By heating the gas, you are adding energy to the particles, and as a result, they will move faster and with more force. If the gas does not have room to expand (set volume), then the pressure will...
Objects expand when heated because the heat 'gives' molecules more energy, so they move around more. This vibration, as molecules are pushed further apart, is manifested as a larger volume.
An...
They are the same because they are comprised of atoms. Most material can assume the three "phases" solid, liquid, gas; and a fourth "plasma". Water can be taken as an example Water is made up of 2...