(i) Rigidity. The tendency of a substance to maintain or retain their shape when subjected to outside force is called rigidity. Solids may break when force is applied but it is difficult to change their shape, so they are rigid. The particles in a solid are very closely packed and have a very strong force of attraction among them. Hence, solids possess high rigidity. Liquids and gases are not rigid because their particles do not have fixed positions. The forces of attraction among the particles of liquids and gases are very low as compared to solids.

(ii) Compressibility. The particles of matter have intermolecular spaces. If an external force is applied, it can bring these particles closer. The property of a fluid or a solid due to which its volume decreases when pressure is applied is called Compressibility. The particles in gas have large spaces between them. When the pressure is applied on a gas its volume decreases to large extent. Hence, gases have high compressibility. On the other hand, the particles in solids and liquids are closely packed, so solids and liquids do not have much compressibility.

(iii) Fluidity. The tendency of the particles to flow is called fluidity. Gases and liquids exhibit the property of fluidity, so they are called fluids. Due to large intermolecular spaces and very weak forces of attraction, gases can flow very easily. Hence, gases have very high fluidity. Liquids have smaller interparticle distances and stronger forces of attraction among the particles. As a result, the fluidity of liquids is less than that of gases. Solid have no fluidity because they have very strong forces of attraction very little intermolecular spaces.

(iv) Filling a gas container. A gas is able to fill the container because its particles are able to vibrate randomly in all the directions. The particles of gas have very high kinetic energy and very negligible forces of attraction. As a result, gases are able to fill the container fully.

(v) Shape. The external form or appearance of a substance is called its shape. The particles in a solid have very strong forces of attraction due to which they are closely packed and occupy fixed positions. As a result, solids have fixed shape. The liquids and gases have comparatively weaker forces of attraction between them. The position of the particles are also not fixed. Hence, liquids and gases do not have fixed shape.

(vi) Kinetic energy. The energy possessed by particles due to their motion is called kinetic energy. At a given temperature, the particles of a gas have the maximum kinetic energy. This is because the force of attraction among the particles is very weak. Liquids have lesser kinetic energy as compared to whereas solids have the least kinetic energy at a given temperature.

(vii) Density. The mass per unit volume of a material is called its density. Solids have high densities. This is because their particles have very strong forces of attraction and are packed very closely. Liquids have usually lower densities than solids because their particles are loosely packed as compared to solids. Gases have the lowest densities because their particles are very far away from one another due to weakest forces of attraction among them.