[linds]

I've been able to do the whole experiment of collecting gas through an inverted graduated cylinder, but am struggling with the detail of how to read the volume of the gas collected.
My question is: why is it necessary to raise or lower the cylinder until the water level inside meets the water level outside?  I would assume it is just to be able to read the cylinder if the volume is under the water, but is there another reason?
Thanks.

[Dan]

I'll give you a hint: pressure

Any ideas now?

[linds]

Not really.  I know volume and pressure are inversely related, but I don't understand how it would change the pressure.  Would it change the internal or external pressure?  And how does it affect the pressure since it's in a graduated cylinder?

[JGK]

If the level of the water inside and outside the cylinder are the same what assumption can you make about the pressure?

[linds]

So if the volume stays the same, the pressure stays the same?  So you move the cylinder up or down to maintain the same volume of water so that your initial pressure is also the final pressure?

[linds]

Nevermind, I got it!  The total pressure is made equal to the atmospheric pressure by adjusting the height of the cylinder until the water level is equal.

Thanks for all the help