As a KS3 science student, you will be familiar with using indicators to tell whether a substance is acidic, alkaline or neutral. Indicators are usually made from plants, for example, you can use beetroot juice as a simple indicator. You have probably used the two types of litmus paper. Litmus is a mixture of dyes that come from lichens, very slow growing organisms that are made from algae or bacteria living in symbiosis (look it up if you don't know what that means) with a fungus. Blue litmus turns red in an acid and red litmus turns blue in an alkali.
Another mixture of dyes is called universal indicator. This is more useful than litmus as it can be used to measure how strong an acid or an alkali is using the pH scale. A pH of 7 is neutral (like for example water); a pH of 1 or 2 is a strong acid (for example sulphuric acid); weak acids like vinegar have a pH of 3 to 6. Weak alkalis (like soap) have pH values of 8 to 10 and a strong alkali like sodium hydroxide will have a pH of 13 or 14. Universal indicator comes as a liquid or as a roll of universal indicator paper.