Fossils formed when an organism dissolves and leaves an empty space in a rock are called molds.

Here's a breakdown:

* Mold: The empty space left behind after an organism dissolves. It's a negative impression of the organism's original shape.

* Cast: If the mold later gets filled with sediment or minerals, it creates a cast. The cast is a positive replica of the organism.

So, the mold is the initial empty space, while the cast is the solidified replica of the organism.