If you take half a head of purple cabbage and simmer it a few minutes in enough water to fully cover it, you can get a two-liter bottle of acid/base indicator liquid. You can use this liquid directly in clear cups or you can soak absorbent paper in it and then let it dry to make test strips which can be dipped in test liquids. The absorbent paper can be paper towels or coffee filters.Because this provides so much indicator liquid, you can feel free to do lots of things with it without running out.I like to begin with letting my younger boys experiment on their own with it. I give them a container of water with baking soda mixed in, a small bottle of vinegar and a container of cabbage indicator. I tell them that they can do whatever experimenting they want with it except they cannot eat any of these materials. I know they are all non-toxic, but they are not particular good just to drink and it sets a good precedent for the chemicals they will use later on in chemistry.
|And so they mixed and poured...|
|and exclaimed over the color changes...|
|It captivated their attention for quite some time.|
The liquids that were slippery when you rubbed them between your fingers turned green and were bases.
This year we reviewed what we had learned about acids and bases and how the indicator worked by having them mix the indicator with a known acid (white vinegar) and a known base (ammonia) and a known neutral liquid (milk) so that they could have color coding to refer to as they tested other unknown liquids.They were eager to find out what colors meant what about the pH of the liquids because they had already experimented until they were satisfied. They were ready to find out what it all meant now.
So we tested some unknown liquids and put them in the correct categories.
For Sam, we explored determining the endpoint of a neutralization reaction. We put a 1/2 cup of vinegar in a jar and then added 3 Tablespoons of red cabbage indicator. We noted the color, and all the color changes, from pink to pinkish-purple, to a purple and then to a bluish-purple and then eventually a blue-green.
We added ammonia a tablespoon at a time, and noted the color. It is hard to see the color perhaps in this photo, but it has turned a blue-green.If you note the color each time, you can chart these on a graph and see that it stays pink for a long time and then suddenly there is a color change and that is the point in which they neutralize and then you see another leap as it goes to the blue-green of base.
We also performed a titration using a polyprotic acid. A polyprotic acid has more than one point of neutralization. We used clear soda (Sierra Mist). We added the red cabbage indicator and recorded the color.
We slowly added tablespoons of baking soda water and recorded the colors as it changed.If you look at the graph of the titration, you can see two "bumps" where there are leaps in the color change.
Alex could not understand how to graph the colors as this is a bit of an abstract concept for a person with autism.
I took the graph that Sam had made an had Alex pick out a Unifix cube of the right color from the graph.
I kept tying the graph and his Unifix cubes to what he saw when we performed the experiment. I lined up the control acid, base and neutral solutions with his Unifix cubes and the graph.
We talked about pH and neutralization and I made up this worksheet while we talked. He then filled in the colors, using his Unifix cubes as a guide. This could also be done with a younger child to help them understand the color changes and pH in a more concrete way.
- Exploring Creation with Chemistry, Jay Wile
- Real Science 4 Kids, Chemistry Pre-Level 1, Level I, Level II
- Se7en shows how she used red cabbage indicator to make clear liquids turn color magically, art projects by painting with it and make pretty drinks with it.