Testing a substance to see if it is acidic, basic, or neutral. Add some acid or base to some indicator till color changes. Then reverse color by adding base or acid. If dry ice is available, add some to basic solution gradual changes as CO2 reacts. But they still neutralize acid in your stomach.
Rain is always slightly acidic from the presence of carbon dioxide.
It dissolve limestone slowly over the eons to form limestone caves. Baking powder and Alka seltzer are mixtures of an acid and baking soda.