The change from liquid to gas is called evaporation. As you increase the heat, the bonds holding the molecules together will break, resulting in molecular dissociation . Hotter still, and the electrons start to break free and the atom is ionized. A plasma is when the substance is so hot all electrons are free from being bound to the nucleus and move freely. The temperature of phase changes differs for each type of chemical substance.
Atoms can have different degrees of ionization, depending on how many electrons they have.
Hydrogen only has one, so it can only be singly ionized. The minimum amount of potential energy of the electron cloud is termed the ground state. As the electron gains more energy, it enters the excited state . Energy must be conserved so it must get it from elsewhere. If an electron loses energy, it must do so either by colliding with another particle or by emitting light.