I built a large volcano model (about 2.5 feet in diameter). To thicken up the lava, I boiled up some 2/3 cup vinegar and 1/3 cup water with cornstarch and red food coloring to get some non-watery lava. I put the "lava" into a 3-gallon insecticide sprayer. (This is the kind of sprayer that you hand pump and then carry around.)
The cone of my model volcano was a plastic champagne glass that you can pick up at most grocery stores. The narrow bottom has the end cut off and makes a good place to clamp on a hose back through the bottom of the volcano back to the sprayer.
The top of my volcano is painted white to simulate snow. I fill the cone of my volcano with baking soda which is also white. Now my volcano doesn't have a cone, it appears as a normal mountain with a nice sharp peak.
After I put my lava mixture into the sprayer, I add some baking soda and screw on the lid. I turn the bug sprayer upside down so that it is pumping only air. The burst of air through the hose comes up through the baking soda in the cone of the volcano. What the students observe is an ash plume and the formation of a cone.
Now, I turn the sprayer right-side up and pump slowly so that lava begins to ooze out of the top. Increase the pressure so that the lava flow increases. Turn the sprayer back upside-down to let air into the line. The air will cause the lava to erupt in a more dramatic manner from the volcano.
Putting model houses (little pieces of wood, etc.) on the mountainside will increase the students interest. Before starting, ask them which houses will be safest.
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