Kitchen Science: Testing the pH of an Unknown Solution

December 10, 2012

The third in our series of science experiments. These are a great way to explore the world around you – and for some schools an experiment for the science fair is mandatory. At a loss for ideas? Here’s another one to test.

We often refer to orange juice, lemon juice, or tomato products as ‘acidic’ but do you (or your child) really know what that means?  If there are acidic items in our kitchen what do we call those items that are NOT acidic?  The acidity of a liquid is measured on a scale of 0 to 14 and is called a solution’s pH.  Items with a low pH number are acidic and those with a high pH number are basic.  Items in the middle of the scale are called neutral.

What does the word pH mean?  pH is a measure of the activity of the hydrogen ions within a solution.  This can get a little complicated but if you want a detailed description of what pH really means you can check out Wikipedia.  Pure water is neutral, with a pH of 7.0.  Vinegar and lemon juice are acidic substances (low pH), while laundry detergents and ammonia are basic (high pH).

It is relatively easy to check the acidity of a solution by using a pH indicator that you can make in your own kitchen from red cabbage!  Red cabbage contains a pigment called flavin that turns different colors based on its pH.  This water-soluble pigment is also found in apple skin, plums, poppies, cornflowers, and grapes.  Very acidic solutions will turn the pigment red (or pink), neutral solutions result in a purplish color and basic solutions appear slightly greenish-yellow.

Once you have the cabbage juice indicator ready it is easy (and lots of fun) to determine which item’s in your kitchen are basic, neutral or acidic!

First, make the cabbage juice indicator:

1. Chop one head of red cabbage into small pieces until you have about 2 cups
2. Place the cabbage in a large glass bowl and add boiling water to cover the cabbage.
3. Allow at least ten minutes for the color to leach out of the cabbage.
4. Alternatively, you can place about 2 cups of cabbage in a blender, cover it with boiling water, and blend it.
5. Filter out the cabbage and you are left with a red-purple-bluish colored liquid. This liquid is at about pH 7 (neutral).

Now check the pH of common household liquids:

1. Add a small amount of cabbage juice indicator into a glass or test tube if you have them (check out Steve Spangler for supplies.  They also offer a premade red cabbage juice indicator if you don’t want to make your own.)
2. Add a small amount of various household liquids and note the color change that occurs.

Take it Further: 

To take this experiment one step further, teach your child the fun of graphing and make a chart of your solutions and their pH.  You can also test to see how much water you need to add BACK to your experiment to neutralize the pH back to pH 7.  Is it possible?

*note:  take care when using household ingredients in science experiments.  If you are using orange juice and milk you are pretty safe but if you choose to use household cleaners like ammonia or other strong solutions please invest in the proper eye protection.  Also, do not mix assorted household products together for this experiment.

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