Science Saturday – pH Indicators for Kids

If you’ve ever played with natural food dyes, you might have already used red cabbage to make things the colour blue. Crazy, right?

Red cabbage has an abundance of pigments (which give it the pretty red colour) which are called anthocyanins. They dissolve well in water, and react to the pH balance–the acidity or basicity–of the water itself and anything that might be dissolved in it (the solution). In ordinary boiled water, which has a relatively neutral pH balance of around 7, anthocyanins are blue-violet.

When acidic substances are added, such as lemon juice or vinegar, the pH drops through a range of purple and to red, which would be a pH of 1 (very acidic). When a substance with a pH higher than 7 is added (a base, or alkaline substance), the colour ranges from green through to yellow and almost clear. Items that are base are considered the opposite of acids: they are items like washing soda, chalk and antacid medicines.

Who knew that cabbage could make such a rainbow of pretty colours?

Making the indicator:

•  2 Cups of chopped red cabbage
• Boiling water

Steep the red cabbage in the boiling water for about 10-15 minutes to get your “indicator” material, and pour it through a sieve into a container. It will be approximately blue to violet in colour, depending on the chlorine levels of your tap water. Then pull out some some beakers (if you have them!), clear jars, or white-bottomed bowls.

Time for some fun!

Have your kids perform some experiments. Mom and dad may need to help with this part. Make a solution for any items that are powdered or solid–that means grind it up if necessary (ie antacids), and add a bit of water until it dissolves! Some great (safe) items to test:

• Lemon Juice
• Baking soda
• Vinegar
• Tums
• Cream of Tartar
• Seltzer water
• Salt

Pour off a little red cabbage water into some “test” bowls. Using a syringe or a dropper, have them add the solutions to the test bowl until a colour change occurs. What colour did it become? Does that mean what you tested is an acid or a base? Show your kids how to “neutralize” a pH imbalance by adding a solution from the opposite end of the spectrum. What happens when you mix an acid and a base like lemon juice and baking soda?

Play safe: If you’ve decided to experiment with the stronger pHs of household chemicals like bleach and washing soda, make sure you’re using gloves and protective eyewear in an area where you can open a window. Mom or dad should supervise when using chemical items. Do not mix these more dangerous substances together–neutralization experiments should be done with food-grade items only. Chemicals can produce harmful gases when mixed.