Well it expands when it is heated because as the atoms gain more energy from the heat, they begin to vibrate more and more which ''pushes'' the surrounding atoms away, causing the spatial volume to...
Air expands due to high temperature water. And nothing happens to the molecules. As they move faster usually from increased temperature, they become farther apart.
If its due to temperature rise, essentially all the molocules move further apart from each other in whats known as volumetric expansion, so when its volume increases, so does its surface area.