Properties of Acids and Bases

Observable Properties

One purpose of this lesson is for you to become familiar with many aspects of acids and bases. Let's start with some of the observable properties of acids and bases. Take a look at exercise 2 in your workbook. In this exercise you will test the reaction of an acid and a base with the various chemicals listed here. For the acid use 1 molar hydrochloric acid (1M HCl). For the base use 1 molar sodium hydroxide (1M NaOH).

The video clip to the right shows the reaction of an acid with blue litmus paper (left) and then with pH paper (right). To start the video clip move your mouse over the image. (You may need to click.)

The video clip to the right shows the reaction of a base with red litmus paper (left), then with phenolphthalein (center), and then with pH paper (right). To start the video clip move your mouse over the image. (You may need to click.)

Base with litmus paper, phenolphthalein, pH test paper.

When you are in the lab you will perform these tests and the others in the exercise, then record the results in the blank spaces on the chart.

When you have completed exercise 2, you will have experimented with several properties of acids and bases. Those properties give you a variety of ways that can be used to test whether a material is an acid or a base (which comes up in exercise 4). Let me take a minute to touch on a few of them.

	Your tests with red and blue litmus paper show that litmus is red in the presence of an acid and blue with a base.
	The hydrion papers or pH papers that you used, are a mixture of different indicators that change to different colors and different combinations of colors, depending on the acidity of the solution. Acids have low pH (below 7) and bases have high pH (above 7). We will get into what that means later.
	Phenolphthalein is colorless with an acid and pink with a base.
	You should have noticed that the base felt slippery. (One of the uses of bases such as sodium hydroxide is to make soap from oil or fat. The slippery feel is from some of the oil on your fingers reacting with the base to make soap.)
	Another thing that can be used to identify acids and bases is that acids have a tart taste. I didn't include that on the list of ways to check but you might want to write that in also; "acids have a tart taste." I don't recommend that you go around tasting things just to find out whether they are acidic or basic. However, there are common examples of the tart taste of acids. Vinegar is an acid. The tartness of carbonated beverages, soft drinks, is from an acid (carbonic acid) that we'll be talking about later on.

Neutralization

Another important characteristic of acids and bases is that they can neutralize one another. To point that out I'd like you to try a short little test when you are in the lab. The instructions are in exercise 3 in your workbook. Put a few drops of 1 M sodium hydroxide solution into a test tube and add a drop of phenolphthalein solution. It will turn pink to indicate that the solution is basic. Then start adding, drop by drop some 1 M hydrochloric acid to that solution, and you will find that after the proper number of drops the acid will have neutralized the base and converted the phenolphthalein back into a colorless solution. These chemicals are available in the lab and I suggest that you try this short experiment yourself. (You may remember doing this in lesson 1 of CH-104.)

Identifying Acids and Bases

The reactions observed in exercise 2 will help you to experimentally identify acids and bases.

When you are in the lab, get three lettered solutions and identify each one as being either an acid, a base, or neutral, using 2 different tests for each unknown from the tests you performed in exercise 2. Record your observations and conclusions in the space provided in exercise 4.

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