A falling candle: kitchen-science
Q: If you place a lighted candle in a sealed jar and drop it, why does the candle go out as soon as someone catches the dropped jar? (Karen, Brainerd, Maine)
A: First, a word about how a candle burns. A candle is a cylinder of solid fuel — paraffin wax — that surrounds a wick. Bringing a lit match to a wick melts and then vaporizes the wax coating the wick. The wax vapor combines with oxygen, and burns.
A burning candle. Within the bluer, hotter region near the base of the wick, hydrogen separates from the wax vapor, burns and forms water vapor. Within the brighter, yellower part of the flame, carbon soot oxidizes, and forms carbon dioxide. Courtesy of Wikipedia.
Second, a caveat: Candles burn nicely, and can catch other things on fire. Keep a bucket of water or, better still, a fire extinguisher handy if you experiment.
Now, to answer your question: The flame goes out when we catch the falling jar for the same reason as it goes out when we blow on a candle — the temperature of the candle-wax vapor drops below the vapor’s combustion temperature. The vapor becomes too cool to burn.
As the jar and candle free fall, the effects of gravity disappear. So, the candle, the flame and the hot gases surrounding the flame all fall together as a unit. When someone catches the jar, however, the cooler mass of air high in the jar continues to descend and replaces the hotter air below (which then buoys up). The descending cooler air "blows out" the candle.
That’s the answer that occurred to me as I mulled over your question in bed at 3 a.m. one night. But is it right? Like a good scientist, I needed to run an experiment to see if my theory fits the observable facts.
The next Saturday a conscript and I got together to determine the facts. Soon, I was scooping peanut butter out of a quart-size jar while my assistant sawed the ends off of a clothes pin. We fitted the trimmed clothes pin into the peanut-butter jar lid, and stuck it in place with double-sided tape. We clamped a birthday-cake candle to the jar lid with the clothes pin, lit the candle, and screwed the lid onto the jar. The result was something like shown in Figure 2.
Figure 2. A quart-size peanut butter jar and a burning birthday candle. Image by author.
The first question was — can we duplicate the reader’s scenario? I dropped the jar with the burning candle; the assistant caught it. The flame stayed lit all the way down, but went out when caught. Good — we observed the phenomenon.
What can put out a burning candle? If it doesn’t have enough air or fuel, or it gets too cool to burn.
So next, we made sure that the candle had plenty of air during its fall. Timing it, as it burned stationary in the closed jar, we determined the candle took more than 15 seconds to go out through lack of oxygen. Dropping it and catching it took only 5 seconds. So we knew the candle had sufficient air.
It had plenty of fuel. The candle was almost whole.
But — maybe the hot wax splashed on the wick when we caught the jar, and put out the flame. To test this, we placed a kitchen match in the jar instead of the candle, lit the match, screwed on the lid and dropped the jar. The flame stayed burning on the way down, and went out when caught. Since the match had no wax to splash onto the wick, that cinched it:
When we caught the falling jar and the candle went out, the descending cooler air mass blew out the candle. We concluded that my idea in the middle of the night was correct.
By the way, we captured the flame in flight with a digital camera. A difficult shot that blurred, but showed the flame shape was an orange ball instead of an upright ‘flame shape.’ A ball just like in space!
A candle burning in the free fall of space, where there is no "up" for hot gasses to buoy above the flame. Courtesy of NASA.
Freefall candle by Paul Doherty, Scientific Explorations and Adventures
Heat convection by Rod Nave, HyperPhysics
(Answered March 28, 2006)