Dr Hall's Chemistry Blog (copy)

\(\require{mhchem}\) Chemists are always being asked how to separate mixtures of things. A mixture is two or more substances put together but not chemically combined. We use different properties of the two substances to achieve separation. Example: you are given a mixture of dry salt and sand. How can we separate them? If you had enough time, a microscope, and a very fine pair of tweezers, you could examined the grains one by one. The cubic grains would be salt, and you could (in theory) sort those out from the others. That would take a dreadfully long time, but this technique of hand picking can occasionally be useful. Louis Pasteur (inventor of Pasteurisation) used it to separate left- and right-handed crystals. In hand picking we are using a physical property of the substances—their appearance—to separate the two substances. An easier way is to use a different property: solubility. If we add water to our mixture, the salt dissolves and the sand doesn't. We can then use...

\(\require{mhchem}\) Amphoteric is a fancy-sounding word, but it has a simple meaning. The word comes from the Greek ἀμφότερος (amphóteros, “each of two”) which gives a clue to the behaviour. An amphoteric compound is capable of acting as an acid or an alkali. We don't see this very often but it crops up most in the chemistry of aluminium, zinc, lead, tin, and beryllium. Let's have an example: aluminium, the metal, will react with an acid $$\ce{aluminium + hydrochloric~acid -> aluminium~chloride + hydrogen}$$ This is our old friend: $$\ce{acid + metal -> salt + hydrogen}$$ $$\begin{equation} \ce{2Al(s) + 6HCl(aq) -> 2AlCl3(aq) + 3H2(g)} \end{equation}$$ It also reacts with alkalis: $$\ce{aluminium + sodium~hydroxide -> sodium~aluminate + hydrogen}$$ $$\begin{equation} \ce{Al(s) + 2NaOH(aq) + 2H2O(l) → 2NaAlO2(aq) + 3H2(g)} \end{equation}$$ Zinc oxide is another amphoteric compound. It reacts with acid: $$\ce{zinc~oxide + sulfuric~acid -> zinc~sulfate + wa...