Hi All, I have always been made to understand that the reason why gas expands or causes pressure build up when heated in a sealed container is that gas molecules speed up when heated. If this is correct, can someone in the know please explain what is happening at the individual molecular level how absorbing heat energy can cause a molecule to change its speed. Assume a single gas molecule in a 1 cubic meter container at temperature T1 moving at speed S1, if the temperature is increased to T2, does its speed increase to S2 resulting in more frequent collisions with the container walls? Exactly what mechanism is at play to cause the increase of speed and in what direction does the speed increase? If the speed does indeed increase, then there must be acceleration which implies a force. What kind of force is this? Why does this force always act in the direction in which the molecule is already moving? Thanks for any help. Marco.