There is not a great deal about acids on this blog, which is odd because they are very common components of the science syllabus. We add them to bases and alkalis to make salt and water; we put metals in them to liberate hydrogen gas; citric acid is used to produce the sour taste in tangy sweets (ahh, happy powder!); dissolved nitrogen and sulphur oxides in the atmosphere cause acid rain; and amino acids are the building blocks of protein. But what makes an acid different to other molecules?
All acids contain the element hydrogen. For example…
…and so on and so forth!
An acid is any molecule that increases the concentration of hydrogen ions when dissolved in water. The molecules in the table above split into H+ ions and a negative ion when they dissolve (for example H+ and Cl– in the case of hydrochloric acid). How completely they separate determines whether they are a strong acid or a weak acid. Carbonic acid, for example, hardly releases any H+ ions into the water when it dissolves so it has a pH of about 4 in a fizzy drink or nearer 6 in rainwater. Hydrochloric acid very nearly completely separates so its pH is very close to zero.
This willingness to give up its hydrogen results in the three classic acid reactions. Firstly, acid plus alkali (or base – more on this later in the week)…
Then there is acid plus a carbonate…
And finally, an acid with a suitably reactive metal…
If you remember these three scenarios you will be ready for most occasions.
Questions…
- What is the chemical name for table salt?
- How does a cell know what order to assemble amino acids in to make a particular protein?
- What is the chemical test for hydrogen?
- What is the chemical test for carbon dioxide?
- What is the common name of calcium sulphate?
- How does nitrogen oxide get into the atmosphere?
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