Question

Short answer. why do gases not have a fixed volume or fixed shape?

Answer 1

Gases not have fixed volume because the molecules in gas is loosely packed and move randomly ( here and there) .
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And in whichever container they are kept they take up the shape of the container because the walls of container restrict molecules movement.

Answer 2

A gas is a substance with no definite volume and no definite shape. Solids and liquids have volumes that do not change easily. A gas, on the other hand, has a vol- ume that changes to match the volume of its container. The molecules in a gas are very far apart compared with the molecules in a solid or a liquid