Equipment needed: 1 floating candle 1 bowl of water 1 clear glass jar or drinking glass that fits over the candle 1 match to light the candle When you light a candle, the heat from the flame melts the candle wax. Liquid wax flows up the wick where it is burned in the surrounding air. So, what happens if you cut off the air supply? And why are we doing this experiment with a floating candle?This experiment can be interpreted at different levels depending on the age of the child. A young child can enjoy the cause and effect result. Cover the candle with the glass. Watch the fire go out, watch the water rise inside the glass, and see fog form on the inside of the glass.Older children, about 10 and above, can appreciate the science at a more complex level. You’ve probably snuffed out a candle. You cover the flame, and when there isn’t enough air for the wax to burn, the fire goes out. Air is made of a combination of gasses. About 80% of air is nitrogen, which has nothing to do with a candle burning. About 20% of air is oxygen (O2), which is necessary for a candle to burn. Air also has small amounts of other gasses, like helium, and argon, which also have nothing to with fire. Fire needs air to be at least 16% oxygen in order to burn candle wax. Candle wax is a combination of carbon (C) and hydrogen (H). When the candle wax burns, the carbon in the melted wax combines with oxygen in the air to become carbon dioxide (CO2). And the hydrogen in the melted wax combines with oxygen to become water (H2O), which makes the glass look foggy. (I’ve posted a picture of the glass without the fog, so you can see the difference.) When the amount of oxygen in the air drops to 16%, the fire goes out, even though there is still plenty of gas surrounding the flame.Now, we come to the reason for doing this experiment with a candle floating on water. Watch what happens to the candle when the flame goes out. Water gets sucked into the glass to take up the room that the oxygen used to fill. The candle now floats on water that is higher than the liquid outside the glass. Water has been pulled up inside the glass. You can lift the glass higher, and see that the water level climbs along with the glass. You are pulling water up.This is a complex experiment. The first time I saw it was in an organic chemistry class. The teacher didn’t know the 16% rule , and couldn’t explain why the water didn’t fill 20% of the glass when the candle flame died. But when my younger daughter was in 2nd grade, she was thrilled that she could make the flame go out and the water rise just by putting a glass over a floating candle. There’s more to learn at every level. For example, carbon dioxide is somewhat soluble in water. Therefore while every Oxygen molecule that forms a Carbon dioxide molecule is replaced one-for-one, (One O2 plus 1 Carbon = 1 CO2) fewer gas molecules are actually in the air because some of them are in the water.In addition, when air is warm (as it is over a flame), the air molecules are further apart that when it is cool. So, when the flame on the candle goes out, the air begins to cool, and the air molecules move closer together, thus taking up less room, thus leaving space for the water to rise. There’s lots going on here. Have fun with it. |
