CAUTION: be sure to stop heating the pot before it is out of water. Heating an empty pot is a fire hazard.
Observe what happens to the cabbage juice when it is poured onto the lemon juice and the baking soda.
In chemistry, many substances change color due to reactions with other substances. Cabbage juice is an example. When cabbage juice is placed onto an acid, such as lemon juice, it will react differently when compared to cabbage juice and a base, such as baking soda. Why cabbage juice changes color remains uncertain. However, with this indicator, you can now test all sorts of things to determine if they are acids or bases.
Even though the nature of the color change is uncertain, we do know that it probably has to do with the composition of acids and bases. All acids have hydrogen ions, H+, while all bases have hydroxyl ions, OH- (notice that the combination of H+ and OH- yields water, which is neutral in acidity). It is most likely the cabbage juice's reaction with these ions that stimulates the color change.
The presence of the hydrogen ion is also the basis of the pH scale. This scale is used to measure the acidity of a substance. The scale goes from 1 to 14 where 7 is neutral (water), anything less than 7 is acidic and anything greater than 7 is basic.