Lesson 4 Page 6
Explore: Indicator
Blues
What is an indicator?
An indicator is a substance that changes colours when in contact with an acid and a base.
A surprising thing happens when you place pieces of a red cabbage leaf in water. When you squish the pieces in the water, the water turns purple! If you add small amounts of various liquids, the cabbage-water will turn various colours – pink, green, blue, orange or remain purple.
Red cabbage juice is a natural indicator.
Acids turn cabbage juice pink; bases turn cabbage juice green. If something is neither an acid nor a base, it is neutral. Cabbage juice is purple in a neutral substance.
Digging Deeper
Many indicators can be used to test the acidity of common substances. View this interactive gizmo that has you use a common indicator called pH paper. Scientists use a pH scale to measure how acidic or basic liquids are. The pH scale ranges from 0 - 14. An acid has a pH under 7 (0-6.9 on the pH scale). A neutral substance has a pH of 7. A base has a pH over 7 (7.1-10).
If you were to place cabbage juice indicator colours on a colour chart, the chart would look like this.
Some alternate indicators are:
beets, blueberries, cherries, curry powder, grape juice, onion, peach skin, pear skin, plum skin, raddish skin, red apple skin, tomato, and turnip skin.
In this activity, you will find what these colours tell you about the substance being tested.
Video
Click here to watch the BrainPop video Acids and Bases. It explains acids and bases in further detail.
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Follow this link and use the following login information: Username: 0099 Password: students
How are acids and bases different?
- red cabbage leaf
- water
- measuring spoons
- plastic bag with zipper closing
- 5 small cups
- vinegar
- laundry detergent powder
- baking soda
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lemon juice
- ammonia
- masking tape
- ball point pen
Notebook
Video
Watch the ADLC Digital Lesson on Acids and Bases. It will provide more instruction for this experiment.
Body Connection
When carbon dioxide is dissolved in water, it forms an acid called carbonic acid. This is why soda pop that has gone “flat” (no longer fizzy) tastes so sweet. There is no longer a (sour) carbonic acid in it to counteract the sweetness of the sugar.
Take a straw and try blowing bubbles in some cabbage juice indicator. You should notice it change colour to pink to indicate that the carbon dioxide you are breathing out is producing carbonic acid in the water.