle however, we can't squeeze the water inside because water is incompressible (i.e. it can't be squeezed into a smaller volume), thus it in turn squeezes the air inside the balloon or pipette diver instead, which can easily reduce its volume. In the case of the balloon fish diver, when the volume of the balloon decreases its density increases (because it still weighs the same amount), and once its density becomes greater than that of water, it sinks. When the volume of air inside the pipette diver becomes smaller, water flows in to fill the space, which increases the divers weight as well as its density (because the total volume of the pipette- with its fairly strong plastic shell- stays the same), decreasing its buoyant force, and it sinks.

The pipette diver is particularly interesting to watch because you can see the water flow in and out as you squeeze the soda bottle. This is exactly how a submarine dives or rises to the surface, by filling or emptying its ballast tanks.

Variations: A bigger balloon (more air inside) has more buoyant force, while the number of BB's inside determines its weight, so by varying the size and weight of the balloons we can make it easier or harder for our diver to work. Try 2 or 3 different combinations in the same bottle. To make it harder to operate, leave a small amount of air in the bottle before screwing the top on. When you squeeze the bottle now, you are squeezing the air in the bottle rather than the water, thus the water can't squeeze the balloon as much. Changes in temperature also affects the density of the water (cold water is more dense than hot water), so a balloon which floats in cold water may sink if the water bottle warms up to much, perhaps by sitting in hot sunlight [to a lesser extent the warm water also warms the air in the balloon, which causes it to expand and increase its volume, thus increasing its buoyancy, so there is a trade off to be considered before determining if the balloon will now sink or continue floating. Generally speaking, the decreased density of the water with warming usually dominates, and the balloon sinks]. On the other hand, if there is a little air left in the bottle, then warming the water also warms the air in the bottle, increasing its pressure, and effectively adding more "squeeze" force to make the balloon sink. You can also make the balloon sink by removing the bottle cap and blowing very hard into the bottle with your lips tight around the neck (as though you're trying to blow it up). Finally, try other objects, such as restaurant ketchup or honey packets. What else can you use to make a diver? Use Fizz-Saver pumps to control buoyancy.

Dancing Raisins

1. Fill a plastic cup ~3/4 full of 7-Up
2. Add a few raisins (including some dry ones, wet ones, and some cut in half)
3. Watch what happens. At first all of the raisins should sink to the bottom of the cup, but within a few seconds at least some of the raisins should begin to rise to the surface, then dive again to the bottom, and repeat for several minutes.

What's happening: Carbonated drinks have a lot of carbon dioxide gas dissolved in the water (as well as some other gasses), the raisins have lots of wrinkles, and the density of a raisin is just a little more than the density of water.  The CO2 molecules in the liquid desperately want to escape, but only those at the surface have an easy way out.  The rest are surrounded by water molecules, so they need to form into a bubble and rise to the surface before they can escape, and to do this they need what's called a nucleation site, something that makes it easier to start a bubble.  This can be a lot of things, like specks of dirt, cracks in the surface of the container, other CO2 molecules if there are enough of them or they are moving really fast, or in this case the wrinkles on the raisins.  [Don't ask me why the wrinkles make good nucleation sites, I know it has something to do with making it easier to push away the surrounding water molecules, but the exact mechanism is a little beyond my studies on the subject.]  In any case, once a CO2 bubble starts to form it's much easier for other CO2 molecules to jump in- just like those up at the surface- and buoyancy allows the bubble to rise to the surface where it pops and the CO2 gas escapes into the atmosphere.  [This is actually very similar to what happens when a liquid starts to boil by the way, which is just when water molecules deep in the liquid (i.e. far from the surface) acquire enough energy to change state from liquid to gas and form a bubble, rise to the surface, and escape.  Watch a pot of water as it starts to boil, and you will see that the first bubbles form on scratches and dents in the metal, i.e. good nucleation sites.]

Getting back to our dancing raisins, when bubbles start to form in the raisin's wrinkles, they tend to stick there and grow. Eventually enough bubbles will form and grow to a big enough size that they create enough buoyant force to lift the whole raisin to the surface (the raisins are too dense to float on their own).  It's almost as if you put a little life-preserver onto the raisin so it could float.  Once the raisin gets to the surface however, some of the CO2 bubbles begin to pop, suddenly there's not enough buoyancy to float it any longer, and it sinks back to the bottom, where the whole cycle starts all over again.

Variations: Try different liquids, which may have more or less CO2 and other dissolved gasses, and more or less sugar and other dissolved solids which affect the density. Try different fruits (including some without wrinkles), rice, spaghetti, etc. Try first soaking the fruits (and pastas) in water, 7-Up and other liquids overnight or longer. Try cutting the fruits (and pastas) into different sizes. Try putting them into Cartesian diver bottles and applying pressure. Try warm or cold liquids. Try first soaking raisins (and pastas) in oil to prevent gas bubbles attaching.