To illustrate the effects of gravity on the burning rate of candles.

BACKGROUND:

A candle flame is often used to illustrate the complicated physio-chemical processes of combustion. The flame surface itself represents the location where fuel vapor and oxygen mix at high temperature and with the release of heat. Heat from the flame melts the wax (typically a C 20 to C 35 hydro-carbon) at the base of the exposed wick. The liquid wax rises by capillary action up the wick, bringing it into closer proximity to the hot flame. This close proximity causes the liquid wax to vaporize. The wax vapors then migrate toward the flame surface, breaking down into smaller hydrocarbons enroute. Oxygen from the surrounding atmosphere also migrates toward the flame surface by diffusion and convection. The survival and location of the flame surface is determined by the balance of these processes.

In normal gravity, buoyancy-driven convection develops due to the hot, less dense combustion products. This action has several effects: (a) the hot reaction products are carried away due to their buoyancy, and fresh oxygen is carried toward the flame zone; (b) solid particles of soot form in the region between the flame and the wick and are convected upward,Activity 10 where they burn off, yielding the bright yellow tip of the flame; (c) to overcome the loss of heat due to buoyancy, the flame anchors itself close to the wick; (d) the combination of these effects causes the flame to be shaped like a tear drop.

In the absence of buoyancy-driven convection, as in microgravity, the supply of oxygen and fuel vapor to the flame is controlled by the much slower process of molecular diffusion. Where there is no "up" or "down," the flame tends toward sphericity. Heat lost to the top of the candle causes the base of the flame to be quenched, and only a portion of the sphere is seen. The diminished supply of oxygen and fuel causes the flame temperature to be lowered to the point that little or no soot forms. It also causes the flame to anchor far from the wick, so that the burning rate (the amount of wax consumed per unit time) is reduced.

MATERIALS NEEDED:

Birthday candles (several) Matches
Balance beam scale (0.1 gm or greater    sensitivity)
Clock with second-hand or stopwatch
Wire cutter/pliers
Wire
Small pan to collect dripping wax

PROCEDURE:
Step 1. Form candle holders from the wire as shown in the diagram. Determine and record the weight of each candle and its holder.
Step 2. Light the "upright" candle and permit it to burn for one minute. As it burns, record the colors, size, and shape of the candle flame.
Step 3. Weigh the candle and holder and calculate how much mass was lost.
Step 4. Place the inverted candle on a small pan to collect dripping wax. (Note: The candle should be inverted to an angle of about 70 degrees from the horizontal. If the candle is too steep, dripping wax will extinguish the flame.)
Step 5. Light the candle and permit it to burn for one minute. As it burns, record the colors, size, and shape of the candle flame.
Step 6. Weigh the candle and holder and calculate how much mass was lost.

QUESTIONS:
1. Which candle burned faster? Why?
2. How were the colors and flame shapes and sizes different?
3. Why did one candle drip and the other not?
4. Which candle was easier to blow out?
5. What do you think would happen if you burned a candle horizontally?

FOR FURTHER RESEARCH:
1. Burn a horizontally-held candle. As it burns, record the colors, size, and shape of the candle flame. Weigh the candle and calculate how much mass was lost after one minute.
2. Repeat the above experiments with the candles inside a large jar. Let the candles burn to completion. Record the time it takes each candle to burn. Determine how and why the burning rate changed.
3. Burn two candles which are close together. Record the burning rate and weigh the candles. Is it faster or slower than each candle alone? Why?
4. Obtain a copy of Michael Faraday's book, The Chemical History of a Candle, and do the experiments described. (See suggested reading list.)