The Ogre Downstairs

A novel by Diana Wynne Jones. As a peace offering, the protagonists' stepfather buys them a chemistry set, which turns out to be magical. Hilarity Ensues.

Tropes used in The Ogre Downstairs include:

Ax Crazy: Johnny when he turns invisible ( apparently, turning invisible and slowly fading from existence while already in a bad frame of mind can cause one to go a bit loopy) and Gwinny, to a lesser extent. She baked the Ogre a foul cake mixed with a little bit of everything poisonous after he hit Johnny and Malcolm. It turned out all right though.
Adults Are Useless: Both parents remain woefully ignorant of the magic going on around them for most of the book (the Ogre even prevents two of the children from finding the antidote to one of the chemicals at one point), but find out at the end, whereupon the Ogre proves to be very useful.
Carrying a Cake: Gwinny
Freaky Friday Flip: Malcolm and Caspar
Freak Lab Accident: Happens quite a bit.
Gone Horribly Wrong: The Toffee bars. Oh, lord, the toffee bars.
Hidden Depths: The Ogre, Malcolm.
Incredible Shrinking Man: Malcolm
Invisible Jerkass: Johnny
Invisibility Cloak: Johnny
The Little Shop That Wasn't There Yesterday: The shop they buy the chemistry sets from, which, though it doesn't appear or disappear inexplicably, is demolished at the end (once its work is done).
Living Toys: Including - among other things - toffee bars, dust bunnies, and dolls.
Missing Mom: Sally leaves after a particularly nasty row with the Ogre.
Off to Boarding School: The Ogre threatens Caspar and Johnny with this.
Professor Guinea Pig: The kids often test the potions this way.
Psycho Serum: Toned down, but all the experiments wind up having some bad side-effects when used incorrectly.
Technicolor Science
Wicked Stepmother: Played with: the Ogre appears to be an evil stepfather, but turns out to be fairly sympathetic, if grouchy. Also with quite a dry sense of humour, Casper realises that they have probably been taking his jokes as serious statements.