Learn English – why are “feet” common and “chemistry” abstract nouns

No, abstract nouns and adjectives are two different things. Anger is an abstract noun, while "angry" is and adjective. "Length" is an abstract noun, while "long" is an adjective.

While specific words can be categorized into different parts of speech depending on how it is used in a sentence, specific parts of a particular sentence can only be categorized as one part of speech. For example, take the word "love".

A mother's love is eternal. (here love is an abstract noun) I love pizza. (here love is a verb)

The conventional definition of a concrete noun is something that we can detect with our physical senses: We can see, hear, touch, taste or smell them. There are several problems with this definition, for example nobody would suggest that music is concrete, but we can hear it.

Fumblefingers has proposed a better definition- things that have mass. This is not a sure-fire way of classifying things, though, because a scientist would say that light has mass: he or she can prove that experimentally that it has mass and, for a given colour, can calculate accurately how much each photon weighs. An artist, however, would say that light is abstract.

Scientists have long dealt with this kind of dichotomy of perspective, for example with wave-particle duality: they view light as either particles or waves, depending on what kind of behaviour they wish to predict. Maybe we should look at why exactly we want to classify things as concrete or abstract (what behaviour we want to predict), then we can identify the best way of classifying them.

We use countable and uncountable nouns differently, for example we use a for countable nouns but generally not for uncountable nouns. What differences in usage are there for concrete and abstract nouns? Absolutely none. I can only think of two things, neither related to usage, that give value to the concept of an abstract noun.

First, when we teach grammar and we want to explain what a noun is, it's easy to explain what a concrete noun is: it's a person, place or thing. We need abstract nouns to explain about all of the other nouns- the things that we cannot touch.

Second, when we wish to communicate with people from other cultures, we have to bear in mind that the meanings of concrete nouns are generally communicable, but those of abstract nouns vary between cultures. I had lived in Egypt for eight years before I came across a word for debt: it's an alien concept.

Maybe you can come up with other ways that absrtact adds value, but as far as I am concerned it is an interesting theoretical concept that is of transient practical value for people learning a language. It's definitely not worth splitting hairs over definitions.

As I mentioned earlier, countable is a much more useful concept because it affects how we construct a sentence. Many nouns, though, can be both countable and uncountable: there is duality here. How do we handle that? We proceed as scentists do, and use the appropriate model for the kind of meaning that we want to convey. Looking at promise, for example:

The air was full of promise - uncountable.
He made me a promise - countable.

A useful guiding principle is whether the thing you want to describe is atomic: this comes from Greek, and means something that you can't cut. If you can cut something in two and it's no longer the same- its nature has changed- it's countable.

If you take a whole fish and cut it in two: its nature changes: it is no longer a fish. The whole fish was countable. If you take a pieces of fish meat and cut it in two, you have two smaller pieces, but the nature of the fish meat in this context is unchanged. The fish meat is uncountable.

You can apply the atomicity rule to abstract concepts too: the first promise is a general concept, and its nature would be unchanged if you cut it in two: it will still be a general concept, so it is uncountable. The second promise would be of little value if you cut it in two, so its nature has changed: it is countable.