Just to expose my ignorance of chemistry...

[El_Spectre]

Question: Sodium reacts energetically w/water. So why doesn't part of dissolved salt (NaCl, I know there are lotsa salts), ya know... blow up?

[Terry]

The Na in NaCl is in the form of an Na+ ion, which is much less reactive than the uncharged atom.

AKA:
Mr electronegative chlorine, and Mr electropositive Sodium meet up. Mr. Sodium lends Mr. Chlorine an electron, and they both go off a lot happier, each having an electric charge, but full shells.

--Terry.

[pgwenthold]

The sodium that reacts violently with water is sodium metal, which has it's full complement of 11 electrons. The sodium in NaCl is sodium cation, Na+, where one electron is missing. In fact, Na+ is what is being formed when Na metal reacts with water.

The technical term is "oxidation state." Sodium metal is said to have an oxidation state of 0 (meaning it has all its electrons) whereas sodium ion has an oxidation state of +1. Different oxidation states of atoms can have very different reactivity.

To understand the reactivity difference, you have to consider the electron configurations. Starting with the +1 state, note that it has 10 electrons. That is similar to the electronic configuration about neon, an unreactive "noble" gas. Other noble gases include helium and argon. Thus, sodium ion is basically like a noble gas with an extra proton in the nucleus. The extra proton means it is charged, and therefore will form electrostatic complexes (like Na+Cl-) but Na+ itself is highly unreactive.

Sodium 0 however, has 1 little electron hanging there all by himself in the next energy level of the atom. Single electrons on the whole lead to reactivity ("promiscuous" if you will), and note, in this case, if that electron gets away, you get to that happy noble gas arrangement, so there is a good driving force to head that way. Hence, if you have something that can take that electron away, it will go that way. Water (H2O) does it, leading to the formation of hydroxide ion, OH-, and a hydrogen atom. If it happens twice, you get hydrogen gas instead of the atom.

Now, the reaction occurs with the evolution of a lot of heat. So if you have a lot of sodium, it tends to get very, very hot (the sodium will glow red). Eventually it gets hot enough to ignite the hydrogen that is coming off, causing an explosion. It can be a pretty big explosion.

Don't try this at home.

[uruk]

That's also the reason why the oxygen and hydrogen in water does not go boom even though we use oxygen and hydrogen as rocket fuel.

[Bronze Dog]

Attempt at a short explanation:

The metalic sodium gives up its electrons and turns some water into free hydrogen atoms (which quickly reform into H2 gas).

2Na + 2H2O -> 2Na+ + 2OH- + H2 (gas) + lots of heat.

The electrons given up fill in the role of the shared electrons from the freed hydrogen. Shortly after the gas forms

Ignition heat + 4H2 + 2O2 -> 2H2O + more heat of combustion.

[casebro]

Question: Sodium reacts energetically w/water. So why doesn't part of dissolved salt (NaCl, I know there are lotsa salts), ya know... blow up?


Simple answer: it already has.

[Bronze Dog]

Simple answer: it already has.
That reminds me: Don't worry about your Lithium ion batteries blowing up when you crack the case of the biochips installed in your right hand.

[El_Spectre]

Thanks for the responses folks... and particularly to Terry for rendering it in cartoon terms :)