This is a classic experiment. On a plate with a little water place a candle and light it. Place a drinking glass over it. The light goes out while sucking up the water.

At least two things happen:

  1. The oxygen O₂ is changed to CO₂
  2. The air is heated up a few degrees
  3. Water is sucked up

But then I tried to change the experiment with a vase (big glass) and opening upwards and place the candle at the bottom. Then lit the candle and place plastic over the opening. The result was that the light went out but it didn't suck down the plastic. So it there is no vacuum in the glass? A strange thing is that the plastic was pulsing like it was breathing. It can be seen here in my video https://youtu.be/aolz8Y27Lsk

Why is the experiment so different from the other?

  • $\begingroup$ Obviously the plastic didn't completely seal the vase. $\endgroup$
    – MaxW
    Mar 19, 2020 at 19:10
  • $\begingroup$ Your chemical description is incomplete. $\endgroup$
    – Karl
    Mar 19, 2020 at 19:41
  • $\begingroup$ There must be a leak somewhere, because the gas volume does not change during the combustion. And the volume must change during the combustion. It must increase or decrease, as explained by Karsten Theis $\endgroup$
    – Maurice
    Mar 19, 2020 at 19:54
  • $\begingroup$ The seal on your vessel has to be perfect for this to work. An inverted glass vessel achieves this; it is very, very unlikely that a plastic seal will do so. $\endgroup$
    – matt_black
    Mar 20, 2020 at 10:49

2 Answers 2


Changes in volume

The chemical reaction for burning a candle is something like this:

$$\ce{C25H52 + 38 O2(g) -> 25 CO2(g) + 26 H2O(g)}$$

For every 38 dioxygen molecules used, you are making 25 carbon dioxide molecules and 26 water molecules (which start out as a gas, but will condense once they reach an area of lower temperature such as the glass surface). So once the flame is out and the water has condensed, the volume should be less than that of the fresh air (20 % of the volume initially are oxygen. Its volume will be replaced by that of carbon dioxide, so the volume will go from 100% to 25/38 * 20% + 80% = 93%). While the candle is burning, however, the temperature is higher, and some of the water will be in the gas phase. Also, while the air was already hotter than room temperature before the container was covered, the temperature will rise as soon as the influx of cooler (fresh) air is cut off.

How hot will it get?

A tea light produces heat at about 30 J/s, and (with a molar heat of combustion of about 15,000 kJ / mol) used up $\pu{80 \mu mol}$ oxygen per second. This means you need about 10 mL of fresh air each second the candle burns. The container has a volume of about 400 mL, so the candle should burn <40 seconds. In the video, it burned about 2 minutes (not bad for an estimate). Using up all the oxygen in the container would give off about 1.2 kJ, which is sufficient to raise the temperature by 1000 degrees Celsius if it were isolated. As it is, most of the heat flows into the container.

What is different when using water?

Water has a higher heat capacity, so it is more efficient in cooling down the gas (and the container). As a consequence, the water level will rise appreciably. As MaxW mentions in the comments, the plastic probably did not seal perfectly. The pulsing could be intermittent flow of gas, or temperature fluctuations because of mixing.

  • 1
    $\begingroup$ So in the next experiment I will 1) put water at the bottom to increase coolant 2) light the candle and cover immediately 3) use more heat resistant plastic 4) use better and more rubber bands. Goal: To see the plastic cover be sucked in the container (will that happen?) (Sorry for making such a lousy experiment) $\endgroup$
    – hschou
    Mar 19, 2020 at 22:06
  • 1
    $\begingroup$ @hschou Instead of a candle, you could use some steel wool soaked in vinegar. It will use up the oxygen without making carbon dioxide (the oxygen ends up being rust). This experiment removes two complications - the carbon dioxide and the large changes in temperature. But I encourage you to try your 2nd experimental design as well - the first try is always lousy, and you only learn by repeating experiments. $\endgroup$
    – Karsten
    Mar 20, 2020 at 0:17

Why do most people want to believe that on burning fuel little or no carbon monoxide is created?

Yes, it could be true, mainly CO2 is formed, but read these qualifying comments from a source:

A properly designed, adjusted, and maintained gas flame produces only small amounts of carbon monoxide, with 400 parts per million (ppm) being the maximum allowed in flue products.

And, per the same source:

Incomplete combustion occurs because of:

  • Insufficient mixing of air and fuel.
  • Insufficient air supply to the flame.
  • Insufficient time to burn.
  • Cooling of the flame temperature before combustion is complete.

Also, per Wikipedia, to quote:

In the presence of oxygen, including atmospheric concentrations, carbon monoxide burns with a blue flame, producing carbon dioxide.[10]

So, many combustion reactions perhaps should be expressed in steps including:

$\ce{2 CO + O2 -> 2 CO2}$

So, perhaps more likely, the presence of CO is more of a reality than many would want to believe.

In the case of your experiment, leakage (from possible thermal exposure damage and/or a poor seal) along with incomplete combustion may be some route causes (as carbon monoxide is not very soluble in water, 27.6 mg/L as compared to CO2 with 1,450 mg/L at 25 °C).


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