Testing for an Acid

Can we make an indicator to see if a substance is an acid?

Materials
Red cabbage Knife Cutting board Stove Metal pot Water Stirrer Spoon Lemon juice Baking soda

Procedure

1. Using the knife and the cutting board, cut a portion of the red cabbage into small pieces.
2. Place the small pieces of cabbage into the pot.
3. Pour enough water into the pot to just cover the pieces of cabbage.
4. Heat the pot until the water just begins to boil. Stir the cabbage while it is being heated.
5. Allow the pot and its contents to cool for thirty minutes.
6. Remove the pieces of cabbage from the pot. You have now made your acid indicator.
7. Using the spoon, pour some of the cabbage juice left in the pot onto the lemon juice and the baking soda.

CAUTION: be sure to stop heating the pot before it is out of water. Heating an empty pot is a fire hazard.

What to observe

Observe what happens to the cabbage juice when it is poured onto the lemon juice and the baking soda.

Discussion

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.

Advanced Discussion

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.