But it's not sodium metal that's dissolved in the water. It's sodium ions! There's a huge difference. In becoming a cation, a sodium ion gains an electron (Na+). In chemistry, we call gaining an electron "oxidation." So you can't further oxidize a sodium ion. To get back to sodium metal would require abstracting an electron away from the sodium ion, which would require a huge energy cost--much more than you'd be able to recoup by burning the sodium.
BUT, you can burn sodium COMPOUNDS, as well as pure sodium, which as you correctly state is a "metal". How do the burn metal sodium? Not by picking up a solid bar and lighting the end, instead they use the finely ground powder form. Just becasue it is a metal, there is no reason to confuse the public into thinking it is fully equivalent in shape and structure to th ecast iron skillet they used this morning to cook breakfast.
What do these directed radio waves do that conduction heat cannot? Not sure, but it may be somewhat similar to the inside of a microwave oven, except that it may possibly heat only specific targeted cancer cells. Backstory http://www.rd.com/content/radio-wave...cancer-cure/0/
The mechanics have proven to heat certain minerals, including carbon nanoparticles, so why not sodium, in any form you wish. What happens when you combine elemental sodium with water?
Reaction of sodium with water
Sodium metal reacts rapidly with water to form a colourless solution of sodium hydroxide (NaOH) and hydrogen gas (H2). The resulting solution is basic because of the dissolved hydroxide. The reaction is exothermic. During the reaction, the sodium metal may well become so hot that it catches fire and burns with a characteristic orange colour. The reaction is slower than that of potassium (immediately below sodium in the periodic table), but faster than that of lithium (immediately above sodium in the periodic table).
2Na(s) + 2H2O → 2NaOH(aq) + H2(g)
Now we know that as found in saltwater, sodium is in a dissolved compound, however, pure sodium can react with water as previously quoted, with enough heat (exothermic) in the reaction to cause it to burn. Well, I guess so will the H2, or Hydrogen gas produced by the reaction.
Now, what IF the directed radio waves involved here "break" the bonds of Na and Cl, thus releasing Na into the water, and as it has extra "heat" added due to the energy from th edevice, the sodium (metal) now burns while the outside energy is applied, and ceases to burn when th eenergy source is removed. The dilution of the sodium in the water is sufficient and the salt bonds strong enough to remain intact at normal temperatures, and without any external input th esalt remains dissolved, for the most part.
Why, when the same or higher amount of energy is applied during cooking to saltwater does it not spontaneously burst into flames. Or when using rock salt to melt ice, why does the asphalt highway not catch on fire? Becasue teh salt is still salt, dissolved in and recoverable from the water it IS in. But seperate the sodium from the chlorine and now maybe we have something.
Is there enough latent energy in the water? To recover simply by whatever process can be proven to ignite it? Not entirely likely - dilute gasoline with water down to a 3% solution and see if you can run a car on it. Think in terms of heating your entire house with only the pilot light in the furnace, never igniting the main burner.