3.06 pH Scale

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Indicators

To know if a substance is acidic or basic, we use an indicator. An indicator is an organic compound that changes color in the presence of acids and bases. Litmus paper is a commonly used indicator.

• Blue litmus paper turns red in the presence of an acid.
• Red litmus paper turns blue in the presence of a base.

In the image above, you can see blue litmus paper and red litmus paper. Each strip has tested an acid and a base. Keep reading to understanding the color changes.

1. When the red litmus paper was dipped in an acid, it stayed red.


2. When the blue litmus paper was dipped in an acid, the blue paper turned red.


3. When the blue litmus paper was dipped in a base, it stayed blue.


4. When the red litmus paper was dipped in a base, the red paper turned blue.

Another way to measure the pH of a substance is to use a pH meter. A pH meter provides a more accurate reading, compared to litmus paper.