Candles
Last updated: September 12, 2008.
Imagine if there were no electricity
and you had to survive up to 12 hours of darkness each night by candelight! It sounds wonderful in our age of
cold, sterile, fluorescent
light. But if you had to live that way all
the time you'd find it an awful lot of bother, especially if your
house had many candles, all burning at once. You'd not only have to
keep the wicks burning brightly, you'd also have to ensure they
weren't going to tip over and cause a fire. Drawbacks aside, candles
will always be a symbol of romance. Look more closely and you'll also
find they're classic examples of ingenious technology. Let's take a
closer look at how they work!
Photo: Who says science isn't romantic... or romance isn't scientific? Candles are a great example of how science adds an extra dimension to the beauty of the natural world—a point the brilliant American physicist Richard Feynman was fond of making. Listen to him discussing the question: Can a scientist really enjoy the beauty of a flower?
How candles use combustion
Candles make light by making heat, so they're crude examples of what
we call incandescent lamps (classic electric filament lamps, pioneered
in the late 19th century by Thomas Edison, are a much more
sophisticated version of the same idea). All the light a candle makes
comes from a chemical reaction known as combustion
in which the wax (made from carbon-based chemicals typically derived from
petroleum) reacts with oxygen in the air to make a colorless gas
called carbon dioxide. Water is also produced
in the form of steam. Since the wax never burns perfectly cleanly, there's also a little
smoke produced. The smoke is an aerosol (tiny particles of solid, unburned carbon from the wax mixed in with the steam) and it often leaves a black, carbon
deposit on nearby walls or the ceiling above where the candle's
burning. The steam is made in the blue part of a candle flame, where
the wax burns cleanly with lots of oxygen; the smoke is made in the bright, yellow part of the flame, where
there isn't enough oxygen for perfect combustion to take place.
Photo: Candles don't burn all by themselves. It takes energy to kick-start the chemical combustion reaction that makes the wax burn. The initial energy you need to start a reaction is called activation energy. You can provide it using a burning match.
How a candle wick works
Candles may look simple but they're remarkably ingenious. Set fire
to the wick (the little string poking up at the top) and heat travels
rapidly downward toward the wax body of the candle beneath. The wax has a
low melting point so it instantly turns into a hot liquid and
vaporizes, funnelling straight up around the wick as though it's
rushing up an invisible smokestack (chimney). The wax vapor catches light
and burns, sending a flame high above the wick. Heat from the flame
travels in three directions at once by processes called conduction,
convection, and radiation. Conduction carries heat down the wick to
melt more wax at the top of the candlestick. Convection draws hot wax
vapors out from the wick and sucks oxygen from the surrounding air
into the base of the flame. The flame also gives off invisible beams
of heat in all directions by radiation. The candle continues to "feed"
on the wax underneath it until it's all burned away—until all the
potential energy locked away in the wax is converted to heat,
light, and chemical waste products.
Which part of a candle flame is the hottest?
Here are some approximate temperatures for the different parts of a candle and its flame. Note that the exact temperatures
vary quite a bit depending on all kinds of different factors, notably the type of wax from which the candle is made but also the ambient (air) temperature, and how much oyxgen is present. Please don't take these values as absolutely definitive ones that apply in all cases—they're just a rough guide.
- Wick: 400°C (750°F).
- Blue/white outer edge of the flame (and also the blue cone underneath flame where the oxygen enters): 1400°C (2550°F).
- Yellow central region of the brightest part of the flame: 1200°C (2190°F).
- Dark brown/red inner part of the flame: 1000°C (1830°F).
- Red/orange inner part of the flame: 800°C (1470°F).
- Body of the candle: 40-50°C (104-122°F).
- Melted pool of wax on top of the candle: 60°C (140°F).
Perhaps surprisingly, the brightest part of the flame is not the
hottest. The
blazing part of the flame gives off three quarters of its energy as
light and only a quarter as heat (so you can see a candle is, at
best, around 75 percent efficient as a light). The hottest parts of a candle
flame are actually the blue, almost invisible area near the base,
where oxygen is drawn in, and the blue/white part around the edge, where the
flame meets the oxygen-rich air all around it. The flame
gets progressively cooler as you move in from the outside edge toward
the wick. Cooler areas are darker and colored orange, red, or brown.
Most of the flame's heat is delivered toward the tip, where a large
volume of gas is always burning and convection is sweeping hot gases
constantly upwards. If you want to heat something with a candle, hold it
near the tip.
Do candles burn in space?
The's answer's no, yes, and maybe. "No", because there's no oxygen in space. "Yes", because you can burn candles in a spaceship where there's an artificial supply of air. The answer's "maybe" because candles don't burn in the microgravity of space exactly as they burn back here on Earth. There's no "up" and "down" in space, so there's no "top" or "bottom" of a candle flame either. Convection doesn't draw cooler oxygen in at the bottom and throw hot exhaust gases out at the top, as it does here on Earth, where hotter gases are less dense (weigh less per unit of volume) than cooler ones. In the microgravity of space, with plenty of oxygen, candle flames are more spherical, as this NASA photograph clearly shows:
Photo: Candles burning on Earth (left) and in space microgravity (right).
Photo courtesy of
NASA Glenn Research Center (NASA-GRC).
