Why does gas expand without temperature increase or external force?

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1. why does gas expand to fill a container when there is no increase in temperature or any force applied to it? ( I am confused with the rising effect of gas in air and the expanding effect. how can we distinguis it?)

2. how can we determine the limitation of gas expansion when there is no increase in
temperature?

3. will the gas expand in vacuum when there is no increase in temperature?
 
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All of your questions can be answered with 1 word. Entropy.

A container with gas molecules packed together at one side and a vacuum at the other is of higher energy than one with even concentration throughout. Hence the air molecules will redistribute themselves to fill up the entire container. (This is one of the methods used to store energy.)

Rising effect of gas would be due to the density instead.

Not quite sure what you mean by: when there is no increase in temperature.
 
when i searched this question in google, most of the answers i found were of the reason of expansion of gas due to heat supplied to it. So i was wondering why there is gas expansion even without the supply of heat.
i searched the term entropy in wikipedia but am struggling to understand it. Also i need the answer of my second question. i have found that gas expands only to a certain limit. But how can we determine it?
 
Well, gas can expand in a vacuum when there's a supply of heat. Otherwise, if in an enclosed container, just use the ol' pV = constant. To find temperature difference, then it's pV = nRT. This equation is for ideal gases, but it usually still works.

Not too sure abou the limit of gas expansion.