Solid
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The molecules in a solid are very closely packed. The inter-molecular force of attraction is very strong in solids. Hence solids have a definite size and shape. Molecules of a solid vibrate about their mean position.
Molecular structure of a solid
Solids cannot be compressed because the molecules are closely arranged. When the solid is heated, molecules gain more kinetic energy and vibrate more, inter-molecular distance increases and the solid expands.
Liquid
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In liquid, the inter-molecular space is larger than that in solids. Therefore, the inter-molecular force of attraction is weaker. Molecules in liquids have vibratory motion as well as strong translatory motion. Thus the molecules of a liquid can move freely within the liquid and also they set up convection currents. As a result, liquids don't have a fixed shape. It takes up the shape of the container.
Molecular structure of a liquid
Liquids cannot be compressed as the molecules are still close together. When a liquid is heated molecules vibrate and move apart. When heated molecules gain more kinetic energy and move apart and evaporation occurs.
Gas
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The molecules of a gas are far apart. The inter-molecular force of attraction is negligible and they have no fixed freedom of motion. Hence gases have neither fixed shape nor volume. They completely occupy the container and take the shape and the volume of the container.
Molecular structure of a gas
A gas mainly consists of empty spaces between molecules and can be easily compressed.
Plasma
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Plasma are much like gases except that their constituent particles have become electrically charged, and their behavior in consequence depends strongly upon electromagnetic forces. Most of the matter in the universe is in the plasma state.
A fluorescent tube contains plasma.
Melting
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It is the process of changing a solid into its liquid state. The temperature at which a solid changes to the liquid state is known as melting point. When a solid is heated, the molecules gain more kinetic energy and start vibrating with a greater velocity and greater amplitude. At a particular temperature, the molecules gain kinetic energy sufficient to overcome the force of inter-molecular attraction. They then start to slip and slide against each other. This process is called melting.
Evaporation
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All the molecules of a liquid are in a state of constant and random motion. They have different velocities. During the motion, molecules collide against each other and some of the molecules gain more kinetic energy than others. The energy rich molecules break through the surface of the liquid. Thus the average kinetic energy of the remaining molecules decreases and this lowers the temperature of the liquid. Thus evaporation results in cooling.
Boiling
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When a liquid is heated, the molecules gain more kinetic energy and the temperature starts rising. After it has reached a specific point though heat is being supplied, the temperature becomes steady. At this temperature the energy supplied gets utilized in breaking the inter-molecular force of attraction. Now the molecules start leaving the liquid and escape as vapor. This process is called boiling and the temperature is called boiling point.
