Which word in this sentence is a demonstrative adjective..that boy scored three goals in the soccer game.

The answer to your question would be that this is an example of a compound sentence.

Explanation:

A compound sentence is a sentence that has at least two independent clauses joined by a comma, a semicolon or a conjunction. In this case, the two independent clauses have been joined by the coordinating conjunction but.

1) There is a swimming pool in our backyard (independent clause I)

2), but it contains no water (independent clause II)