Image credit: Scott Brande
The Bottom Line on the Acid Test - Here on Top!
Calcite - CaCO3 - is a common mineral and the major compound in limestone. Among the common minerals, only calcite (or its polymorph, aragonite) react strongly to a dilute solution of cold hydrochloric - HCl - acid. If a sample fizzes immediately producing large bubbles (I call this the "rice crispy" reaction, for the audible sound produced as bubbles pop), then the sample is rich in calcite.
Bottom Line: The "acid test" is fast and easy to interpret. A positive acid test result is diagnostic for the identification of calcite. For this reason, why not do the acid test first before others?
Errors/Suggestions: Contact Scott Brande (see footer) or post on Discussion Forum.
What is the "Acid" Test?
How DO you test a sample with acid?
Watch the video demonstrations below.
Video of Positive Test for Reaction to HCl
Video of Negative Test for Reaction to HCl
Instructions: The "Acid" Test
Background - Minerals are chemical compounds, and a number of minerals are to some degree unstable in the presence of an acidic solution. That is, an acidic solution can break the chemical bonds of a mineral compound, and the mineral dissolves. One chemical test is made with a solution of cold, dilute (10%) hydrochloric acid - HCl. Only one common mineral reacts strongly under this solution.
A solution is made of about 10% hydrochloric acid (90% water and 10% concentrated HCl). The solution is stored at room temperature in a plastic, safety "drop bottle" for use.
Caution - The use of a dilute solution of hydrochloric acid should not be attempted without safety instructions normally provided by the instructor.
Materials for test
- mineral sample
- cold (room temperature), dilute (about 10%) hydrochloric acid in a plastic, safety "drop bottle"
Procedure for the test
- Squeeze the safety bottle gently to release just one or two drops of cold, dilute HCl is onto the sample.
- Observe the sample for any evidence of a chemical reaction.
- Blot with a paper towel any drops of acid from the sample.
Possible test results and interpretation
- The sample responds immediately by rapidly producing numerous bubbles which pop and fizz. We interpret this reaction as "strong" or "vigorous". We conclude that the sample is made of the chemical compound CaCO3 - calcium carbonate, the only common mineral that vigorously reacts with cold, dilute HCl. This compound is the formula for the mineral calcite.
- The sample does not respond, or responds only weakly, to the presence of the cold, dilute hydrochloric acid. The drop of acid simply sits on the surface of the sample and appears stationary like a drop of water. A weak response is indicated by the presence of a few, tiny bubbles. We observe no reaction, or a weak reaction, and we conclude that the sample is made of a chemical compound that is NOT CaCO3 - calcium carbonate, and that calcite is largely absent from the sample.