What Is Thermal Expansion?

It is a matter of fact that except few substances almost all the substances expand on heating provided external pressure remains same. Solids on heating increase in dimensions like length, area and volume. Such expansion is called thermal expansion where as liquid and gases expand in volume due to heating.

Due to heating if a solid expands in length it is called linear expansion. A solid expands in length and breadth (area) then such an expansion is called superficial expansion. if a solid expands in length, breadth and height (volume) then such an expansion is called volume or cubical expansion.

Due to heating the average kinetic energy of molecules of solids, liquids or gases is increased, so the average distance among the molecules is increased thus molecules vibrate with larger amplitude, which causes solids to expand. In liquids and gases the volume increases due to heating. Similarly, on cooling the average kinetic energy of molecules is decreased, they come closer to each other, which decreases the dimensions of substances.

The properties of substances, which change uniformly with the change of temperature, are called thermometric properties. For example volume of liquid in a vessel, pressure of fixed mass of gas in a container etc

Solids, liquids and gases generally expand when heated and contract when cooled. All solids expand on heating and if there is not sufficient space for expansion, large forces may set up within solids resulting in their bending or cracking.

Gaps have to be left in railway tracks to make allowance for expansion, otherwise the rails will buckle. Allowance is made for the expansion of long steel bridges. One end of such bridge is fixed while the other rests on rollers. Telephone wires sag more in summer than in winter due to expansion.

Thermal expansion of solids has many useful applications too. Iron and steel tyres are tightly fitted on cartwheels by first heating them and then slipping them onto the wheel. On cooling, these contract and have a firm grip on the wheels.

Thermal expansion is made use of in riveting metal places together. A rivet is heated and through the holes of plates to be riveted till its head holds tightly against one plate.

A basic meaning of this is when something is heated it changes the matter leading to a change in volume.

All solids,liquids and gases expands when we heat them and contract when we cooled them.
  In solids  there may change occur in length, area and volume. If the solid in the shape of rod there may be change occur in length. If the solid is in the shape of thick solids there may change occur in the area. If the solid in the shape of cube, cuboid,cone etc., there may be change occur in the volume.
  The change in the liquids occur with respect the container, which was filled by the liquid. There are two types of expansion in liquids. They are apparent expansion of liquid and real expansion of liquid.
  The gases expands with respect to their pressure, volume.
The gases have more expansion than solids and solids. The liquids have more expansion than solids.
  In summer season solids are expands very easily due to high temperature is present in atmosphere.

Let me clarify marine-eng answer. Thermal expansion does occur when something is heated. When commissioning a water tube or fire tube boiler and firing it for the first time with cold water in it, you must fire it in this sequence or damage will result and you will have a boiler room occurance. From cold start, you fire the unit 5 min on and 5 min off for the first hour and then 10 min on and ten min off the second, and 15 on and 15 min off until it reaches temp. Thermal expansion puts stress on the metal and over time will fail causing serious property damage or death.there is no boiler made to withstand the damage from thermal shock. Knowing how to operate and maintain is key. Most big boilers rated in horse power have a safety called the low fire hold which is a safety that does not allow the boiler to modulate higher than low fire until the water temp rises to a safe point so as to not cause thermal shock.