|Besides pedagogy or psychology, it can be also relevant just to get the facts right. An illustrative example.|
|Experiment: Cover a burning candle with a pitcher so that the candle is in an air-tight room sealed by the water at the ground.||Observations: After some time, the candle dims and goes out. Just before the candle dies, the water level rises to almost 1/10 th of pitcher height. No air bubbles are seen. The water level stays up for many few minutes more.|
The chemical aspect: oxygen O2 and
Cn H2n+2 react. The burning produces
water H 2O and carbon dioxide C O 2. For n=1 we balance
the equation as follows:
|The physical aspect: the candle heats the air and expands it. This cancels the depletion of the oxygen temporarily and the water level stays down. When the oxygen is depleted, the candle goes out and the air cools. The volume of the air decreases and the water rises. The temporary temperature change delays the rise of the water. As several readers have pointed out, also the water condensation should be mentioned. While water is initially gas, it condenses and helps to delay the effect.|
|Summary: There are two different effects. Both a chemical and a physical reasoning are needed to explain what we can see. Both physics and chemistry matter. The initial cancellation effect can confuse the observer. Mathematics plays a role when the chemical equations are balanced.|
|O2 + x Cn H 2n+2 = y C O 2 + z H 2 O|
n x = y ( C atom balance ) (2n+2) x = 2 z ( H atom balance ) 2 = 2 y + z ( O atom balance )
|(1+3 n) O2 + 2 Cn H 2n+2 = 2 n C O 2 + (2+2n) H 2 O|
|p V = N k T|
Many people try to explain the problem with physics alone with a different
argument. They argue that less hot air is captured in the cup. In other
words, the cup covers a volume of less dense air because the air is heated
around the candle. When the air cools after the candle goes out, the
pressure decreases almost entirely from less dense air cooling. |
Regardless, some may argue that the chemical aspect is very minimal because the water level sometimes rises to one third of the volume, but under perfect conditions reaction condition, the reaction chemistry can only account for a maximum ten percent water level rise. You suggest that the water level rises to one tenth of the height, however it can be much higher if more candles are used.
I agree that the chemical reaction can have an effect, but how would you rank the contribution of each? Is one effect minimal or more important? How does the size of the candle or container play a factor?
|What is happening in this experiment? When we ignite the candle, the hydrocarbon reacts with oxygen (in excess) to produce carbon dioxide and water. The burning sets an air current which gives dome shape to candle flame and it helps to get complete combustion at the bottom and the outer surface of the flame. The hot air and products of combustion rise up above the flame. As soon as the gas jar comes over the flame, the hot gases moving upward enter the jar and air inside the jar expands pushing some of the air out of the jar. This process goes unnoticed. As soon as the jar touches the water, the burning occurs in a closed environment. Further pressing the jar into water helps to retain the air in jar which is less in quantity than at room temperature and pressure. However, due to thermal expansion, the pressure is higher than atmospheric pressure which is balanced by pressure from the water. The burning of hydrocarbon in the jar produces about 30% more molecules of carbon dioxide and water than the molecules of oxygen consumed in the reaction (see below the title expectedchemical reaction). The increased heat and number of molecules increases the pressure in side as a result if not careful some bubbles of gas will escape from the jar. Over the time the oxygen in the jar is reduced and conditions for burning are changed. Burning under reduced oxygen may not produce carbon dioxide rather carbon monoxide (very little). When the candle is put out, the temperature decreases followed by also a decrease in pressure due to condensation of water vapour and decreased quantity of air due to thermal expansion during the process of placing the jar on the candle. The overall situation is a decrease in pressure inside the jar as compared to atmospheric pressure. Therefore, despite water is heavier that air, it is pulled into the jar. How much water rises as a result of dissolving of carbon dioxide? Very little practically negligible during 30 - 40 minutes, the time the experiment usually takes for performing in a classroom situation. If the number of candles is increased in the jar, the heat produced is more therefore more air is likely to escape from the jar due to thermal expansion during the process of pacing the jar over them. Therefore, more water will rise in the jar with more candles. The nature and quantity of the products will depend upon the composition of candle material. However, it is assumed that combustion of saturated hydrocarbons is taking place during burning. CnH2n+2(s) + (1.5n+0.5) O2(g) = n CO2(g) + (n+1) H2O(g) For n=1, two moles of oxygen reacts with a mole of CH4 to produce three moles of product molecules. Assuming that supply of methane was controlled and it is stopped as soon as the flame is put out, otherwise there will be an explosion. The number of moles of the product molecules is 1.50 times that of oxygen. As n increases, the multiple factor decreases from 1.50 and approaches 1.0 at n = ? For n=30 (a typical paraffin wax), the factor will be 1.34. The overall understanding of the experiment is that all the oxygen is not used up (I have rested the presence of oxygen after the candle is put out in our laboratory using yellow phosphorus) and the consumption of oxygen does not create empty space rather the number of product molecules in the jar increases over that of the consumed oxygen. Thus giving rise to an increase in overall pressure in the jar (see above equation). Moreover, almost equal number of molecules of CO2 and H2O are produced. A quick rise of water in the jar after the candle is extinguished is mainly due to a decrease in pressure as a result of a decrease in amount of air in the jar due to thermal expansion during the process of placing the jar on the candles, bubbles escaping (if any) through the water and may be the condensation of the water vapour. The amount of condensation of water will depend upon the temperature difference between initial and final temperature of the air in the jar. Since air is above water, therefore saturated water vapour pressure is considered in the beginning of the experiment. Increase in temperature, during the candle burning, will make air unsaturated to accommodate additional water vapours especially produced as a product of burning. A decrease in temperature over time after the candle is off to the initial temperature will help water vapour to condense. This condensation will decrease the pressure inside the jar and will help water rise in the jar. The amount of water vapours condensed during a small change of temperature as usually occurs in this experiment may even be small to notice. The amount of CO2 dissolved in water is minimal in the 30-40 minutes during which experiment is conducted.|
Question by a reader, November 20, 2011:
We have two setups, one is with 1 candle and the other with 4
candles. We see that that level of water will rise more in 2nd setup.
