Lesson 5: Properties of Acids & 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). You'll also run the tests on a neutral solution, deionized water.
When you are in the lab you will perform a series of tests, 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. You'll be using these tests in exercise 3 when you try to identify some unknowns as "acid,"base," or "neutral" solutions. Let me take a minute to touch on a few of them. Pay attention to what conclusions can be drawn from the test results - a negative result may not help you to positively identify an unknown as an acid or a base, but it should be able to narrow it down.
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Your tests with red and blue litmus paper show that litmus is red in the presence of an acid and blue with a base. If you have red litmus and it doesn't turn blue, you know that you don't have a base... but it could be neutral or an acid. |
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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. |
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Phenolphthalein is colorless with an acid or neutral and pink with a base. Again, if it doesn't turn pink with an unknown solution, you know that it isn't a base but you don't know whether it is an acid or neutral. You would need to do another test to identify it conclusively. |
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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.) |
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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. Bases are bitter; if you've ever gotten soap in your mouth then you know what that bitterness tastes like. |
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 4 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, by using 2 different tests for each unknown from the tests you performed in exercise 2. You should use 6 different tests in total - 2 for each unknown. For example, you might choose to use pH paper for one of your tests on your first unknown; you won't be able to use the pH paper for a second unknown, but you could use the litmus paper or any of the other tests.
Remember that if you get a "negative" result for one of your tests then you should pick a second test that will be conclusive. For example, if the result of my first test was that phenolphthalein stayed colorless (showing that this unknown is acid or neutral), I would want to pick a second test that could confirm the presence of an acid (like the carbonate test). A negative for both tests would then show that this unknown was neutral; a positive for the second test would confirm that this unknown was an acid. If I picked a second test where a negative just indicated that the unknown wasn't a base, I wouldn't have learned anything new from the second test and still wouldn't know whether I had an acid or a neutral solution. Record your observations and conclusions in the space provided in exercise 3.
Lab Report: You'll be typing up an individual lab report this week over exercises 2 and 3. The usual format will be used: Purpose, Data/Observations, Results. You don't have any calculations but you have some tables of observations about the tests you performed (on the known and the unknown solutions) and in your Results you should state the identities (acid, base, or neutral) of your three unknowns. There aren't any additional questions to answer for this lab report.