Stine, R. L.
Book 8.0 of Ghosts of Fear Street
The stink bomb in Al's chemistry set doesn't explode...and it doesn't smell. It just sits there. Until it starts to ooze. It oozes all over the cat. It oozes all over Al's best friend. Everything the ooze touches...changes. Forever. And now the ooze is coming right at Al!