Answer: theoretically, if you assume that the candles will burn up all
the oxygen in the container, and assume the room is completely air tight
and assume that both water and air incompressible, it does not matter.
You will have the same water level at the end in both setups after the
candles have burned out and the situation cooled down. |
In real experiments, there are differences but they depend on the actual experiment:
|Simo Tolvanan from Helsinki kindly informed me about the Article of Vera, Riviera & Nuez in "Science and Education". The article explains things very well and also contains much history and references. This paper makes the story again interesting. It points to the fascinating story of Lavoisier, who first realized that the total mass does not change during this process and who noticed that only a fraction of the oxygen reacts before the candle goes out by demonstrating that a mouse still can breath afterwards. The authors of the article provide also experiments The classical candle experiment is compatible what is seen by everybody else and which matches the stoichiometry computations. The artificial wick experiments demonstrate only a one percent increase. The authors conclude that bubbling and hot air trapping are responsible for the rising water. The setup for the candle experiment and the artificial wick experiments are very different. In the later case, the candle burning is violent and the container is very long. Heavier CO2 (which the ignition already produces in the first moments) can kill the candle before much of the oxygen is out.|
January 27: the bubbling effect. Here is an illustration why many teachers report bubbles. If you place
the pitcher flat on then bubbles escape initially. One can avoid this by tilting the glass first.
It does not matter. We just want initially to have the same level of water and the same pressure
inside and outside. The experiment starts then. |
How can the Vera-Riviera-Nuez (VRN) experiments be explained? They report only a 2 percent water increase. Here are some additional effects, which includes comments some readers have mentioned over the years:
Peter Dureen had a great idea to modify the experiment. He wrote:
I wanted to test Avogadro's hypothesis by doing the following. I would take a piece of burning charcoal, throw it in a glass, and immediately cover the glass with plastic wrap. I hoped that some carbon monoxide would be produced. In that instance, I expected an increase in volume, or pressure, as the case may be, since every oxygen molecule that entered into the reaction would produce two carbon monoxide molecules. In any case, to the extent that carbon dioxide was produced, there would be no change in pressure, I thought, since each molecule of oxygen would be replaced by a molecule of carbon dioxide. fully expected the plastic wrap to bulge upward with the increase in gas pressure, even after the piece of charcoal ceased to burn due to the lack of oxygen, and the temperature fell to room temperature again. What happened amazed me. The plastic wrap bulged downward into the cup. This indicated, to me, a reduction in pressure exerted by the gas in the glass, and thus indicated a reduction in the number of molecules. You may try this interesting and simple experiment yourself. I have no explanation for what happened, although one conjecture is that the carbon absorbed the carbon dioxide.The stoechimetry for coal is different than for paraffin. In the case of only carbon, one has
C + O2 = CO2and one would indeed expect that the volume would stay the same. Since the pressure decreases afterwards, this could indicate that indeed some air has gone out when the heat has expanded the inside. After cooling, the plastic wrap collapses. Peter Dureen again:
The second experiment is a parallel one to the candle experiment. I and an associate made a little stand from aluminum foil, so that it could support a piece of burning charcoal. This little stand basically replaces the candle in the burning candle experiment. We had a shallow reservoir of water in a pan, the water surrounding the little stand. I took a piece of charcoal, which I had fetched earlier from my fireplace, and took a propane torch to all sides of it to ensure it was well lit, and then placed it on the stand. My accomplice then quickly placed a jar over the assembly. Remarkably, water eventually rose in the jar after the apparatus had a chance to cool down. It, in fact rose to precisely the same level as it had in a candle experiment using the same jar. So it seems that charcoal has the same effect as a candle. Now in your analysis, you mention that the candle burns hydrogen, and produces water, and that seemed quite reasonable to me at the time. But after conducting this experiment, I must ask why burning carbon alone should produce precisely the same result. It is hard for me to explain to my students these two experiments. I now await further discussion of this matter, and hopefully, an analysis that explains the results of these recent experiments.I think this is more indication that some hot air has left the container before it started to cool down. I have repeated the experiments also with different type of containers and seen also some air, as other teachers have observed too.
I thank you for posting your explanation. I hope the above adds something to the discussion. Maybe we should consult Faraday. Did he not write something called the history of the candle?Faraday had been a fantastic experimenter and assisted a chemisist before for a long time. Lavoisier was definitely also a great pioneer in this matter.