Experiment
Red cabbage - acid and base
Check pH values in this simple experiment! Can you tell if something is acidic or basic?
Curriculum objectives - Science:
- Wonder, ask questions and create hypotheses and explore these to find answers
- Use tables and figures to organize data, create explanations based on data and present findings.
- Explore and describe how some substances can change when mixed with other substances.
You need:
- Knife and blade
- Pot
- Glass or bottle
- A peel of red cabbage
- Water
This is what you do:
Cut a skin of red cabbage into small pieces. Place it in a saucepan and fill with water so that it barely covers the cabbage pieces. Let it simmer for 5 minutes. Cool and pour the water into a glass or bottle.
The color should be dark bluish purple. Now you can try dripping it on various foods and detergents in your kitchen.
The juice turns red when you mix it with an acid and green or yellow when you add a strong base!
Explore and observe:
- Nice variety of food around the kitchen. Make a hypothesis about what you think will happen when you mix foods with red cabbage juice.
- Do all foods change the indicator to the same color? What do you think this means?
- Does adding a larger amount of acid or base change the color more?
- Do you see any patterns in what the acids or bases are used for?
Record your observations.
What is going on?
Some substances are classified as either an acid or a base. Think of acids and bases as opposites - acids have a low pH and bases have a high pH. For reference, water (a neutral) has a pH of 7 on a scale of 0–14. Scientists can tell whether a substance is an acid or a base using an indicator. An indicator is usually a chemical that changes color if it comes into contact with an acid or a base.
As you can see, red cabbage juice turns purple when mixed with something acidic like lemon juice and turns green when mixed with something basic like baking powder. Red cabbage juice is considered to be an indicator because it shows us something about the chemical composition of other substances.
What is it about red cabbage that makes this happen? Red cabbage contains a pigment called anthocyanin that changes color when mixed with an acid or a base. The pigment turns red in acidic environments with a pH less than 7, and the pigment turns blue-green in alkaline (basic) environments with a pH greater than 7.
Red cabbage is just one of many indicators available to researchers. Some indicators start out colorless and turn blue or pink, for example, when mixed with a base. If there is no color change at all, the substance you are testing is probably neutral, just like water.
