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 include:

 * Ax Crazy: Johnny when he turns invisible and Gwinny, to a lesser extent.
 * 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.