Chemistry Experiments - Effect Of Strong Acids And Alkalis

First of all, it needs to be said that this experiment is not really suitable for schools or as a demonstration. It just takes too long. The video works very well, though, to help explain safety rules in the laboratory. So what is this experiment about?

Alkalis and acids are known to be highly corrosive in their concentrated forms. I can say from my own experience that extreme care with these substances is certainly justified. Chemistry teachers, students and laboratory workers who have ever had a drop of concentrated Sulphuric acid hit their trousers will agree. This experiment gives a graphic illustration of what happens when concentrated acids and alkalis come into contact with flesh or metal. Another very nice experiment, designed to ensure that the ladies will wear their laboratory coats is the effect of concentrated Sulphuric acid on tights, which it destroys in a matter of seconds.
The solution in the small cylinder takes on a blue/green colour and bubbles rise to the surface. At the end of the experiment, the watch has completely disintegrated and the remaining metal pieces are thin and brittle. The caustic soda corrodes the pig's trotter slightly, the solution becomes cloudy. The trotter in the Hydrochloric acid dissolves completely, leaving behind a dark solution and a shapeless, pale mass.
Acids react with metal, such as the iron contained in the stainless steel watch, forming metal salts and Hydrogen:

Reduction: 2 H3O+ + 2e- -> H2 + 2 H2O

Oxidisation: Fe -> Fe2+ + 2e-

2 H3O+ + Fe -> H2(g) + Fe2+(aq)

The watch contains metals other than iron, and it is these that are responsible for colouring the solution.

The reactions of the acid and alkalis with the pig's trotters are manifold. A few of the possible reactions are listed here:

The cells of living bodies such as ours mainly consist of proteins, which are degenerated by the acid, meaning that their angular structure changed irrevocably. The bigger protein constructs in the cells are destroyed in this process. Enzyme activity is now taking place in an uncoordinated and chaotic way, so that the body's own enzymes speed up the process of degeneration. The high concentration of the acid also breaks up the peptide bonds between the amino acids making up the proteins, thus destroying the proteins further. Bones consist of carbonates and proteins. Proteins are destroyed as shown above, and carbonates in the presence of strong acids change into Carbon Dioxide and water. At the end of the process, only the fat is left, which will be found floating on the solution.

The alkali has a much less spectacular reaction with the pig's trotter, but a degree of destruction is visible. The strong caustic soda first neutralises the natural, acidic protective layer and then forms a soapy coating with the fat – this may be what protects the trotter from further destruction. A few proteins are also hydrolized (disintegrated) by the alkali.

The alkali is decidedly not as harmless as it may seem in comparison to the acid. Just a few drops of highly diluted caustic soda on the retina of the eye can lead to blindness! The corrosive effect of alkalis can be much stronger in other circumstances and it is very difficult to wash it off in case of a laboratory accident.

As we come into contact daily with concentrated acids and alkalis in our homes (essence of vinegar, drain blockage remover, etc.) we should be most careful to comply with any safety notifications on the packaging!