How atoms and molecules are affected by heat?
Their speed, energy, and the total volume increases.
When we heat up substances, we are transferring energy in the form of heat to them. The molecules have to do something with the energy, so usually they will start to vibrate faster (more kinetic energy), and in some cases, lose electrons (become ionized), but that process takes up a lot of energy.
Also, the volume of a substance will increase, while keeping the mass the same, and so its density decreases. This is why hot air floats.