Why is the bottom of a candle flame blue?

The bottom of the flame is getting plenty of oxygen, allowing the fuel vapor to burn completely. That produces a blue flame. If you look very closely at a candle flame, you can sometime see that the entire flame is surrounded by a thin layer of blue flame, because the surface where flame meets the surrounding air also gets plenty of oxygen, allowing complete combustion.

On the inside of the flame, limited oxygen levels result in incomplete combustion, which produces particles of carbon soot. Those particles are heated by the flame, and glow, much like the filament of a light bulb.

You can see the same blue flame on a gas stove, because vents allow the gas to mix with air before it burns. If your burner has a yellow flame instead, the vent is blocked. That can be dangerous, as incomplete combustion can produce carbon monoxide instead of carbon dioxide.

For more information on this, watch the Flame Color video.

With the holidays upon us, I am once again reading through Michael Faraday's Chemical History of a Candle. You can find it online at http://www.gutenberg.org/etext/14474. Truly a wonderful book. I was trying to think of a new candle experiment and came across a package of the "magic relighting candles" in the birthday card section at the grocery store. These are the ones that relight themselves a few seconds after you blow them out. How do they work? Let's find out. You will need:

• a candle
• a lighter
• one of the self relighting candles

First, lets burn the regular candle. Place it in a secure holder, so it does not fall over. Light the candle and let it burn for a couple of minutes. Check to see that it has formed a nice pool of melted wax around the base of the wick. Then blow out the candle. You should see a column of white smoke rising from the wick. Blow strongly on the wick, and you should see an ember glowing at the end of the wick. That ember and the white smoke are two of the important parts of the relighting candle.

The white smoke is really vaporized paraffin, the stuff the candle is made from. The glowing ember is hot enough to continue vaporizing the paraffin, but not hot enough to set the vapor on fire to relight the candle. That calls for a third ingredient.

Place the relighting candle in a holder. I have found that a cupcake or brownie works very well for this. I also put a little ice cream around it, just for safety. Light the "magic" candle, and let it burn for a few seconds. Then pretend it is your birthday, and blow out the candle. Watch the candle carefully. You should see the same rising column of white smoke. You will probably also see the glowing ember, but do not blow on the wick this time. Instead, watch closely. After a few seconds, the candle relights itself. Just as it relights, you should see something else. There should be tiny, bright sparks that jump from the wick. That is the third thing that we need to have a relighting candle, but what makes the sparks?

The sparks are caused by tiny bits of the metal magnesium. Magnesium is a very light metal. It also burns with a very hot flame. Tiny bits of magnesium are mixed into the wick. While the candle is burning, liquid wax flowing up through the wick keeps the magnesium cool enough not to burn, but once the candle is blown out, the wax cools and stops rising. That lets the glowing ember heat the magnesium bits enough to set some on fire. They burn hot enough to set the paraffin vapor on fire, relighting the candle.

If you relight the candle several times, you will probably get some nice bursts of sparks. Repeated melting can cause some of the particles to concentrate in one place. When they get hot enough, you get a nice, miniature fireworks display.

Once you are done, but sure that the "magic" candle is out. Put it in some water for a little while to be sure. It would not be a good thing to put it into the garbage, and then have it relight again.

Be sure to dispose of the cupcake or brownie properly too, preferably with a little hot fudge sauce and a fork.

To go into this subject deeper, try the following:

The Fire Diamond: To understand what we need to have a fire.

This week's experiment comes from Burning Questions, my program on the science of fire. When teaching about fire, educators frequently talk about the fire triangle to teach about how things burn. Just as a triangle has to have three sides to be a triangle, you need three things to have a fire: Fuel, heat, and oxygen. Recently, fire educators have started teaching about the fire diamond instead of the fire triangle, because sometimes having fuel, heat and oxygen is not enough to get a fire. To see this, you will need:

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As much as I would have liked to do another experiment with chocolate, I don't really have time to go buy a larger belt, so this week's experiment is related to fireworks instead. If you are in the USA, you are probably going to see fireworks for July 4th. Have you ever wondered how they get the different colors into the fireworks? If you want yellow fire, do you add yellow paint to the mixture? No, that would not work. To see how the colors get into fireworks, you will need:

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Straining a Flame

Why can't a flame go through a metal screen? Try this fun experiment to explore the science of combustion and heat.

For this week's experiment, I wanted something that related to hurricanes. I settled for one that is based on fast moving air and differences in air pressure. You will need:

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This experiment was sent in by Leilah, an 11 year old list member from Indiana. It is exactly the kind of experiment I like, because it is simple, it makes you think, and it’s interesting enough to get you to actually try it, instead of just saying, "Wow, I'll have to try that some time."

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This project has science fair potential.

Warning! This experiment uses fire. Be very careful and be sure an adult is around to help.

This time we are going to explore a gas called carbon dioxide. It plays an important role in the lives of plants and animals, and it has very specific chemical properties. We can use those properties to do some surprising things.

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Which will go out first, the tall candle or the short one?

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The answer to the last video's challenge.

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Why do fire crackers make such a loud sound?

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A little extra oxygen can make a big difference in combustion!

It is amazing how many books get this one wrong!

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