As discussed in the Fire Extinguisher experiment, fire is a chemical reaction that requires a fuel source, heat, and oxygen in order to sustain itself. Without one or more of those components, the reaction will stop and the flames will go out. The combustion reaction consumes both the fuel and the oxygen and reorganizes their molecules into new substances, including carbon dioxide.
In a candle, the fuel source is actually the wax of the candle, not the wick. The heat from the flame melts the wax surrounding the wick. The wick “sucks up” the melted wax, like a drink through a straw, delivering it to the flame. The fire then burns the melted wax, creating more heat, which melts more wax. Traditionally, candles were made from substances found in nature that burned very slowly, like animal fats or beeswax. Today, candles can also be made with paraffin wax (a byproduct of oil refining) or plant oil-based waxes like soy wax. Wax burns much slower than other fuel sources, like wood, so candles can produce sustained light for a long period of time.
Have you ever wondered why a candle goes out when you blow on it? It is not because of the carbon dioxide in our breath. The current from blowing lowers the temperature of the area around the reaction and separates the flame from the fuel source – the melted wax. Without anything to burn and without the requisite heat, the reaction stops.
The Experiment
Supplies: A tealight candle, a saucer or shallow bowl, water, food coloring (optional), a clear glass or jar, a lighter or match, an adult helper.
What to do: Ask your adult assistant to set the candle in the saucer and light it. Add a few drops of food coloring to your water, if desired. Pour about 1/4 cup of water into the saucer so that it surrounds but does not extinguish the candle. Turn the glass upside down and carefully place it over the candle. What happens to the candle? What happens to the interior surface of the glass? What happens to the water in the saucer? Listen carefully as you lift the glass up.
What is happening: Three different principles are being demonstrated in this experiment. The first has to do with the mechanics of the chemical reaction. The water forms a seal between the upside-down glass and the dish, limiting the reaction to only the air available inside the glass. At first, the candle will continue to burn as normal, but the longer the flame burns, the less oxygen is available in the confined space to sustain the reaction. The candle will only burn as long as there is oxygen inside the jar.
The second thing happening inside the glass is proof the the displacement reaction taking place. In displacement reactions, the compounds or molecules that contribute to the reaction are different than the ones that result from the reaction. In this case, the carbon and hydrogen from the wax is combining with the oxygen to fuel the reaction. The during the reaction, the oxygen molecules get shifted around, and the end products of the reaction are carbon dioxide gas and water. While the flame is burning, the water remains in the air inside the glass in the form of water vapor, but as soon as the flame goes out, the air inside the glass cools down, and the water vapor condenses on the interior surface of the glass.
Did you notice the water creeping up the sides of the glass as the flame went out? This is related to the third principle being demonstrated, differences in air pressure. While the flame was burning, the oxygen inside the glass was being consumed by the reaction and replaced with carbon dioxide, but not in equal quantities. The heat from the flame causes the gas inside the glass to expand in volume, but once the flame goes out, the gas inside the glass cools down quickly. This creates a vacuum inside the glass because the air pressure outside the glass is greater than the air pressure inside the glass. The sound created when you moved the glass was the greater outside air pressure rushing in to fill the space inside the glass where the air pressure was lower.
Links
To learn more about oxygen and Antoine Lavoisier (the scientist who came up with the name “oxygen” and made this experiment famous), head to the Kiddle article on Oxygen Facts.