Cloud Formation: Make Your Own Cloud! demonstration
One very fun and unexpected event happened while we were doing our What Combustion Needs demonstration and I thought that it merited a post of its own. To review what we did in the demonstrations, for the first one, we just lit a candle and then put the glass upside down over the candle so that it covers the candle completely.
As expected, the candle went out and we noticed that water vapor condensed on the inside of the glass. I expected this, but didn't make too much of it at the time as I planned to go over this when we next talked about the products of combustion.
Then we did the second demonstration in which we put the cup of vinegar into the bowl.
We then placed the candle in the candleholder into the bowl and lit the candle.
Next, we slowly sprinkled baking soda into the vinegar, all around the candle so that the bubbles surrounded the candleholder.
As we expected, the air that was in the bowl was pushed out by the carbon dioxide, and since combustion needs the oxygen in the air, the flame went out.
Lastly, we tried relighting the candle but when we moved the match towards the candle, the match went out. Even though the foaming of the vinegar and baking soda had stopped, the carbon dioxide stayed in the bowl for quite a while and continued to extinguish matches.
It took about five minutes before we were able to relight the candle.
But, here is where the unexpected thing happened, which on retrospect, I feel I should have anticipated. Between when the first attempt to re-light the candle and the time in which the candle actually was able to re-light, I had let the boys try lighting the candle as many times as they wanted to, which for my boys was about once every five seconds (no, I am not exaggerating). It was pretty quick into this process that suddenly dense water vapors, or a cloud began to form.
No, that isn't just smoke from the matches, although there is a little of it in the clouds because in order for clouds to form, there has to be particles in the air. Cloud formation is caused when water vapor, air pressure and something for the water droplets to condense on come together.
The water vapor was around because it was a bi-product of the combustion from the candle and the matches. The smoke from the matches created a medium for the water droplets to condense and lastly, the change in air pressure was caused by the variance in the relative densities of the carbon dioxide from the vinegar-baking soda reaction with that of the ambient air.
According to Wikipedia, at standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.67 times that of air, which is also why the carbon dioxide hung so low in the bowl for so long. A drop in air pressure was the third factor that was needed for the clouds to form.
We have seen this sort of combination occur before when we did the Cloud in a Bottle demonstration, but this Combustion demonstration outdid the Cloud in a Bottle by far, with it's long lasting and dramatic clouds. 
Since I have not heard of anyone doing a cloud demonstration in this way, I am not sure whether there were other factors that came into play (such as the pressure of the air on that day, or the amount of moisture that was in the air that day) or whether this is a pretty reliable way of students observing cloud formation.
I would be thrilled to hear if any of you try this demonstration what your results were.
Since I have not heard of anyone doing a cloud demonstration in this way, I am not sure whether there were other factors that came into play (such as the pressure of the air on that day, or the amount of moisture that was in the air that day) or whether this is a pretty reliable way of students observing cloud formation. I would be thrilled to hear if any of you try this demonstration what your results were.
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