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How to Use Muriatic Acid on Rocks

Geologists use acid tests to help determine the identity of certain rocks. Muriatic acid, also known as hydrochloric acid, or HCl, acts as a strong acid. Rocks that contain carbonates react with strong acids to produce carbon dioxide that emerges with a distinctive "fizz." Therefore, rocks that "fizz" when contacted with muriatic acid are probably limestone, calcite or carbonate-cemented sandstone. Limestone and calcite represent the rock and mineral forms, respectively, of calcium carbonate, or CaCO3. Carbonate-cemented sandstone contains small particles of sandstone bound together with a lime-based cement in an agglomeration that resembles concrete.

Things You'll Need

16-oz. measuring cup
Concentrated muriatic acid
Eyedropper
Small rock samples

Instructions

- 1
  Fill an 16-oz. measuring cup to approximately 8 oz. with tap water. Working in a well ventilated area, pour about 1 oz. of concentrated muriatic acid into the water. Add additional tap water to bring the total volume to about 10 oz. This represents a 10-percent solution of muriatic acid --- the recommended strength for performing carbonate tests on rocks.
- 2
  Transfer the muriatic acid solution into an 8- or 16-oz. plastic bottle, cap it tightly and label the bottle.
- 3
  Test a rock sample by drawing some 10-percent muriatic acid solution into an eyedropper and placing two or three drops directly onto the rock sample. The presence of carbonates should be immediately apparent by the formation of small bubbles where the acid contacts the rock.

Rocks & Minerals