 We can classify substances based on their properties, how they react, what they do, what we use them for. For instance, fuels are chemicals that we burn to provide heat which can be used for warmth or turned into other forms of energy like kinetic or electrical energy. These other classes of chemicals here have their own properties and uses as well. Two very common and important classes of substances though that we encounter a lot in ordinary life and which we're going to look at here are acids and bases. Acids and bases have some properties that you may already be familiar with. Acids taste sour. In fact, the word acid comes from the Latin word for sour. Think of lemon juice and vinegar and plain yogurt which has acids in it that are caused when bacteria ferment the milk. In contrast, bases taste bitter. Acids turn litmus paper red whereas bases turn it blue. Litmus is a natural dye that's derived from a lichen and it acts as an acid base indicator. Other indicators will also show a colour change between acids and bases. Acids and bases neutralise each other. In general, if you add an acid to a base then the result, the products will be neither acidic nor basic. There are exceptions to this that we'll look at in Year 12 when we talk in more depth about acids and the idea of chemical equilibrium. Both acids and bases can be corrosive if they're concentrated enough but the chemical reactions that they undergo that do the corroding are actually different. We're going to look at some of the characteristic reactions of acids later. So what is an acid and what is a base? It was first recognised in the 18th century that there existed a group of chemicals that had related properties including the fact that they tasted sour and so they became known as acids. At the time though chemists didn't know what the formulae of these acids were and there was a great deal of discussion and argument relating to what was responsible for them all being acidic. The first generally accepted definition for acids and bases came from Svent Arrhenius, a Swedish chemist. In 1890 he defined an acid as a substance delivering hydrogen cations to the solution and a base as a substance that delivered hydroxide and ions to the solution. This is the same as saying that when you dissolve an acid in water it increases the concentration of hydrogen ions in the water whereas if you dissolve a base in water it'll increase the concentration of hydroxide ions. We can explore this definition by looking at what happens to some acids and bases when we put them in water. Let's take nitric acid. A nitric acid molecule is covalently bonded but when it dissolves in water it behaves almost as if it were ionic. It separates, we say dissociates, into a hydrogen ion and a nitrate ion. So putting nitric acid molecules into water does increase the concentration of hydrogen ions in that water. A similar thing happens for sulfuric acid but here each molecule has the potential to release two hydrogen ions. In contrast a base is something like sodium hydroxide. This really is an ionic substance. It's made of a metal ion and a non-metal ion. And when it dissolves in water it releases a sodium ion and a hydroxide ion. So dissolving this compound in water does increase the concentration of hydroxide ions in water. And similarly for magnesium hydroxide which releases two hydroxide ions. However, Arrhenius's definition runs into problems when you probe a bit more deeply. Because what about ammonia? Ammonia has the formula NH3 and if you play around with it you find that it has the properties of a base. But how does it increase the concentration of hydroxide in water? It has no hydroxide ions in it. So Arrhenius's definition was clearly incomplete and it needed some further work.