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How do you make a volcano with chemicals?
Most of the volcanoes made for grade-school science fair projects are made using a baking soda and vinegar reaction to create a foam to represent the lava. Dishwashing soap is often added, along with food coloring, to get a red or orange appearance.
Baking soda is a salt made from a strong base and a weak acid. The strong base is sodium hydroxide, and the weak acid is carbon dioxide dissolved in water, the same thing you drink in soda water.
When a stronger acid (such as vinegar) is added to the baking soda, it replaces the weak acid. Since the weak acid is the same fizzy water in sodas, it fizzes in the volcano and makes tiny bubbles. The dishwashing soap helps these bubbles form a foam instead of just popping when they reach the surface.
If you like your volcanoes with real flames and sparks, you can use ammonium dichromate. A small pile of the compound can be lit with a match but should be done with adult supervision, outside in a fire pit, barbecue, or fire-safe location. It then burns with pretty red flames and orange sparks, and the ash expands, producing a typical volcano cone.
The ammonium dichromate is not actually burning. It is undergoing thermal decomposition. The atoms in the molecule are rearranging themselves to be in a lower energy state. If the compound is heated, it breaks down into green chromium oxide ash, water vapor, and nitrogen gas.