Chemical Reactions

One of the most fun parts of chemistry is looking at chemical reactions. In 2008, I gave the boys four substances: lemon juice, vinegar, baking soda mixed with water and milk. Their task was to systematically mix two of the ingredients and record the reactions.  

They learned about precipitates (solids forming from a chemical reaction as two liquids mix), color changes and bubbles. They were particularly surprised when their cups bubbled over!

Now that they are a little older, I decided to try this experiment again. I picked six items for my students to test: baking soda, milk, vinegar, iodine solution, corn starch and red cabbage juice. I made the red cabbage juice by taking a red cabbage and after slicing it up into quarters and simmer it in a quart or two of water. Distilled water is best as the minerals in tap water can change the quality of the cabbage juice. You will need only a small amount for this experiment, but I knew I was going to do a series of acid/base experiments soon, so I made up a large batch of it. Once the cabbage has simmered for about 5 minutes, the juice can be drained off and put in the refrigerator until needed. The cabbage can be eaten.

Other items you could use include calcium chloride (which is salt you put on your steps to melt the ice), lemon juice, water, or egg whites. If you try other samples, be sure you know what the reactions will be so that you do not accidentally create a hazardous condition. For example, ammonia and bleach give off a noxious gas when they react together. I put my samples in disposable clear plastic cups and put a plastic spoon in each cup.

Since I still have some beginning readers and I did not want them to struggle with that aspect of the experiment, I color coded the cups of samples and I made up a sheet that made all the combinations possible. Now I was ready for my students.

They could look closely at each sample and they could smell them by wafting. In wafting a person takes an open hand with the palm towards the body and moves their arm in a rapid circular manner over the substance so as to lift vapors of the substance towards the nose. This method allows for a lower concentration of vapors to be inhaled and is particularly useful in safely smelling unpleasant or dangerous chemicals. Even though we were not using anything that was dangerous to smell, it is a good habit to get into for chemistry experiments. For this reason, I did not let my students taste any of the samples either, even though many of them could have tasted. After they examined the samples, we began the testing. I chose not to make a big deal about them spilling some of the samples on the table this time, but this is a good habit to teach them early, so I will work with them on how to handle materials without spilling them with future experiments. Part of the problem this time was that in the beginning we used hard plastic test tubes, washing them between uses, and these have very small openings. Later we used plastic disposable cups and we found they worked better than the test tubes because it had a larger opening and also they could see the reactions better. As each combination was tested, they noted how the chemicals had changed from their previous condition. We agreed on the result, and I noted it down. Through this experimenting they experienced most of the signs that chemical has reacted. They were able to see bubbles, color changes and precipitations. They did not experience the other indicator that a chemical reaction has taken place, which is a solution giving off or taking in heat. We will be doing experiments that have that component later.

Bubbles form when one of the molecules produced is a gas. Color changes occur, among other times, when an acid/base reaction occurs. Precipitations occur when one or more products of a reaction are no longer soluble in the solution.

These reactions are caused by the different ways the molecules react with each other. In combination reactions, two or more molecules combine to form a single product. We saw this in our previous experiment, when hydrogen and oxygen combined to form water. In a decomposition reaction, molecules break down to form other molecules. We saw this in the previous experiment when we broke water down into hydrogen and oxygen. A displacement reaction is when bonds are breaking and reforming all at once. In an exchange reaction, the atoms change places. These last two reactions are more difficult to show at home because they often cause volatile reactions.

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.