Kids, a baking soda and vinegar volcano is fun but there are better models for how a volcano actually works.  In this activity, wax "lava" forms a volcano in sand, eventually erupting into the atmosphere, which is water in this model. A real volcano forms and erupts because molten rock (magma) and hot gases push up from the Earth's mantle into the crust. This material pushes up through the weakest spot in the crust to be released as an eruption.  In this model, wax in the bottom of the cup is heated and becomes molten.

You’ll need a candle as a source of wax (try red or orange wax for realistic lava), sand, water, and a heat-safe clear glass cup or glass.

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

Make the Volcano

1. Have an adult partner light the candle and drip wax into the bottom of the cup.
2. Cover the wax with a layer of sand. The thickness of the wax and sand layers will affect the way your volcano erupts. If you have a thin sand layer, the wax will readily rise up through it. If you have a thick sand layer, the wax will have a harder time erupting through the sand. Thicker layers will produce a more cone-shaped volcano, but you may need to apply more heat or the "magma" may be unable to erupt.
3. Add water to nearly the top of the cup and let the sand settle.
4. Have an adult partner carefully and gently heat the bottom of the cup. A safe (though slow) method is to set the cup in a shallow pan of water. Heat the pan over low heat on a burner on a stove or hot plate. Don't apply too much heat or raise the temperature too quickly or else the cup may shatter! The water will offer protection for the glass. Heat the cup until your volcano erupts.
5. This project is repeatable. After your volcano erupts, turn off the heat and let the cup cool. Remove the sand and wax from the cup. You can re-attach the wax to the bottom of the cup by melting a few drops of wax into the cup. Stick the other pieces to the melted spot. Add sand and water and try again.
6. The wax pushes its way through the gaps between the said grains, much like magma pushes up through rocks.  You’ll get clouds of wax “ash” in your watery atmosphere.  You can experiment with the amount of wax, quantity and type of sand and intensity of heat to form different types of volcanoes.

Watch a video of the wax volcano project. 

--------------

Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
May 2012

----------------

References: 

Anne Marie Helmenstine at About.com: Chemistry
http://chemistry.about.com/od/chemicalvolcanoes/a/Wax-Volcano-In-A-Cup.htm