Thermite

thermite, cannon balls

As two rusty cannon balls (one covered in aluminum foil) are smashed together, sparks and a cracking sound are produced.
Fe2O3 (s) + 2Al (s) → Al2O3 (s) + 2Fe (s)

Hydrogen Balloons

hydrogen balloon explosion

This demo shows the explosiveness of hydrogen, by igniting a balloon filled with hydrogen.
2H2 (g) + O2 → 2H2O (g)

Electrocute a Pickle

pickle electrocution

When electricity is passed through a pickle, it is conducted through Na+ and the Cl- ions that are present in the pickle. The electricity excites the electrons to a higher energy state, and when the fall back to ground state they emit a yellow/orange light.

Colored Flames

manganese strontium sodium copper potassium salts burning in ethanol

When different ionic compounds are heated, electrons around the metal atom become excited and jump energy levels. When the electrons fall back to ground state they release light and heat energy. The amount of energy released determined the wavelength and color of the light released, resulting in different colored flames.