Try the animation and then read the text for a complete explanation of pH.

This is a rough measure the acidity of a solution. The “p” stands for “potenz” (this means the potential to be) and the “H” stands for Hydrogen. So you must write pH with a lower case (little) p and an upper case (capital) H. Writing PH would mean a public house! Writing Ph would mean philosophy as in PhD which means a Doctor of Philosophy.

The pH of distilled water is 7, this is neutral. Any solution with a pH below 7 (i.e. pH 1.0 to pH 6.9) is an acid and any solution with a pH above 7 (i.e. pH 7.1 to pH 14) is an alkali.

Acidic solutions have a pH between 1 and 6.9 === your stomach contains HCl it is pH2.

Alkaline solutions have a pH between 7.1 and 14. === your small intestine is pH 9.

Neutral solutions are neither acidic nor alkaline so their pH is 7.

The pH scale was invented a long time ago and sometime it seems a little silly that the more acid it is the lower the pH is. Really it is quite easy. Acids all produce Hydrogen ions (H+). Acids like Hydrochloric acid produce lots of Hydrogen ions; this is because when Hydrogen Chloride gas dissolves in water the molecules of Hydrogen Chloride dissociate into Hydrogen ions and Chloride ions.

HCl(aq) = H+ + Cl

Water also dissociates to produce ions, this time it is Hydrogen ions and Hydroxyl ions.

H2O(l) = H+ + OH

Sodium Hydroxide also dissociates to produce ions when it is dissolved in water, this time it is Sodium ions and Hydroxyl ions.

NaOH(aq) = Na+ + OH

In each case we can measure or calculate the concentration of Hydrogen ions present. We use the symbol [H+], we use square brackets to mean that it is the concentration of Hydrogen ions.

In HCl(aq) Hydrogen Chloride solution or Hydrochloric acid [H+] = 0.01

In H2O(l) water [H+] = 0.0000001

In NaOH(aq) Sodium Hydroxide solution [H+] = 0.00000000000001

As you can see, these numbers are very silly (well the first one might be OK but not the second two). So what we do is count the decimal places. It is much easier to write and remember pH7 than 0.000000, so wee use pH values.

HCl(aq) ~ pH2

H2O (l) ~ pH7

NaOH ~ (aq) pH14

There is a lovely advert on TV (or there was) in which the advertisers said that their shampoo was the best because it had very low pH. This means it is very acid, but you would not put it on your hair if they told you that it was acid. I think that the advertisers think that you must be stupid and that you will use their shampoo because it has low pH.

So to recap if the pH is low, it means that there is a high concentration of Hydrogen ions and if the pH is high it means that there is very low concentration of Hydrogen ions or none at all. Water and other neutral solutions are in the middle at pH7.

The gentleman who invented the pH scale wanted to measure how acid his beer was. As you know you have to use yeast to make beer or wine, and you also know that yeast uses enzymes. Enzymes only work if the pH is right. What he discovered was that acids and alkalis make the colours in plants change. The easy one is Litmus paper. Try grinding up some red cabbage or a beetroot. You will get a red or blue liquid. You can make the liquid change colour by adding acids or alkalis. All that happens is that the colour of the plant dye depends upon the concentration of Hydrogen ions. If you have used universal indicator paper you know that it can change from blue to green to yellow to red. Universal indicator is a mixture of dyes which all change when you change the pH.

(If you have a Nobel Prize for chemistry or would like one, please do not complain about this page because you know a more complicated explanation of pH. This page is intended for good GCSE science students to help them remember which way round the pH scale runs.)