tomato soup / a tomato soup

[jesus]

What is the difference?


- it could have been tomato soup.
- it could have been a tomato soup.

Thanks.

[Pete]

What is the difference?


- it could have been tomato soup.
- it could have been a tomato soup.

Thanks.
In most contexts, "it could have been tomato soup" would be the natural choice. That phrase simply identifies a substance or a kind of food.

You might use your alternative phrase, "it could have been a tomato soup" if the discussion was much more specific and the subject was what kind of soup, especially if you want to suggest that there are different kinds of tomato soup and this was a particular one of these kinds.

Occasionally, someone might also use "a tomato soup" as a shortened form of "a bowl or serving of tomato soup".