In chemistry, light gases are collected using upward deliver method and heavy gases are collected using downward delivery method. Does it make any difference when, according to kinetic theory, air molecules from the surrounding will mix the gasses collected? Are the above two methods incorrect?

Since you are collecting gas, it means that when the delivery is being done, the wanted gas is being constantly 'poured' into the vessel. Some of the gas might escape to the surroundings, but practically no gas will enter the gas jar because gas collected is constantly pushing it away. As soon as the experiment is done, you will have to close the vessel. (or if you close it before the reaction is done, but since I never did that in practise, there might be some method to do this)

There will be no doubt some other gases mixing with the 'wanted' gas then only.

In chemistry we usually collect gasses by displacement of water. A filled vessel is inverted over the bubbler in a pneumatic trough and, as the gas flows in, the corresponding volume of water is released. Other than water vapor (whose vapor pressure is known at the operative temperature) this assures that there are no extraneous gasses present.

Hi DrBob1, thank you for your reply. Thanks for the reply. The one you mentioned is based on water displacement. I have no problem with that. The idea of collecting gases based on density(light gasses rise, heavy gases sink) seems illogical since overtime air will go into the jar and mix with the collected gas(kinetic theory). This is what troubles me. For reference see: http://scienceaid.co.uk/chemistry/applied/gases.html.

Your answer makes sense but this is not what textbooks are saying. They do not say it is a purging method. They claim the method is based on density.

It all depends on the reason you are collecting the gasses. If you just want to do the tests the website shows in order to demonstrate the identity of the gas, the upward/downward method is quite adequate. There will be losses and mixing with air, but the results will be completely satisfactory.

I have had times that I wanted to collect the gaseous products of a reaction for further analysis. This is where the displacement technique works best. Solubility differences can raise quantitative questions, but by positioning the delivery tube at the top of the collecting vessle, bubbling and gas adsorbtion are minimized. The results are pretty good both qualitatively and quantitatively.

I'm not sure that the kinetic theory of gasses is a big deal here.

Yes, it is based on density, but being denser or not, your question will remain the same. There will be diffusion of gases.

Upwards delivery is for collecting less dense gases.
Downwards delivery is for collecting denser gases.

This way, diffusion of the gases in or out of the collecting vessel is minimised (but not eliminated). And the gas jars are closed with a lid after the collection of the gas, of course to reduce the gas molecules to escape or air molecules to enter.

EDIT: I merged your threads so that it's easier to keep track of your responses.