Air Pressure (Joseph Lewis)
Title: The Rising Water Experiment
Principle(s) Investigated: The Ideal Gas Law
Standards: -HS-PS1-5. Apply scientific principles and evidence to provide an explanation about the effects of
changing the temperature or concentration of the reacting particles on the rate at which a
reaction occurs.
Materials: Beaker (100mL is best), matches or lighters (matches are best), candles (birthday candles are best), plate or petri dish, and water.
Procedure:
Light the candle and let the candle wax drip down onto the plate to make a puddle.
Place the still-lit candle in the puddle of wax so that it stands upright.
Once the candle is secure, pour water onto the plate until water surrounds the base of the candle.
Take an empty beaker and place it upside down on top of the candle, placing it onto the plate.
Make observations as the candle goes out and the water rises up into the beaker.
Student prior knowledge: Gay-Lussac's Law: The Pressure Temperature Law. This law states that the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature. With an increase in temperature, the pressure will go up.
Explanation: The candle heats the air and expands it. The chemical reaction inside the beaker is: oxygen O2 and paraffin Cn H2n+2 . The burning produces water H2O and carbon dioxide CO2. Balancing the equation reveals:
2 O2 + C H 4 = C O 2 + 2 H 2 O
Twice as much oxygen is burned than carbon dioxide released, which also causes the number of molecules to decrease. When the oxygen is depleted, the candle goes out and the air cools. Cool air has lower volume and pressure, and now the inside of the beaker has fewer gas molecules as well, which causes the air to compress, pulling the water up and into the beaker. The water wants to rise because the air pressure outside the beaker is higher than inside the beaker since the candle went out; the air pressure outside the beaker pushes the water down and up into the beaker.
The relationship between these variables can be measured by the Ideal Gas Law, which states that:
PV=nRT
P = pressure
V = volume
n = amount of substance
R = ideal gas constant
T = temperature
This formula can be used as a way of validating claims and theories about why the water rises.
Questions & Answers:
Q: Why does the water rise once the candle goes out?
A: The drop in temperature causes a drop in air pressure within the candle
Q: Why doesn't the water rise right away, if the chemical reaction is reducing the number of gas molecules in the air?
A: While the chemical reaction is resulting in a decrease of n and V, and thus P, the increase in T from the candle has a greater impact on the P. Physics is playing a larger role than the Chemistry.
Q: What would happen if we added multiple candles instead of just one?
A: It would effect several variables. It would result in quicker depletion of O2, but it would not change the Chemical formula, so theoretically would produce the same amount of CO2 - however, more candles means less V to begin with, because they are taking up space, which means less O2 to burn. This would mean the Chemistry side of the experiment would produce a smaller outcome on the change in Volume, and a smaller influence on the rising water.
Still, if Physics and Pressure Differential is playing the larger role, there could still be a rise in water. However, because the candles will extinguish faster, and less combustion takes place due to less V, it is possible the Temperature will not rise as high as it would with a single candle, and thus the Physics aspect would result in a smaller rise in water. However, this can be adjusted by allowing the candles to burn for a period of time before placing the beaker on top of them, allowing them a head-start on heating the air around them. Though, this could ALSO result in the air just outside the beaker to be heated more than in the original experiment, which would result in very hot air outside the beaker. This hot air would have even higher pressure, and be able to push down the water down and up into the beaker, possibly resulting in more water rising inside the beaker.
The degree to which these variables effect the speed and amount the water rises is a great task to give students to investigate.
Applications to Everyday Life:
Taking a soccer ball to a high elevation town: If you inflate a soccer ball at sea level, where air pressure is high, and then take it with you to visit a friend in the mountains, the ball will explode/expand. This is because the higher elevation town has less air pressure. The higher pressure inside the ball, then, attempts to compensate and escape. This 'escape' is the result of the Ideal Gas Law, wherein if you solve for V, P is in the denominator, meaning it will have an inverse relationship; this means that a drop in P results in an increase in V. Thus, the gas inside the ball will try to increase their Volume.
Refrigerators: Coolant gas is compressed, causing its temperature to increase. The hot gas is passed through a radiator, allowing excess heat to escape, and then the gas is allowed to expand into the refrigerator. As it expands, it cools off, and heat is drawn from the interior of the refrigerator.
Exploding aerosol cans left in the sun: A can of spray paint contains lots of gases, and leaving it in the sun causes the temperature of those gases to increase. As described by the Ideal Gas Law, this means that the pressure of those gases on the can also increases. When the temperature, and thus the pressure, of those gases increase to a point where the pressure of the gas on the can is more than the can is able to withstand, the can of spray paint explodes.
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