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

(a ) Acids are those compounds which can dissociate and produce hydrogen ions in water. Thus, hydrogen is the element most common among the acids.

(b) Activity
Alcohol and glucose have hydrogen but releasing hydrogen from them is not easy. Thus, they are not called acids.
Take alcohol, glucose and HCl in small amounts. In a 100 mL beaker place the nails. Now put the cork inside the beaker and attach two terminals of a 6 V battery and a bulb with the two nails. Add the HCl and switch on. Repeat with alcohol and glucose. We observe that the bulb glows in case of acid but not in case of alcohol and glucose because acid can release hydrogen ions which conducts electricity but not the latter

Final answer:
Alcohol and glucose have hydrogen but releasing hydrogen from them is not easy. Thus, they are not called acids.
Take alcohol, glucose and HCl in small amounts. In a 100 mL beaker place the nails. Now put the cork inside the beaker and attach two terminals of a 6 V battery and a bulb with the two nails. Add the HCl and switch on. Repeat with alcohol and glucose. We observe that the bulb glows in case of acid but not in case of alcohol and glucose because acid can release hydrogen ions which conducts electricity but not the latter