(POS) Indicators Lesson

Indicators

Indicators are substances that can be used to determine the relative acidity or basicity of a solution, generally through a very distinct color change.  One common type of indicator is litmus paper. If a piece of blue litmus paper turns red when dipped into a solution, it means that the solution is acidic. Like acids, bases can be identified by the use of an indicator too. For example, if red litmus paper is dipped into a basic solution, it will turn blue.

A natural indicator is an indicator that is a naturally occurring substance. For example, the juice from red cabbage can be used to prepare indicator paper. It contains the chemical anthocyanin, which is the active ingredient in the indicator. Red beets, blueberries, and cranberries are other great examples of naturally occurring indicators. These are all due to the same anthocyanin molecule found in red cabbage. The juice of the red cabbage is red in acidic solution, purple in neutral solutions and green to yellow in basic solutions.

A universal indicator makes use of a mixture of indicators so that an entire rainbow of colors is displayed when going from low pH to high pH. A universal indicator is used to make pH paper, which can be used to quickly test solutions for their approximate pH.

The pH Scale

This scientist is collecting and testing samples of water from the river. One of the properties of the water she is testing is acidity. She wants to know how acidic the water is because water that is too acidic can harm the health of water organisms.

To begin thinking about the strength of acids and bases, let's think in terms of some common examples.  Common examples of acids are vinegar and battery acid. The acid in vinegar is weak enough to safely eat on a salad. The acid in a car battery is strong enough to eat through the skin. Examples of bases include those in antacid tablets and drain cleaner. Bases in antacid tablets are weak enough to take for an upset stomach. Bases in drain cleaner are strong enough to cause serious burns.

To measure how acidic or basic a solution is, we use the pH scale. This scale is shown in the picture below. Acids have a pH less than 7.  Pure water, which is neutral (neither acidic nor basic), has a pH of 7.  Bases have a pH greater than 7. The lower the pH, the more acidic. The higher the pH, the more basic.  It's important to realize that the pH scale is based on powers of ten. For example, a solution with a pH of 8 is 10 times more basic than a solution with a pH of 7, and a solution with a pH of 9 is 100 times more basic than a solution with a pH of 7.

Look at the following animation to learn more about measuring the pH level of a solution.

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