In this demonstration, different acids are compared in terms of:
1. Electrical conductivity
2. Effect on damp universal indicator paper
The strong acids, hydrochloric acid and sulfuric acid, both exhibit high electrical conductivities. This reflects the large number of ions present these solutions: both acids are completely dissociated.
The weak acid, ethanoic acid, shows a much more modest electrical conductivity. This acid is only partially dissociated and there are fewer ions in solution that can move and carry the current. As a rough comparison, out of 1000 molecules, every HCl will be dissociated in water, but only 4 ethanoic acid molecules will be dissociated.
Solid citric acid does not conduct electricity. This is because the ions are not free to move. This acid will only become an electrical conductor once it has been dissolved in water. Notice that the electrical conductivity is still very small, despite being dissolved in water. This is because citric acid is an example of a weak acid.
Similar trends are seen with the universal indicator paper. Strong acids give a deep red colour; weak acids give lighter red. Citric acid will not give any colour change until it can donate H+ ions, this is not possible until it is dissolved in water.
In nature, citric acid is found in citrus fruits. The concentration in some varieties of lemons and limes can be as high as 0.3 mol/dm3. The acid was one made by grinding up these fruits and adding calcium hydroxide to the juice. This led to the precipitation of solid calcium citrate, which could then be filtered. Sulfuric acid would then be added to regenerate the citric acid, which would then be crystallised.
