Thermal expansion and cold contraction is a basic property of an object, that is, the volume of the object will expand after heating and shrink when cooling. Most objects in the world have this property.
Matter is made up of atoms, which are constantly moving, and the speed of movement will change with temperature, which is the fundamental reason for the expansion and contraction of objects. For a solid, its basic unit is a lattice composed of particles (atoms or ions), which vibrate around their equilibrium position in the lattice lattice. This kind of vibration is not left-right symmetrical simple harmonic vibration, but non simple harmonic vibration.
When the solid is heated, the particle vibration intensifies. Due to the existence of anharmonic effect, the equilibrium position of the particle will move, resulting in the increase of the average distance between adjacent particles, so the volume of the solid becomes larger. Conversely, when the temperature decreases, the average distance between particles decreases and the volume of solid decreases due to the weakening of particle vibration.
We can often see the cold shrinkage of solids. For example, metallic iron has the characteristics of thermal expansion and cold contraction. Therefore, when laying the rail, we must keep a certain gap to prevent the rail from pushing and deforming each other due to thermal expansion and elongation when the temperature rises. Similarly, the cement laid on the road will expand and contract with heat, so we will find that there will be a gap at intervals on the road.
Liquid also has thermal expansion and cold contraction. For example, our commonly used thermometer uses the thermal expansion and cold contraction characteristics of liquid mercury (or alcohol). The phenomenon of thermal expansion and cold contraction of gas can also be seen everywhere. If a table tennis ball is accidentally crushed, as long as it is scalded in boiling water, the deflated place will swell up, indicating that the volume of the air in the table tennis ball becomes larger after being heated.
We can observe the change of the temperature of the gas in the whole space (if we need to expand the gas in a space), but if we need to increase the temperature of the gas in a space, it will not increase.