Matter Falling Out: Precipitation

I shall now introduce you to one of the simplest kinds of chemical reaction: precipitation, the falling out from solution of newly formed solid, powdery matter when two solutions are mixed together. The process is really very simple and, I have to admit, not very interesting. However, I am treating it as your first encounter with creating a different form of matter from two starting materials, so please be patient as there are much more interesting processes to come. I would like you to regard it as a warming-up exercise for thinking about and visualizing chemical reactions at a molecular level. Not much is going on, so the steps of the reaction are reasonably easy to follow. There isn’t much to do to bring about a precipitation reaction. Two soluble substances are dissolved in water, one solution is poured into the other, and—providing the starting materials are well chosen—an insoluble powdery solid immediately forms and makes the solution cloudy. For instance, a white precipitate of insoluble silver chloride, looking a bit like curdled milk, is formed when a solution of sodium chloride (common salt) is poured into a solution of silver nitrate. Now, as we shall do many times in this book, let’s imagine shrinking to the size of a molecule and watch what happens when the sodium chloride solution is poured into the silver nitrate solution. As you saw in my Preliminary remark, when solid sodium chloride dissolves in water, Na+ ions and Cl– ions are seduced by water molecules into leaving the crystals of the original solid and spreading through the solution. Silver nitrate is AgNO3; Ag denotes a silver atom, which is present as the positive ion Ag+; NO3– is a negatively charged ‘nitrate ion’, 1. Silver nitrate is soluble because the negative charge of the nitrate ion is spread over its four atoms rather than concentrated on one, 2, as it is for the chloride ion, and as a result it has rather weak interactions with the neighbouring Ag+ ions in the solid.