a block of aluminium at room temp is 20cm when heated it increases to 25cm when back to room tempthe volume returns to20cm why does this happen
20 cm is not a volume. Either you mean 20 cubic cm or that the length returns to 20 cm.
Almost all materials get let dense when the temperature is increased. The reason is that the atoms or molecules start "jiggling around" more as they absorb heat energy, and they tend to push neighboring molecules farther away.
The only exceptions I know of are water between 0 and 4 degrees C (which gets denser when heated), and certain types of low-expansion glass (Zerodur; ULE). Glasses can have quite anomalous behavior.