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The pH Scale
The pH scale is used to measure how acidity and alkalinity. Something which is neither acid nor alkaline is said to be neural. The neutral pH is pH7. Distilled water is neutral.
Any pH value below pH7 is acid. Weak acids will be around pH6 or pH5. Strong acids will show pH1 or pH2 when tested.
Alkalis how a pH value above pH7. Weak alkalis are around pH8 or pH9. Strong alkalis will have a reading of pH13 or pH14.
What is an acid?
You normally have a good idea about acids because of the taste of vinegar and lemon juice, for example. These solutions taste sour and can hurt if they touch a cut in your mouth or your hand. Strong acids are corrosive and they can ‘eat’ through metal. There are many examples of weak acids in our everyday lives. Rainwater is slightly acidic because it has carbon dioxide gas dissolved in it. Fizzy drinks are acidic for the same reason (and bad for your teeth if you drink too many of them!). You will not often come into contact with strong acids, fortunately. A car battery contains concentrated acid (pH1 or pH2) and you might be surprised to know that your stomach produces an acid at pH2 which helps you to digest your food
What is an alkali?
Alkalis are less well known but, in fact, you use an alkali every time you pick up a piece of soap. Alkalis are just as corrosive as acids even though they feel smooth to the touch. Soap cleans you by taking away your outer layer of skins cells (and with these the dirt!). Strong alkalis can feel ‘soapy’ if you get them on your fingers but beware, a strong alkali would take off more than just the outer layer of skin cells. One alkali, ammonium hydroxide, is used in household cleaning fluids. The strongest of these are those that are used to clean the toilet bowl. Ammonium hydroxide is also a bleach (a chemical that whitens anything it touches). Bleaching agents are able to kill harmful bacteria.
How to test pH
You can test the pH of a solution usingn indicator paper or indicator liquid. An indicator is a chemical which will change colour as the pH changes. There are quite a few indicators in use in the laboratory. This is why each type of indicator paper comes with its own colour chart. When you use indicator liquids, such as universal indicator or bromothymol blue, you are always given pH standards for these solutions to compare your results to.