(a) Which element is common to all acids?
(b) Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

Hint::A substance will be known as an acid if it gives out positive hydrogen ions, that is, ${H^ + }$ ions when dissolved in water. An acidic compound donates a proton. The acid is regarded stable if and only if its conjugate base is also stable.

Complete answer:
(a) Hydrogen is the element which is common to all acids. When an acid is dissolved in water, it gives out \({H^ + }\) ions which are protons.
(b) Let’s take a cork and place it in a 100 mL beaker, after fixing two nails on the cork. Make a circuit consisting of a 6 six-volt battery, a bulb and a switch. Join the two nails stuck in the cork with the circuit. Then make solutions of each glucose, alcohol, hydrochloric acid and sulphuric acid in water. Now pour the hydrochloric acid solution in the beaker with cork and turn on the circuit. The bulb glows and this proves that hydrochloric solution can conduct electricity. Even sulphuric acid taken in the beaker conducts electricity and make the bulb glow. We can say that aqueous solutions of acids can conduct electricity because they give out charged hydrogen ions in the solution. On taking glucose solution and alcohol solution, the bulb does not glow which means they do not conduct electricity and hence do not produce positive hydrogen ions when dissolved in water.
Therefore, alcohol and glucose are not considered as acids because they cannot not produce \({H^ + }\) ions.

Note:
There are three types of acids depending on the number of hydrogen atoms or ions it can replace or donate in a reaction with other substances. They are monobasic acids, dibasic acids and tribasic acids.