Red Cabbage pH Indicator Activity

Here is a wonderful activity for all ages that can be used as an introduction to acids and bases.

Acidity and basicity are a property that many chemicals possess. We use them to refine oil, bleach textiles, in our industry, in our food, in our house, and in our bodies. The way the mitochondria (the powerhouse of the cell) produces energy is by utilizing an acidic gradient across a membrane. The effects of acid mine drainage and acid rain on the environment can be devastating.

Acidity and basicity can be measured by chemicals called “pH indicators”, which measure how acidic or basic a chemical is on a scale called the pH scale (pH stands for power of hydrogen). The scale goes from 0-14, and substances are acidic if they have a pH less than 7 and basic if they have a pH greater than 7. Acidic substances release hydrogen into solution whereas basic substances absorb it. The scale also operates like the richter scale used to measure earthquakes – an acid of pH 3 releases 10 times as much hydrogen into solution as an acid of pH 4, and a base of pH 13 absorbs 100 times as much hydrogen as a base of pH 11.

184phdiagram

To create the red cabbage indicator, simply buy a red cabbage and boil it in a couple litres of water for approximately 15-30 minutes. The longer you boil it, the stronger your indicator will be. I recommend creating 2 litres of concentrated cabbage juice and diluting it by a factor of 10 when using it in class. This is not necessary but will make your juice last much longer.

Collect a bunch of household chemicals, or better yet, ask your students for suggestions on what to collect for the experiment. Not every chemical will induce a color change but the majority of soaps and cleaning supplies will. Allow your students to make predictions about what will happen based on what they know about the household chemicals. Be sure to pre-load information about the traits of acids and bases (acids taste sour, corrode metal, bases taste bitter, feel slippery, etc). This activity can be done at any grade level, and you can even use a crude color scale to identify the pH of your materials.

averillfwk-fig16_013
Picture from: http://www.discoveryexpresskids.com/blog/test-ph-levels-with-red-cabbage

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