Here's why:
* Abstract nouns refer to concepts, ideas, qualities, or states of being.
* "Union" can represent the concept of joining together (like in a labor union or a marriage).
* It can also represent the state of being united (like in the union of two countries).
However, "union" can also be a concrete noun, depending on the context. It can refer to a specific organization (like a labor union) or a specific group of people (like the European Union).
So, whether "union" is abstract or concrete depends on how it's used in a sentence.