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Question Video: Identifying a Sedimentary Rock from Its Appearance, Structure, and Reaction with Hydrochloric Acid Science

On a field trip, a student finds three different rocks. They note the appearance and structure of each rock and record their observations when a small amount of hydrochloric acid is added. Which of the rocks is likely to be a sedimentary rock?

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Video Transcript

On a field trip, a student finds three different rocks. They note the appearance and structure of each rock and record their observations when a small amount of hydrochloric acid is added. Which of the rocks is likely to be a sedimentary rock?

Sedimentary rocks are formed from the deposition, accumulation, and solidification of small mineral or organic particles. When looking at a sedimentary rock, we can often see the individual layers that have been deposited and solidified together. Sedimentary rocks tend to be softer than igneous or metamorphic rocks. So, it seems that rock A might be a sedimentary rock. But let’s take a closer look at the descriptions and observations the student recorded about each rock.

One of the key observations about rock C is that it contains small holes. Rocks that contain small holes are typically igneous volcanic rocks. Igneous volcanic rocks are formed when lava above the Earth’s surface quickly cools. Gases that were trapped in the magma below the Earth’s surface are released as the lava cools, leaving small holes in the rock. So, rock C, which is very hard and contain small holes, does not appear to be a sedimentary rock.

Both rock A and rock B react with hydrochloric acid, producing an effervescence or bubbles. The mineral responsible for this reaction is calcite. Calcite is the crystalline form of calcium carbonate. Calcium carbonate reacts with hydrochloric acid to produce calcium chloride, carbon dioxide gas, and water. We see the carbon dioxide being produced as effervescence or bubbles.

There are two common rocks that are a white color and contain the mineral calcite. Limestone is a soft, smooth, white rock that contains calcite. When limestone is exposed to high temperature and high pressure, it can be transformed into marble, a hard, white rock with a rough texture. Limestone is a sedimentary rock. Marble is a rock formed from the conversion of existing rock. So, marble is a metamorphic rock.

From our discussion, we now know that the rock, which is likely to be a sedimentary rock, is rock A.

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