Bubbler: How Can You Identify Carbonates?

Problem

How can you identify carbonates?

Materials

• long-handled spoon
• raw egg
• quart (liter) wide-mouthed glass jar
• 1 quart (1 liter) white vinegar

Procedure

1. Use the spoon to place the egg in the jar, being careful not to crack the egg.
2. Fill the jar with vinegar.
3. Observe the eggshell immediately and then periodically for the next 2 days.

Results

Bubbles start forming immediately on the surface of the eggshell and increase in number over time. After about 2 days, the shell is no longer present, and a thin membrane holds the egg together.

Why?

The main ingredient of the eggshell is calcium carbonate. When calcium carbonate combines with an acid, such as vinegar, it dissolves as new materials are produced, including carbon dioxide gas. The bubbles seen rising in the jar are carbon dioxide gas.

Minerals containing combinations of carbon and oxygen and some other element, such as calcium, barium, or manganese, are called carbonates. Carbonates can be identified by the fact that they, like the eggshell, produce bubbles of carbon dioxide gas when combined with an acid. Some carbonates require a very strong acid to produce carbon dioxide gas but many react with weaker acids, such as vinegar.

The most common carbonate is calcite, which contains calcium carbonate. Calcite, like most carbonates, tends to be fairly soft. It has a hardness of 3, cleaves perfectly in three different directions, is usually whitish or colorless, and has a white to grayish streak. The rocks limestone and marble are made from calcite.

Let's Explore

Some types of school chalk are a form of calcium carbonate mixed with a clay binder, while other types are made mostly of gypsum (a mineral made of calcium sulfate). Strong acids will cause the gypsum to bubble, but weak acids, such as vinegar, will cause little or no reaction. Even weak acids, however, will cause calcium carbonate to produce large amounts of bubbles. Collect different brands of school chalk to identify which contain calcium carbonate and which contain gypsum. Repeat the experiment, using a baby food jar filled with vinegar and a small piece of chalk instead of the egg. Perform the experiment for each chalk sample. The presence of many bubbles means the chalk contains calcium carbonate, but if there are few to no bubbles, the chalk is made from some other material, such as gypsum.

Show Time!

1. You can identify calcium carbonate without allowing the sample to totally dissolve by removing the chalk as soon as you determine whether or not a carbonate is present. Do this by tying a 12-inch (30-cm) piece of string around each piece of chalk. Lower one piece of chalk into the jar of vinegar. Wait 5 to 10 seconds, or as soon as bubbles are seen, then remove the chalk from the vinegar. Rinse the chalk in water. Test each of the chalk pieces using this method. Record your observations on a chart similar to the one shown.





2. For soft minerals, use the edge of a spoon to scrape some of the mineral into a mound on a saucer. Add 2 to 3 drops of vinegar to the mound of powder. Use this method to test for the presence of calcium carbonate in school chalk. Also test soft minerals such as talc and gypsum.



3. A more common method of identifying a carbonate is to use an eyedropper to add a few drops of acid, such as vinegar, to the mineral sample being tested. Choose a spot on the sample that is not particularly interesting, as acid may damage the surface. After 5 to 10 seconds or as soon as the bubbles are seen collecting on the surface of the sample, you can rinse off the acid with water.

Check it Out!

The most common carbonate, calcite, contains CaCO₃, calcium carbonate. The mineral witherite contains BaCO₃, barium carbonate. Use a rock and mineral field guide to find out more about the names of different carbonates and the carbonate compounds they contain.

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Janice VanCleave's Rocks and Minerals
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