Re: can you measure air pressure in a closed container?

Hi Marissa,

It's good to see your interest in physics, your experiment is showing 
very good thinking, you have correctly assumed that if the bottle is 
tightly closed and the amount of oxygen changes, the water must 
experience the force and move accordingly. All this things, so far, are 
correct. But there is more.

There are at least two problems with the way you are measuring pressure.

The first problem is that you are assuming the loss of oxygen as the only 
factor, when the production of carbon dioxide and monoxide are also 
there. 

As you probably know, combustion is a chemical reaction where a fuel is 
broken down by oxigen, and there are byproducts, mostly carbon dioxide 
and carbon monoxide. So, without doing the math, I don't know if the 
pressure is going to go up because of the production of those two gases, 
or go down because the candle uses up the available oxigen. 

Certainly, the net effect is the result of the individual pressures of 
each gas combined. Perhaps your can settle this matter once you figure 
out a way to make your experiment work. So I am gonna try to help you 
make it work. Your experiment will only measure the NET pressure effect.

The second, and most important problem, is that water is not an easily 
compressible fluid. Compressible means squeezable, compliant. Because 
water is not compressible when a force pushes on a water volume, the 
volume changes very little (practically nothing) for a given force. You 
would need to measure very tiny changes (almost invisible) or have 
incredibly large forces acting on the water. From your experiment 
results, I suppose the pressure change is not that enormous to change 
water volume.

So, even though, in fact when the air pressure changes, the water 
experiences a force either pushing or pulling on it, the force has little 
effect on the volume of the water (is just like pushing on a concrete 
wall and trying to measure the displacement, you are doing things right 
(at least in principle), but it will move very little to see the 
difference) It will move a tiny bit though. If you push with something 
stronger like a big monster truck...well.. better not try that!

Air, for instance is compressible, you can squeeze an air balloon, or if 
you take an empty soda bottle (its really full of air) and put the cap 
back on it (tight), you can still squeeze it. Try that with water 
instead! its so hard it barely moves! If its half full of water, its 
somewhat easier to push on.

Perhaps if a small amount of air could be placed inside your water... 
that might just work! The trick is to put ALL of the air separate from 
the candle's oxigen supply, and completely under the water, so that the 
pressure can still be SEEN as a change in water level. This air cavity 
can be called a "bladder".

A bladder full of air, like this, is a cavity that can change its volume, 
while the pressure inside it changes very little.

Here is something you can try to add to your experiment to increase how 
far the water moves for a given change in pressure. In your water jar, 
submerge under the water a small balloon. Inflate the balloon with some 
air, just enough so its not limp, but not so much that it does not fit 
completely under the water. Also, tie something very heavy (perhaps a 
small metal object like a metal bolt or fishing weight) to the balloon to 
prevent it from floating. Remember that air is very light and it will try 
to float back up. You may even have to GLUE the balloon's "tie rope" to 
the bottom of the jar with very good waterproof glue from the hardware 
store. Avoid white glue or the super glue. Also, wait for it to dry, the 
floating force (buoyant force) will pull too hard on it)

Now, here is what happens: your water is still not compressible, but the 
air inside the balloon submerged under the water is compressible, as you 
can see from the soda bottle experiment. Water that has an air balloon 
inside is compressible because you are really squeezing the balloon, the 
water is just there to help push.

So, when your candle finishes the oxigen (and finishes producing its 
carbon byproducts) the change in pressure will be more visible because 
the balloon is being squeezed (or pulled) by that change.

With this "modified" version of your experiment, the role of water is:
1. To make the floating candle "float"
2. To transmit the force of the pressure change to the balloon. The study 
of how water can be used to "transmit" forces is hydraulics. Your jar is 
a hydraulic system.

Yet, another variation on your experiment can be made by drilling a hole 
on the lid for the jar, then glue with silicon based glue a straw that 
goes down into the water in vertical position. You still want to keep the 
lid airtight, and the straw better be a good quality hard straw, not the 
cheap soft ones. Try the Wendys or McDonalds one, not the ones from 
Burger King :) If you find a straw that lets you see the water inside it, 
all the better.

When pressure pushes on the water, it will try to push the water "out" of 
the bottle" via the straw... Only, it will not push that hard to drive 
the water out, just enough to move a bit up or down the straw. This is 
just like the ballon idea, but instead of a small air volume in the 
water, your straw makes sure the pressure in the bottle remains at 
atmospheric pressure. In essence, the whole atmosphere has become the 
balloon! With the candle, as oxigen gets used up (and other gases 
produced) the volume will change just like with the balloon idea.

Without a candle, the straw jar trick makes a neat barometer. Leave it 
closed very airtight for some days, and see what happens as the weather 
changes.

Fortunately, its barometric abilities will not interfere with your candle 
experiment because weather does not change that fast. Were the 
atmospheric pressure to change in a few seconds, just when your candle 
burns, that could interfere, but that's nearly impossible. You can do the 
*same* experiment 5 or 10 times and compare the results just to make sure.

You could even make the balloon jar and the straw jar and see side by 
side. I bet the jar with the straw shows the most difference in water 
level. Try to find out why...


Good luck and keep up the good work!

your mad scientist,

Aurelio R. Ramos