Posted by Jeff on 4/16/2013
The Colors of a Candle Flame
If you look closely at a candle flame, you’ll see a blue area at the base of the flame. Above that is a small dark orange-brown section, and above that is the large yellow region that we associate with candle flames.
The oxygen-rich blue zone is where the hydrocarbon molecules vaporize and start to break apart into hydrogen and carbon atoms. The hydrogen is the first to separate here and reacts with the oxygen to form water vapor. Some of the carbon burns here to form carbon dioxide.
The dark or orange/brown region has relatively little oxygen. This is where the various forms of carbon continue to break down and small, hardened carbon particles start to form.
Interesting ReadingAs they rise, along with the water vapor and carbon dioxide created in the blue zone, they are heated to approximately 1000 degrees Centigrade.
The Chemical History of Candles
(Michael Faraday’s 1860 series of lectures in London)
Candles in Microgravity
(NASA’s space program research on candles)
Candlestick Rocket Ship
(NASA’s experiments using paraffin wax as rocket fuel.)
The Physics and Chemistry Underlying the Infinite Charm of a Candle Flame
(By Jearl Walker. Originally printed in The Amateur Scientist Column, Scientific American, April 1978.)
At the bottom of the yellow zone, the formation of the carbon (soot) particles increases. As they rise, they continue to heat until they ignite to incandescence and emit the full spectrum of visible light. Because the yellow portion of the spectrum is the most dominant when the carbon ignites, the human eye perceives the flame as yellowish. When the soot particles oxidate near the top of the flame’s yellow region, the temperature is approximately 1200o C.
The fourth zone of the candle (sometimes call the veil) is the faint outside blue edge that extends from the blue zone at the base of the flame and up the sides of the flame cone. It is blue because it directly meets with the oxygen of the air, and is the hottest part of the flame, typically reaching 1400o C (2552o F).
Why a Candle Flame Always Points Up
When a candle burns, the flame heats the nearby air and starts to rise. As this warm air moves up, cooler air and oxygen rush in at the bottom of the flame to replace it.
When that cooler air is heated, it too rises up and is replaced by cooler air at the base of the flame.
This creates a continual cycle of upward moving air around the flame (a convection current), which gives the flame its elongated or teardrop shape.
Because “up” and “down” are a function of the earth’s gravity, scientists wondered what a candle flame would look like in outer space, where the pull of gravity is minimal and there really isn’t an up or down.
In the late 1990s, NASA scientists ran several space shuttle experiments to see how candle flames behaved in microgravity. As you can see from the NASA photos below, a candle flame in the microgravity is spherical instead of its elongated shape on Earth. Without gravity, there’s no “up” direction for warm air to rise and create a convection current.
A candle flame in normal gravity
A candle flame in microgravity