In this video you'll find out what acids are and what the strange pH scale
is all about.
Acids are substances that taste sour, like the citric acid in lemons and vinegar
which is sour wine or vin-egre in french. There are many other natural acids such as
lactic acid in sour milk and the poisonous oxallic acid in rhubarb. Two
hundred and fifty years ago early chemists found that non-metals burnt to form acids,
so they named the gas in the air
Oxygen, meaning acid maker. Sulphur burns to form sulphur dioxide which dissolves
in water to form sulphuric acid. Carbon dioxide forms carbonic acid, making rain
always slightly acidic and nitrogen oxides dissolve in water to form nitric
acid. However oxygen also reacts with reactive metals to form alkalis, such as
sodium hydroxide and calcium oxide. So oxygen is not only an acid maker but
also an alkali maker. In water acids give up a hydrogen ion or proton to the water
molecule, forming a hydroxonium ion, H3O+ or example hydrochloric acid
produces chloride ion and hydroxonium ion. Acetic acid, the acetate ion. Sulphuric acid,
the sulphate ion. The amount of acidity can be measured on the pH scale
a measure of the concentration of hydroxonium or hydrogen ions. Little p
for power or strength and capital H for the hydrogen ion. Acids can change the
colour of indicators. Try making your own by extracting the colour from red cabbage
leaves or other coloured plants. Add lemon juice then garden line and watch the red
colour gradually change through green to blue as the acid is neutralised. Perhaps
the most famous indicator is litmus
which is extracted from likens, red in acid and blue in alkali. A full range indicator
contains a mixture of dyes allowing you to measure the pH from 0, a strong
acid, getting less acidic through neutral water, pH 7, getting more alkaline to
strong alkali at pH 14. The pH scale is logarithmic, meaning that each change of
one unit of pH there is a ten-fold change in acidity. To understand its
logarithmic nature, consider this: you have collected a bucket full of acid
rain with a pH 4 and you want to make it less acidic for watering your
garden. How much neutral water must you add to make the pH change from 4 to 5.
Pause the video and think. Well the answer is that you need to act 9 buckets of neutral
water just to change the pH by one unit since each change in pH is at 10 fold in
acidity. To summarise, most acids are naturally occurring substances which
dissolve in water and are sour to the taste (warning: only taste food substances).
They are also formed when oxides of nonmetallic elements, like sulphur dioxide,
dissolve in water. There are several different indicators which can help us
determine the acidity of a substance and the degree of acidity is measured on the
pH scale which is logarithmic.