What is the thermal expansion of matter?

Answer 1

With one notable exception, water, all matter expands (gets larger) without pause during increasing temperature. [See NOTE at the bottom.] This results because with the higher temperature the particles that make up the matter have more kinetic energy, which is measured by temperature or heat. And that heat is the result of the atoms and molecules that make up the matter moving faster and farther than when the heat is lower.

The "farther" part of the increased agitation with heat is what results in the matter expanding. In fact there will be a point where matter is no longer a solid, it becomes a liquid, as the atoms and molecules can partially disassociate from each other because they are moving so far away from each other. But they are not totally free from each other as a liquid.

For total disassociation and total freedom to move about and away from each other, the particles have to have more kinetic energy that manifests as more heat and temperature. And when that happens, we call the matter a gas.

Solid ice, for example, becomes water as its heat content and consequent temperature rises. Add even more heat and that water turns to steam, which is a gas. And as a gas the atoms and molecules can move so far from each other that there is almost no interaction among them

NOTE: Water (H2O) is the notable exception to the continual expansion with added heat. In fact, water is at its highest density (least expanded) at 4 deg C. This is why ice floats atop water; the liquid water is denser than the solid ice.