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.
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.)
