Properties of Acids and Bases

Before we dive into a bunch of formulas, equations, and concepts around acids and bases, it will be useful for you to get a grounding in how they behave at the human scale. Acids and bases are a classic chemistry topic, in large part because they are extremely relevant to humans, both in their biochemistry and day-to-day lives. The acid and base properties of substances like vinegar, baking soda/baking powder, ammonia, lye, and bleach are encountered in everyday life, and while you may not be aware of them, your cells rely on a host of reactions to maintain a very tightly controlled environment with respect to acid/base properties. Now, let's dig into some of the physical and chemical properties of acids and bases.

This section is somewhat unique, in that much of the information you learn in this section will be gained in your lab work itself. This text will lay out the kinds of things that can be observed about acids and bases, but it will be up to you to actually make those observations in lab. The goal is for you to learn the properties of acids and bases in a real-world, tactile way.

Indicators

If you don't know whether a particular substance is an acid or base, an indicator is a great place to start. Indicators are substances that can change their color in response to the presence of an acid or base. Indicators commonly come in two forms.

The first is a solution you can mix in with a substance. Examples include bromothymol blue, methyl red, and many many others. In this lab, you will use the indicator solution phenolphthalein, which you will add a drop or two of to an acidic, a basic, and a neutral solution. In the lab, note how phenolphthalein responds in each of these three cases.

The second form indicators often take is that of a paper strip, or sometimes a solid sample adhered to a plastic backing. These strips are easier to handle than solutions, and can allow you to test a sample for acid/base properties without contaminating it with indicator. Universal pH strips are an example of a strip-type indicator you will use in this lab, as are red and blue litmus paper strips.

Visible Reactions

Many of the reactions that acids and bases undergo are invisible, so you shouldn't always assume that just because you can't see something happening in the lab, there's no reaction going on. However, there are some reactions of acids and bases that cause visible changes, which can be used as a way of identifying if a solution is acidic, basic, or neutral.

For instance, reactions with metals can sometimes be observed. In the lab for this lesson, you will take the metal magnesium and mix it with acidic, basic, and neutral solutions and look for visible signs of a reaction in each case. While not all metals undergo reactions with acids and bases, many of them do, especially when concentrations are high.

In addition, reactions with carbonates and bicarbonates also often produce visible signs. As you should remember, the carbonate ion has the formula CO₃^-2 and appears in compounds such as Na₂CO₃, K₂CO₃, and CaCO₃. The bicarbonate ion is closely related, having the formula HCO₃^-, and appears in compounds such as NaHCO₃, commonly known as baking soda. It is this last compound that you will use to test an acidic, basic, and neutral solution in this lab.

Physical Sensations

You should be very cautious about ever smelling or directly touching anything you encounter in a chemistry lab, and you should never under any circumstances taste anything in the chemistry lab. However, the sensations of touch and smell can sometimes be useful in learning about compounds, including acids and bases. In the limited context of this lab, you should touch and smell only the solutions you are directed to.

You will directly observe how acids, bases, and neutral solutions feel on your skin. This type of test should only ever be performed with dilute solutions, and at the explicit direction of your instructor.

You will also treat acidic, basic, and neutral solutions with the compound ammonium chloride and smell what happens. In this case, you are not smelling the acid/base directly, but rather smelling a compound it produces when reacted with ammonium chloride (NH₄Cl).

Testing Unknowns

You will be given three unknown solutions in lab, each with an ID number you should write down. For each one, you will use two tests to determine whether it is an acid, a base, or a neutral solution. You can use each test only once. For each test result, you will record what it indicates.

Note: results will not always be definitive, and your interpretations should reflect this. For example, the indicator known as methyl red is red in the presence of acids and yellow in the presence of both bases and neutral solutions. If you were to test a solution with methyl red and observe a yellow color, then your interpretation should be "either basic nor neutral," and you would want to choose a second test that would allow you to differentiate between a basic solution and a neutral solution.