Sodium polyacrylate and water
Sodium polyacrylate can absorb over 200 times its mass in water.
Fluorine reacting with hydrogen in a balloon
A neodymium magnet falling through a copper pipe
Magnetic field equilibrium.
A kernel of corn popping
You can also do this (to a lesser extent) by rubbing a balloon on your hair, then holding it next to your running sink.
Mercury(II) thiocyanate decomposition
Heating mercury(II) thiocyanate causes it to release toxic gases and decompose into an insoluble brown mass.
Hydrogen peroxide mixed with potassium iodide
The iodide catalyzes the decomposition of the hydrogen peroxide. A small amount of dishwashing soap is usually added so that the resultant oxygen is caught in the bubbles, forming a tower of foam.
Copper reacting with nitric acid
Firework explosion in slow motion
A slowly propagating chemical oscillation containing bromine and an acid.
Lithium on fire
A candle lit by its vapor trail
Orange LED light immersed in liquid nitrogen
The wavelength & frequency of the light changes as it loses thermal energy.
Flammable fluid in a glass jug
Isopropyl alcohol, most likely.