Make Water Rise

Though you can make the water level of any body of water rise by adding more mass to it (e.g. more water or another material), there is a common experiment designed to make water “rise” in a glass. This experiment is what people refer to when they talk about making water rise. With just a few items, you can use the principles of Charles’s Law to impress and intrigue an audience with this experiment.

Steps

Setting Up the Experiment

  1. Fill a pan with water. The pan should be an inch or two deep at least. You need a pan that can hold enough water so that it isn’t all sucked up into the glass. If all of the water gets sucked up into the glass, then air will be able to get in. Since the water is heavier than the air, it will fall back out of the glass because of gravity. [1]
  2. Add food coloring to the water. This step is optional. The food coloring does not contribute to the science involved at all. It does make the water easier to see as it rises up the glass. This is especially important for a large audience that might not be able to get close to the glass.[1]
  3. Collect all other materials needed. Make sure that you have a glass, a candle, and a lighter or match before starting. It does not matter what kind of match or lighter that you use. You must use a glass and not a plastic cup. Using a short candle also keeps the flame from directly touching the glass.
    • If you try to substitute a glass with a plastic cup, it will melt and the experiment will fail.
    • Tea light candles work well for this experiment.

Making the Water Rise into the Glass

  1. Put the candle into the pan of water. Make sure not to get the wick wet. Light the candle in the center of the pan of water.[1]
    • If you are using a tea light candle, it will sit in the pan on it’s own. If you are using a tall slender candle, it may tip over. To avoid this, use a short candle holder to stabilize the candle.
  2. Place the glass over the candle. Flip the glass upside down and set it overtop of the candle. Make sure that the opening of the glass is completely submerged in the water. This provides a seal that prevents air from entering the glass and allows the water to rise into the glass.[1]
    • If the candle is touching the glass, you can either get a shorter candle or a taller glass. This will ensure that the experiment goes well.
  3. Watch for bubbles. As the candle burns, it heats the gas inside the glass. This will cause the gas to expand, raising the pressure inside the glass. Some gas will need to escape the glass to relieve the pressure.. The only way for gas to escape is through the water at the bottom of the glass. Once the pressure gets high enough, the gas will have no choice but to bubble out through the water. [2]
  4. Allow the candle to burn out. The burning candle converts two oxygen molecules (2O2) and one molecule of paraffin wax (CH4) to one carbon dioxide molecule (CO2) and two water molecules (2H2O). With no air entering the glass, the candle will burn all of the oxygen available. When the oxygen runs out, the fire on the candle can no longer burn.[3]
    • Though there may be oxygen left in the glass, the candle will go out when it has used up all of the oxygen that is close enough to fuel the flame.
  5. Watch the water rise into the glass. When the candle burns out, the remaining gas will cool quickly. The cooling gas contracts (shrinks its volume) and creates a vacuum in the glass. The outside pressure will force the water into the glass to fill the vacuum. This will be easier to see if you chose to color the water with food coloring. Keep in mind that the water will only fill part of the glass, since there is still some gas trapped in the glass.[1]

Understanding Why the Water Rises into the Glass

  1. Know what chemical reaction is taking place. There are many incorrect explanations for this experiment. In order to understand what is really going on you need to know what chemistry is taking place. The combustion reaction occuring is:[3]
    • 2O2 + CH4 → Co2 + 2H2O
  2. Recall Charles’s Law. Charles’s Law is a piece of the ideal gas law that says that the volume of a gas will change proportionally with temperature (and vice versa) when outside pressure and the mass of the gas remain constant. Since the walls of the glass do not change the amount of pressure that they exert on the gas inside, the pressure in this experiment remains constant. The gas cannot enter the glass because it is upside down in the water, creating a seal.
    • Technically, since the combustion reaction converts two O2 molecules into one CO2 and two H2O molecules, the amount of gas temporarily increases. However, the candle will not keep the temperature inside the glass above 100 degrees Celsius (212 degrees Fahrenheit) and the two H2O molecules will condense into liquid water. This means the amount of gas in the glass effectively decreases ‘’’while the candle burns’’’.[3]
    • Charles’s Law applies in this case only because the amount of gas in the glass remains constant from the time that the candle burns out. Without the combustion, no more gas is consumed or produced. The heat source simultaneously disappears, allowing the gas to cool and decrease in volume.[3]
  3. Consider what happens when the gas cools. Since the volume of your gas is proportional to the temperature, it decreases as the gas cools. This decrease in volume creates a vacuum inside the glass. The pressure outside the glass remains the same, so this creates a pressure differential.[2]
  4. Understand pressure differentials. Within any given system, fluids move from the high pressure end to the low pressure end. With air sealed out of the glass, the water is forced from the high pressure end of the system (outside the glass) to the low pressure end of the system (inside the glass) until the pressure inside the glass is equal to the pressure outside the glass.[2]
    • This is a little like opening a faucet. The pressure outside the water line is lower than the pressure inside the line, so the water is forced through the line and out of the faucet (even if it has to go against gravity to do so).

Tips

  • Try the experiment alone before conducting it for an audience. This allows you to work out any bugs in your system with no one watching.
  • Use the food coloring to make the water visible.

Warnings

  • Do not use plastic cups. The flame will melt the cup and cause the experiment to fail.

Things You’ll Need

  • A Pan
  • Water
  • Food Coloring
  • A Glass
  • A Candle
  • Matches/Lighter

Sources and Citations

  1. ↑ 1.0 1.1 1.2 1.3 1.4 https://www.questacon.edu.au/outreach/programs/science-circus/activities/thirsty-candle
  2. ↑ 2.0 2.1 2.2 http://www.stevespanglerscience.com/lab/experiments/why-does-the-water-rise/
  3. ↑ 3.0 3.1 3.2 3.3 http://www.math.harvard.edu/~knill/pedagogy/waterexperiment/
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