“Matter is anything that occupies space and has mass.”
Matter is composed of small building units like atoms, molecules etc which are packed or arranged in various ways to give matter various states, physical and chemical properties etc. These molecules have forces of interaction between them which are responsible to keep matter bound or else the molecules would have been free. These intermolecular forces depend on the distance between the molecules. In case of gases the molecules are far apart from each other and that’s why the forces are very weak and gases are most loosely bound state of matter. They don’t have fixed shape or volume of their own. They take the shape and volume of the container they are kept in. In case of liquids the intermolecular distances are lesser than gases and that’s why forces are stronger and empty spaces are also less. That is why liquids have a fixed volume if not shape of their own. The molecules are closely packed in case of solids reducing the empty spaces drastically and increasing the intermolecular forces. This gives solids a fixed shape and dimensions and also a volume.
Ideal Gas is a hypothetical state of matter in which there are no interactions between the molecules. All other states of matter are studied comparatively to an ideal gas. There are some assumptions taken in case of Ideal Gas which are:

1. The intermolecular distances are very large and that’s why forces are negligible.
2. The volume occupied by the molecules is negligible as compared to the volume of the container that is most of the space in an ideal gas is occupied by “voids”.
3. The molecules of the gas are in a continuous and random motion.
4. The molecules keep colliding among themselves and with the walls of the container, these collisions are elastic, and due to the momentum change associated with them there is a force applied on the walls which appears as the pressure of the gas.
5. Laws of classical mechanics (Newton’s Laws) are applicable on the molecular motion of the Gas.

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