In this activity liquid colors change unexpectedly! It is easily modified by age – young children enjoy watching the colors change, while older children can make predictions. Parents can enjoy being a magician!
Child-friendly acids – Lemon juice, vinegars, lemon-lime soda, milk, buttermilk, crushed aspirin, pool water
Child-friendly bases – Baking soda, Liquid soap, egg whites
Chop cabbage finely. Cover with boiling water and let sit for 30 minutes. Collect and store the purple water in an airtight vessel. Compost remaining cabbage. Place water and acids and bases (mix powders into a bit of water) into glasses. Add a few drops (or as much as a teaspoon) of cabbage juice to each glass and watch colors change. Water will remain purple, acids will turn pinks to orange, and bases will turn blue to green. Give your child a confidence boost with this addition: Set aside a glass of water, a glass of clear (white) vinegar, and a glass with clear soap (mixed with some water, such as All Free & Clear). After changing the known liquids to their colors, let your child help you “remember” which one is safe to drink. This teaches children that science can be applied to real-world problems.
What’s Going On
Indicators are tools that tell us if something is in a liquid. The purple cabbage can tell us what kind of “chemicals” are in a liquid. For younger children, explain that pH is a way of measuring whether liquids are “clean” or whether they have some sort of chemical in them. Liquid chemicals come in two basic types – acids, which include sour things like lemons and vinegar, and bases, which are slimy like soap. Water is right in the middle.
For older children, explain that liquids vary based on their pH, which is a measure of how the molecules in the”chemical” break down when in water – into Hydrogen (H+) or Hydroxide (OH-) Ions. Acids produce H+ ions and bases produce OH- ions when in water. pH measures the ion concentration in the liquid; it is on a non-linear scale that goes from 1 to 14. Pure water is neutral (neither or both an acid or a base) and has a pH of 7. Bases are between 7 and 14. Acids are between 1 and 7. Red cabbage has a pigment (anthocyanin) that has a chemical reaction with acids and bases. The chemical reaction changes the color of the solution. Chemical reactions occur when the molecules in two liquids come into contact with each other and break apart to form something new.