If you want to think about it on a chemical level, Earthbending is the control of solid particles (though not all), waterbending is the control of liquid particles (of H2O anyway), airbending is gaseous particles and firebending is energy (heat specifically). Of course, this all gets destroyed by the fact water benders can manipulate the heat of water to turn it into ice or steam, but it's the closest I could come up with. And again, it only applies to certain solids/liquids/gases, not all.