Chemistry: Acids and Bases

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...

and bubbles.

I think it brought back memories of the last time we experimented with acid/base indicators. In 2008 we had cups of lemon juice, vinegar, water with baking soda dissolved in it, water with crushed Tums mixed in and milk. We talked about each of these liquids and whether they tasted sour or not.

Then we used our indicator to determine whether the liquid was an acid or a base. They could easily see that all the ingredients that were sour also turned pink, indicating that they were acids.

The liquids that were slippery when you rubbed them between your fingers turned green and were bases.

I encouraged them to experiment with the liquids and indicator, pouring the contents of one cup into another and they discovered that after much experimentation that all of the cups turned a purple, and indicated that they had neutralized and were not either base nor acid.

 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.

 Now that we had enough data, we could look at what made the acids similar, what made the bases similar and what were the differences between the two. They came up with the ideas and I helped them put it in more scientific language. Quentin started first with the tastes, the textures. James added what he had remembered from what we read about pH and covalent compounds. Sam added additional information of precipitates and how acids and bases turn into salts and water when combined. They all talked about how acids contained hydrogen atoms (H), and bases have a oxygen and hydrogen atom combination (OH).

 

 For my oldest student, 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 my oldest student had made and had my special needs student 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.

sources:

Sources:

GEMS: Of Cabbages and Chemistry
Grades 4-8
4 Sessions
88 pages

Students explore acids and bases using the special indicator properties of red cabbage juice. The color-change game Presto Change-O helps students discover the acid-neutral-base continuum. They learn that chemicals can be grouped by behaviors, and relate acids and bases to their own daily experience. An “Acid and Aliens from Outer Space” extension activity can be presented to reinforce student learning or as an assessment. The unit is an excellent lead-in to the GEMS guide Acid Rain.

• 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.