Question

On dissolving salt in water, salt splits into Na and Cl. As i was taught that Na instantly reacts with water and burns. Then why doesn’t Dissolved Na burn in water?

Answer 1

Because the sodium ion has already been OXIDIZED.....

Explanation:

As you say, the dissolution of sodium chloride in water does indeed represent a chemical reaction...

`NaCl(s) stackrel(H_2O)rightleftharpoonsNa^+ + Cl^-`

Bonds are broken, and new substances are formed. However, in this scenario, the sodium ion has ALREADY been oxidized...in the initial formation of salt.

`Na(s) rarr Na^+ + e^(-)`

...and this is conceived to react with HALF an equiv of chlorine gas...

`Na(s) + 1/2Cl_2(g) rarr NaCl(s)`

Answer 2

You are talking about two different reactions.

Explanation:

Dissolving sodium chloride in water is a chemical change forming a solution of sodium and chloride ions. This solution is an electrolyte (can conduct electricity) because the sodium and chloride ions dissociate due to the polar nature of water. The sodium ions will not react violently with the water because the sodium is already oxidized (has lost an electron, forming a cation) as pointed out by anor77.

`"NaCl(s)"` `stackrel("H"_2"O")rightleftharpoons` `"Na"^+("aq") + "Cl"^(-)("aq")"`.

When a piece of elemental sodium metal is placed in water, it does react instantly and violently. This is a chemical reaction in which sodium metal is oxidized into sodium cations, and that produces sodium hydroxide and hydrogen gas, which can ignite during the reaction. The balanced chemical equation is:

`"2Na(s) + 2H"_2"O("l")"` `rarr` `"2NaOH(aq) + H"_2("g")"`