• Among solid, liquid and gas the molecules of solid are less speedy.
• The molecules of liquid are speedier than the molecules of solid.
• The molecules of gas are speedier than the molecules of solid and liquid.

*When the molecules of one matter spread between the molecules of another matter; this action is called diffusion.

Know by experiment

1) Take two smooth pieces of different metals. Put them together clinging. See them after some time. What do you observe? What does it show?

You see the small particles of one another metal on the both pieces. In this way it is clear that diffusion in solid is very slow.

2) Take some water in a beaker. Put some particles of potassium permanganate in it. Now see the reaction in the beaker. What do you see? The particles of potassium permanganate dissolve in water by and by. It means that particles of both scatter slowly among themselves. In this way it is clear that diffusion in liquid is speedier than in the solid matter.

3) Lit a perfumed stick and put it in a corner of a room. After some time the smell of he perfumed stick spreads in the room. Why does it happen so? The molecules of gas rubbing the molecules of air spread in the room.

*Among the molecules of solid, liquid and gas the molecules of gas have fast movement. So the diffusion in gases is speedier than solid and liquid.

No molecules of any matter can be separated from other. Why does it happen so?

In fact other molecules present around a molecule attract or pull it towards themselves. In the same way one alone molecule also attracts the other molecules towards itself. It proves that molecule has a force of attraction. It is called intermolecular attraction or cohesive force.

In solid matter the molecules are very close and they have much intermolecular attraction. So the solid matters are stout and they have a certain form.

In liquid matter the molecules are not very close to each other, like in solid matter. In the molecules of liquid the intermolecular attraction is less than in solid matter. So, the liquid can show a tendency to flow.

In gaseous matter the molecular separation is very large. There is little intermolecular attraction among these molecules. So, Gaseous matter can spread itself at any place.