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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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,
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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.