why are feet common and chemistry abstract nouns?
Why feet is considered as common noun?
why chemistry is considered as abstract noun?
First of all, a common noun is any noun that isn't a (proper) name.
What you are probably talking about are concrete nouns, or nouns that refer to things directly sensed by any of your five senses (see, touch, smell, taste, hear).
fire, music, guns, computer, kitten, chair, nuclear warheads, oxen, cows, meat, wood, car, street, mirrors, light
Abstract nouns are nouns that are not directly felt by the five senses.
society, bravery, awesomeness, anger, mathematics, concept, time, mentality, definition, programming, self-worth
Feet can be seen (at the bottom of the human body) or touched, so it is a concrete noun.
You can't directly touch chemistry, or taste chemistry, or directly see chemistry, or smell chemistry, or hear chemistry, so it is an abstract noun.