Yesterday, we had an urge to do some experimentation and I had a red cabbage that had overstayed its welcome in the refrigerator crisper drawer.
So of course, we made cabbage-water indicator.
An indicator is a substance that produces a color change that gives you information about whether the stuff you're testing with it is acidic, basic, or neutral. Some indicators have just two states. For example, phenolphthalein, the indicator beloved by chemistry students (in part because of the fun of spelling it), is frequently used to find the equivalence point in acid-base titrations. In acidic and neutral solution, phenolphthalein is colorless. When you are one drop of base past a neutral solution (in which acid and base are perfectly balanced), phenolphthalein gives a pale pink color. The more basic the solution, the pinker the phenolphthalein will make it.
Other indicators -- including the one we made -- give a range of colors.
How to make the indicator:
Shred some red cabbage. (We used 3/4 of a very small head. If you don't have a sharp knife, tearing up the cabbage leaves will work, too.)
Put the cabbage pieces in a clean container. (We used a Pyrex 4-cup measure, but you can use a bowl or even a plastic zipper bag.)
Cover the cabbage with very hot water. Let it steep until that water has cooled (to somewhere between lukewarm and room temperature).
The purple liquid you've made is your indicator. Pour it into a container and compost the cabbage. (If some flecks of cabbage remain in your cabbage water, it's not a big deal.)
Next, cast your eyes around the kitchen for substances that might be acids or bases.
Liquids are good, as are fruits that can be made to give up juice without too much urging.
Solids that you have around for baking are also good. (Think salt, baking soda, sugar, cream of tartar, ...)
We stuck to edibles in our experiment. You could check out non-edibles that you have around for cleaning, but some of them may introduce more danger (e.g., from caustic ingredients) than you want to deal with.
There is, after all, something appealing about being able to do chemistry without having to locate safety goggles and nitrile gloves.
Besides your cabbage-water indicator and the substances you will be testing with that indicator, you will also need small containers for mixing. We used tea cups because they were small, shallow, and white inside -- which makes it easier to see and compare color changes. If you use clear glass containers (or test tubes, if you have them), you will probably want a piece of white paper to put under (or behind) them. Otherwise, you might lose the color-changes in your table top.
We found it handy to keep a teacup of cabbage-water indicator as a reference for color comparisons.
So, we poured more indicator into the rest of the tea cups and started testing our kitchen reagents. (Actually, we had to stop to wash out tea cups part way through because we had more things we wanted to test than we had tea cups. Thanks goodness for digital cameras!)
Here's what we found:
Fresh lemon juice turns the cabbage-water indicator a lovely pink.
This was the first substance we tested, and we were actually surprised at how dramatic the color change was.
Fresh orange juice turns the cabbage-water indicator an orangish-pink. (Orange juice is itself orange.) There's definitely a color change from purple to pink, but the pink is not as dramatic as what we got from the lemon juice.
Next time, we might try using full-strength lemon juice and comparing it to diluted lemon juice in terms of the color when added to the indicator. (For that experiment, we'd actually want to measure the amounts of cabbage water and lemon juice being used. After all, the water in the indicator solution also dilutes the lemon juice.)
Rice wine vinegar turned the indicator pink. It also made us think of sushi.
Apple cider vinegar also turned the indicator pink.
Potentially, you could test as many different kinds of vinegar as you have in your pantry, but after a couple vinegars both give the same color, you have a pretty good hunch that other vinegars will do the same thing. (Some of your other vinegar options -- like balsamic vinegar -- may be dark enough to make the color change hard to notice if you add significant volumes of the vinegar to your indicator. Luckily, you only need to add a wee glug of vinegar to see something happen.)
If you hadn't drawn the conclusion before you get to the vinegar, the vinegar gives you good reason to infer that cabbage-water indicator turns pink in the presence of an acid. (The vinegar bottle labels usually say something about the vinegar being diluted to achieve a particular % acidity.)
Having worked out what to expect from indicator + acid, we added a known base to the cabbage-water.
A bit of baking soda added to the indicator gave a distinct blue color. Next time, we might try adding different amounts of baking soda to identical volumes of cabbage-water to see if the results display a range of blues.
Seeing an unguarded mug of tea on the counter, we added a glug of that to the indicator. It got darker, but was still purplish rather than pinkish or bluish. We concluded that the tea must be neutral.
Tonic water turned the indicator pink. Tonic water must be acidic. (We think this is most likely due to the carbonation. However, we didn't test any seltzer water to follow up on this hunch because we didn't want to open a fresh bottle just for this experiment. Tight funding is a problem all the way around!)
We added a bit of milk (nonfat) to some indicator. The result was a bit more opaque and milky-looking, but still purple. We concluded that milk is neutral.
Then, we got a little fancy and separated an egg.
We poured the eggwhite into a tea cup of cabbage-water indicator. The awesome thing about the eggwhite is that it doesn't immediately go into solution with the indicator like the other substances we tested.
In other words, the eggwhite and indicator don't give a visible reaction until you stir them together.
After some stirring, the indicator with eggwhite turned blue. We concluded that eggwhite is basic.
This is a very easy experiment to set up, and the color changes are surprisingly vivid. I'm now inclined to keep red cabbage on hand at all times. (And, we still have about 12 oz. of cabbage-water in a jar in the fridge. Obviously, we're going to have to test it in a day or two to see if the indicator quality persists or degrades.)