Chemical Volcano (Demonstration)
(Reference: Kelterís Favorite Demonstrations)

Acid Base Chemistry   :   Indicators

Illustrates an acid/base reaction, and how indicators work

Safety goggles, gloves and a chemical resistant apron should be worn.  I also recommend doing this in the hood since you are using concentrated HCl and it does spatter

Large tub to set beaker in
200 mL water
1 600 mL beaker
50 mL concentrated HCl
Bromocresol purple indicator (.5g in 50 mL ethanol)
100 g sodium bicarbonate
Dissolve about 10 g of NaHCO3 in 200 mL of the water in the beaker.  Place in center of tub, in the hood.  Add about 20 drops of the indicator (it will be purple in water or orange in the ethanol).  This solution will turn purple.  Add 50 mL concentrated HCl directly to the purple solution and an orange volcano will form.  The solution will overflow.  Now sprinkle the sodium bicarbonate on the acidic solution, allow some to react and change colors back to purple.

In basic solution like the sodium bicarbonate and water the indicator will appear purple.  By adding the concentrated acid the indicator turns orange.  The volcano forms due to the release of CO2 gas.  By sprinkling more sodium bicarbonate on the reacting volcano you will get localized acid base reactions which will again cause color changes.

  HCO3-  +  H+   H2CO3   H2O  +  CO2

Add enough baking soda so solution turns to purple again, and when the reaction is complete rinse down the drain with lots of water.  Be very careful to only handle the tub if you are wearing gloves- there could be some non-neutralized acid on the tub from the spattering.

Ginger Chateauneuf, 2000.