When is a noun concrete or abstract?

A concrete noun is a Word for something that can be experienced by any of the five physical senses; something that can be seen, heard, smelled, tasted, or touched.

queen, Queen Elizabeth II

city, Miami, FL

coffee, Maxwell House Coffee

An abstract noun is a Word for something that can't be experienced by any of the five physical senses; something that is known, learned, thought, understood, or felt emotionally.

grace

secret

independence

knowledge