PICTURES COMING SOON

Air, not air, and super-air: how does a candle burn in air, carbon dioxide, and pure oxygen? It’s easy to create these three different conditions and learn a bit of chemistry. If you have a vacuum chamber you can even fine tune the combustion.

Air that’s not air: extinguishing a candle, or …:

Equipment: a candle, and matches or a lighter to light it; a cup (optional) to put it into; another cup for mixing; baking soda; and vinegar. For an option you can also get some hydrogen peroxide and an old alkaline battery (cell).

How candles burn: Chemicals that allow burning of common fuels and chemicals that don’t: We light a candle and the flame on the wick heats up the wax around it, even making into vapors. Those vapors are composed of the elements carbon and hydrogen, both of which readily combine with oxygen when they are hot enough. (I put the chemical equation at the end, so as not to interrupt the narrative here.) Ordinary air has 21% oxygen when dry, 78% nitrogen, and 1% argon, a noble gas (very loath to react with any other chemical). Water vapor dilutes these a bit, up to about 6% in the most humid livable conditions for us humans. Candle wax, and most things we think of as combustible, don’t react with nitrogen in the air, but, hey, there’s enough oxygen for most fires that we want… and for our “controlled fires” in our bodies, our respiration that’s done with the help of many proteins in our cells. That’s another very detailed story that I won’t go into here.

Carbon dioxide is a gas that looks just like air, that is, transparent, invisible to the human eye, even if it’s extra-visible in the infrared that we can’t see. It’s present in ordinary air at generally low concentrations. Averaged around the globe it’s at about 415 parts per million in free air. In a closed room just our breathing may raise it to several percent; it had better not reach 10% or we can die from a few discrete effects on our bodies. Our breath is about 2% CO2. If we hold our breath we can get it to about 20% CO2, not a great idea to keep doing. CO2 does not support the burning of candle wax. In fact, it’s one of the final products of burning candle wax, the other part being water vapor.

The set-up: Basically, we can make pure CO2 readily by reacting common household chemicals, vinegar and baking soda. We can collect it in a cup and then pour it onto a candle. It can collect because it is denser that air and sits at the bottom of the cup. Its molecules weigh more (have a higher mass) than air molecules. So, light the candle. In a cup, say, a coffee cup, put some baking soda in it; use about ½ teaspoon; even ¼ tsp is enough if you’re careful. Slowly pour in about a tablespoon of vinegar. The mass will foam; don’t let it overflow. Let the bubbles pop. You might cover the cup with a piece of paper to avoid losing too much CO2 to air currents in the room. Now carefully hold the cup over the candle flame, safely high enough not to get burned or to burn your cup if it’s paper. Tilt the cup reasonably quickly to let the CO2 fall right onto the top of the candle flame. It will go out, because CO2 won’t support combustion.

PICTURES

Variations: First, put the candle into a cup that is, say, a few cm (an inch, plus) taller than the top of the wick. Instead of pouring the CO2 directly onto the wick, pour it into the cup that’s holding the candle. Do this slowly, so that the CO2 will rise as a layer. When the CO2 reaches the height of the wick the flame will go out. Of course, you won’t see the CO2 but you’ll see its effect when the candle flame goes out. Second, try adding more oxygen instead of replacing oxygen. There are several ways to do this, but BE CAREFUL. An easy one is to pour about 5 milliliters (a teaspoon) of common hydrogen peroxide solution into the mixing cup. Open a packet of dry yeast and sprinkle some into the hydrogen peroxide. The enzyme catalase in the yeast cells will cause a huge release of pure oxygen gas. Pour it onto the candle flame, very carefully – the flame will rise higher and hotter, so keep a decent distance above the flame. The rate of combustion increases with the concentration of oxygen, as this shows.

There’s a way to decrease the amount of oxygen available to the candle flame in any amount. We can put the candle into a vacuum chamber and slowly draw out the air. That demo is described in a bigger demo about using the vacuum chamber.

The chemical equations: For wax burning in the oxygen in the air: the chemical makeup of candle wax is closely CH2, in units all joined together into somewhat long chains. Let’s look at just one unit:

A couple of things: First, there’s that water on the side showing the results, or products of combustion. Second, note that I use what is called an improper fraction, in which the numerator is bigger than the denominator. That’s almost the universal practice in science. It’s so much easier and less prone to error than using two-part proper fractions such as 1-1/2, when you multiply or divide numbers. Third, the fraction is a fraction of “jillions” of molecules; there are no half molecules. The number of molecules in, say, 1 gram of wax (about 1/5 of a teaspoon) is greater than ten to the 21st power, the digit 1 followed by 20 zeroes. The number of oxygen molecules involved is much greater.

What’s happening with hydrogen peroxide? That’s H2O2 – water with an extra oxygen atom in each molecule. It’s prone to break up and give up that oxygen:

 


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