- PK – grade 3 students: How does a lemon taste? It taste sour. Lemons have a chemical called vitamin C which is an acid. This acid is the reason lemons are sour. Acids are sour. If something has an acid then something called its pH will be low. The pH will be higher if very little acid is present. Red cabbage juice can be used to see how much acid is in something. If there is acid the red cabbage juice will turn red. A base, like soap can make there be less acid. When something has lots of base in it, it turns the red cabbage juice blue or green. Some things do not have any base or any acid. These things are neutral and turn the red cabbage juice purple.
- Grade 4 – 6 students: A pH indicator is something that changes color when mixed with a substance depending on that substance’s acidity. Red Cabbage can become a pH indicator when boiled in water. If you cook red cabbage in hot water you end up with a purple liquid showing that the water is neutral in its acidity. When you add lemon juice, which is an acid, to the indicator solution it turns slightly red and gets redder if you add more. An acid makes the pH low. If you then mix in a base like baking soda, the liquid turns blue and eventually green when you add even more baking soda. A base makes the pH high.
- Middle school students: A pH indicator is something that changes color when mixed with an acid—like citric acid or vinegar–or a base—like baking soda or soap. When an acid is mixed in water it forms a solution of hydrogen ions (H+); however, when bases are mixed into water they form a solution of hydroxide ions (OH–). Red Cabbage is one of many kinds of pH indicators. If you cook red cabbage in hot water you end up with a purple liquid showing that the water is neutral. This mix is your pH indicator. When you add an acid to the indicator solution it lowers the pH and the indicator turns slightly red and gets redder if you add more acid because of the high concentration of hydrogen ions. If you then mix in a base like baking soda the pH goes up and the liquid turns blue and eventually green when you add even more baking soda because of the high concentration of hydroxide ions. Adding baking soda to an acidic solution also results in bubbles of carbon dioxide gas that result from a chemical reaction between hydrogen ions and bicarbonate ions from the baking soda.
- College-level/ scientific description: The measure of the hydronium ion (H3O+) concentration in a solution is its pH. Actually the pH is the negative log of the hydronium ion concentration. Strong acids dissociate to form high concentrations of hydronium ion and therefore the pH will be low. Bases dissolve in water to form hydroxide (OH–) ions. A high concentration of hydroxide ions lowers the concentration of hydronium ions and therefore results in a higher pH. A pH of 8 or higher indicates the presence of a base. A pH of 7 is neutral with equal concentrations of hydroxide and hydronium ions. One way to quickly tell if something is an acid or a base is to use a pH indicator like phenolphthalein. A pH indicator can be made at home using red cabbage boiled in water. It will be red in the presence of acids, purple in a neutral solution, and blue/green in a basic solution
For instructions on how to make the red cabbage pH indicator click here. For more information on acids and bases click here.
Here is a video with more information on acids and bases and red cabbage pH indicator.