We made salt by mixing two clear and colourless liquids: hydrochloric acid and sodium hydroxide. Sodium hydroxide is an alkali. We started by placing a small volume of alkali in a test tube and added Universal Indicator, which went purple showing pH11. We counted drops of acid until the Universal Indicator went green. It was a neutralisation reaction. We had made an aqueous solution of sodium chloride. To get the crystals, we needed to evaporate away the water. The problem was that the salt would have the colour of the Universal Indicator, so we needed to repeat the experiment without the indicator. Here's the end product.
The experiment works because of the reactivity of the ions in the solutions. Sodium hydroxide contains Na+ and OH- ions. Hydrochloric acid contains H+ and Cl- ions. Na+ is more reactive than H+ because it finds it easier to lose electrons. It bonds more easily to the Cl- ion because that is the smallest of the two negative ions. The result is a cubic crystal lattice like this:
This lattice is a strong shape and that makes the molecule very stable. Stable means lower potential energy. In the same way that water flows to the place of lowest potential energy, so chemical reactions happen most often when they head towards lower potential energy. That's why this neutralisation reaction is able to occur and it is why acid and alkali make salt. Finally, notice that if the Na+ bonds with the Cl-, that leave behind H+ and OH-. Of course, that's H2O!!
