Lesson 2

Chemical Reactions Creating Materials with New Colors

Fourth Example

The next example is to put a few drops of sodium hydroxide solution into a test tube and add a drop of phenolphthalein solution to it. (Exercise 3.a.) Notice that we get a change in color. We start off with two things that are colorless, and we end up with a material that has a pink color to it. So there is a change. You get a new material that has different properties than the materials that you started with.

Fifth Example

Next, we take that same solution that we just created and start adding, drop by drop, some hydrochloric acid solution to it. (Exercise 3.b.) Notice that after enough hydrochloric acid is added, the pink color disappears. Again, there was a chemical reaction. We started with something that was pink and something that was colorless, and we ended up with something that was colorless. Whatever it was that was pink has disappeared. It is no longer present. It must have been changed to something else.

Sixth Example

You cannot always count on a change in perceived color as being a chemical reaction. (Exercise 3.c.) These two dye solutions are just food colors. If we mix together a little bit of the yellow and a little bit of the blue in a test tube we end up with green. We started with yellow and blue, not green. We put them together and get green. So in a sense, there is a change. However, this change is not from a chemical reaction. There is not a new material that has green as one of its properties. Instead we have the mixing of the yellow and the blue colors.

Comments

Sometimes a color change is the result of a chemical reaction. Sometimes it is just the result of mixing colors. One way of telling which is the case is your own personal experience. If you are familiar enough with mixing colors, you know that yellow and blue together make green. Therefore, when you mix the two together and you get green, it is just mixing the colors. On the other hand, if you were to mix yellow and blue and get red, you would know you are not supposed to get that color from mixing; so therefore, you must have created something new. Therefore, that would be a chemical reaction.

Another way of investigating things like this is to use a spectroscope to look at what colors are present. When you mix two colors together, you get a new color. Now look at the last column of this data sheet, "Colors in the Spectrum." Each solution has a visible color because it absorbs some colors and lets others pass through. The mixture of solutions 1 and 2 absorbs the colors that either solution 1 or 2 will absorb, and lets through only the colors that both solutions 1 and 2 will let through. The green and some of the red in this case. Thus, we get a different color (or color mix) without having a new and different chemical present.

Now or when you are in the lab, do these exercises so that you can see the reactions first hand and, of course, clean up your glassware.