Explanation:
Sodium is a silver-colored metal which is soft enough to cut with a knife. It is an extremely reactive metal, and is always found naturally in ionic compounds, not in its pure metallic form. Pure sodium metal reacts violently (and sometimes explosively) with water, producing sodium hydroxide, hydrogen gas, and heat:
2Na(s) + 2H2O(l) ——> 2NaOH(aq) + H2(g)
Chlorine is a poisonous, yellow-green gas, with a very sharp odor, and was used in gas warfare during World War I.
Sodium and chlorine react with each other, however, to produce a substance that is familiar to almost everyone in the world: sodium chloride, or table salt:
2Na(s) + Cl2(g) ——> 2NaCl(s)
It is easy to see why this reaction takes place so readily when we look at it on an atomic level: sodium has one electron in its outermost (valence) shell, while chlorine has seven electrons in its valence shell. When a sodium atom transfers an electron to a chlorine atom, forming a sodium cation (Na+) and a chloride anion (Cl-), both ions have complete valence shells, and are energetically more stable.
The reaction is extremely exothermic, producing a bright yellow light and a great deal of heat energy.
In the following demonstrations, a 2.5 liter bottle is filled with chlorine gas. A coating of sand on the bottom of the bottle absorbs some of the heat energy produced during the reaction, and prevents it from breaking. A small piece of freshly-cut metallic sodium is placed in the flask, and then a small amount of water is added, which reacts with the sodium and causes it to become hot. The hot sodium then reacts with the chlorine, producing a bright yellow light, a great deal of heat energy, and fumes of sodium chloride, which deposits on the walls of the bottle.
In the first video clip, the sodium flares up almost immediately upon reaction with the water, and "burns out" quickly. (Don't blink, or you'll miss it.) In the second, water is added twice, to produce one short flash, followed by a much longer one. (This reaction can also be done with molten sodium, but I've never been brave enough to try that.)
