 As far as the properties are concerned, if you think about lemons, lemons contain an acidic solution, or acid in solution, and they have a sour taste. Some of these acids, particularly some of the very strong acids, are corrosive to the skin, and acids change colour in indicators, and you've probably most commonly dealt with things like litmus, which is red, universal indicator, which is often a reddish orange colour in acid solutions and so on, but we'll look at a few more indicators and some different sorts of colours as we go through this unit. They ionise to form this donateable proton, and this is a really important point that we're going to come back to later on, and acids neutralise bases.