I ran across an amazing book by Helen Czerski called Storm in a teacup (link to review of book by The Guardian). Here is her TEDtalk about it. In both she talks about measuring acidity with red cabbage. This was, of course, something I had to try with my older daughter (almost 5 years old).
As chemistry was not among my strong subjects in school, I had to do a little digging around on the internet so I could understand this well enough to explain it to her. My sources can be found at the end of this post. But this opened up the world of chemistry, I think, to both of us. In this post, there is the list of items and guidelines to the test first for the busy parent/teacher and how our test went after.
- Red cabbage
- Baking powder
- Other (preferably edible) substances for testing
The experiment briefly:
- Buy some red cabbage
- Chop some into a kettle
- Add water and boil the cabbage
- When the water turns purple, take off heat and strain the cabbage pieces out. Allow to cool
- Pour the now purple liquid (indicator liquid) into transparent containers
- Add other substances into the indicator liquid of red cabbage and water. See how the colour changes. red equals base/alkali (emäs) while blue/green equals acid. Safest to use edible substances with small children. Vinegar or lemon juice is a good acid. Baking powder is a good base. Water, milk etc. is a good neutral.
- Allow the child to choose some substances and taste the outcome
- Discuss about what they are experiencing
- Have fun. It promotes learning.
How we fared:
I bought us some red cabbage, because as I learned, the colour of red cabbage is due to a substance called anthocyanin which reacts to acidity by changing colors. We were testing for acidity.
Acidity is a basic flavour we can taste for instance in lemon juice. Acidity is also something which can corrode metals. The acidity of the soil affects what can grow in that particular place. For instance rhododendrons like an acid soil. Acids are also used as a preservative (pectin and citric acid). We measure acidity with the Ph-value. There are three categories of substances: acids, bases and neutrals (in Finnish/Suomeksi: hapan, emäs ja neutraali). To explain very briefly: Acids donate hydrogen ions into a solution. They taste sour and the solution has more hydrogen ions than pure water. Substances like lemon and vinegar are some examples of edible acids. Bases on the other hand react to acids and neutralise them. The amount of hydrogen ions in a base solution is lower than in pure water. Some bases are detergents and soaps. Baking powder is an edible base. A good (and brief) history about the topic can be found here courtesy of Chemistry Chronicles and Mark Lesney. I explained a little of this to my daughter at this point. However, it remained too abstract. Luckily we continued.
We cut up some red cabbage and put it in a kettle on the stove, added some water and boiled for a bit. It did not take long before the water was very purple indeed. I then strained the contents of the kettle to get the bits of cabbage out and poured the liquid into a jug for easier management. This liquid I then poured into glass cups for testing as seen in the image above. In chemistry it would be more proper to use exact measurements, but this was kitchen chemistry and kitchen chemistry done in between feeding my baby and putting her down to her nap, so we did not bother with exactness that much. It worked anyway.
Then I decided to test the cabbage just a bit and poured some lemon juice on the cutting board as it was on its way to be washed. The purple colour turned pink, which was very exiting especially to my daughter. Pink is her new favourite colour.
We then tested different items and liquids in the kitchen to see how the colour would change (image below). I found plenty of acids, but could not find a good alkaline/base, so I used detergent. It did not change colour immediately, but did so when my daughter tipped the whole thing on the floor. We then had a beautiful blue/green liquid on the floor. Luckily it was easily both cleaned and test repeated. But edibles are better than detergents. I was nervous that she would taste it or it would spill on her etc.
I, then, did some googling and found that baking powder was a good base. Now, on the internet I found the terms alkaline and base used pretty interchangeably. There is a difference though. All alkaline solutions are bases but not all bases are alkaline as Susan Parrott explains in her text Alkaline vs. Basic. For more information about this, please check that. To continue on our experiment: I then tried them out by myself first and took the following picture:
It worked wonders so we tried it the next day with my daughter as well. This time she was the one pouring and mixing. She liked it a lot more this way. We first tested vinegar and baking powder. Then she could choose what substances she wanted to test and I explained again what we were testing and the background of it (briefly). She wanted to test sugar and cacao. As sugar did not change the colour, we poured the cacao in the same bowl. That of course did change the colour to brown. She then promptly asked if she could eat the end result. She could and found it to taste good. My husband promptly asked her if all following hot chocolates should be made to cabbage water. She actually thought about it for a second or two before saying no. She also tasted the vinegar solution and baking powder solution (just a little bit) and found the vinegar to be sour but the baking powder to be ok. This is why I wanted to use edible substances. That allows us to not be so careful and to use all senses. We then tested black pepper, which did nothing. And we tested another form of baking powder, which caused the liquid to bubble. This allowed me to go a bit deeper into chemistry. I explained to her that the substance reacted with water and as the bubbles stopped I explained that the reaction had run its course.
Our final test was to add the vinegar/red cabbage-solution to the baking powder/red cabbage-solution. That bubbled really well. Luckily our cup was big enough to contain the reaction. It also changed the colour back to purple, which indicated neutral in our experiment with red cabbage.
Now my daughter has seen reactions of a few different kinds. She was engaged, exited and at the end, also in charge (mostly) of the experiment. This is what good learning looks like. Next we’ll try something else and I will report here.
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Copyright text and images: Satu Korhonen. You are free to try these experiments out, use them in your teaching. But instead of copying the text or images, link back to this page.
Helen Czerski 2017. Storm in a teacup. W.W. Norton Company.