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# Gravity of Rock

based on 6 ratings
Source:
Author: Janice VanCleave

So You Want to Do a Project about Rocks!

### Purpose

To determine the specific gravity of a rock.

### Materials

• Scissors
• Ruler
• 2-liter plastic soda bottle
• One-hole paper punch
• Flexible drinking straw
• 2-cup (500-mL) measuring cup
• Pitcher
• Tap water
• 18-inch (45-cm) piece of string
• Apple-size rock
• Gram food scale

### Procedure

Example: A rock with a mass of 150 g displaces 60 mL of water.

1. Ask an adult to cut about 4 inches (20 cm) off the top of the soda bottle. Keep the bottom and discard the top.
2. Use the paper punch to make a hole about 1 inch (2.5 cm) from the rim of the bottle.
3. Insert about 1;2 inch (1.25 cm) of the flexible end of the straw into the hole.
4. Bend the straw so that it forms a 90° angle. Place the measuring cup under the free end of the straw.
5. Use the pitcher to pour the water into the bottle until it is just above the straw. Water will flow through the straw and into the cup.
6. When the water stops flowing into the cup, empty the cup and then set the cup in place under the straw. The water-filled bottle and the empty cup are your specific gravity instruments.
7. Tie the string around the rock, then place the rock on the scale to determine as accurately as possible its mass in grams (g). Record the mass.
8. Holding the rock by the string, slowly lower it into the bottle.
9. When the water stops flowing into the measuring cup, record the amount of water in the cup in milliliters (mL). Use the following example to determine the specific gravity (S.C.) of your rock specimen.
• specific gravity (S.C.) = mass of rock ÷ mass of water displaced by rock
• mass of rock = 150 g
• volume of displaced water = 60 mL
• 1 mL of tap water has a mass of about 1 g, so the mass of displaced water = 60 g
• S.C. = 150 g + 60 g
•         = 2.5

### Results

The specific gravity of a rock is determined. For the example, the specific gravity is 2.5.

### Why

Specific gravity (S.G.) is a physical characteristic used to identify rock types. Specific gravity is the ratio of the mass of a substance, such as a rock, to the mass of an equal volume of water. Your specific gravity instrument allows you to determine the volume of water that is equal to the volume of the rock. This is done by collecting the water displaced by the rock when placed in the instrument. Since the mass of 1 mL of water equals 1 g, the volume of water measured in milliliters is equal to the mass of water in grams. Specific gravity of the rock is calculated by dividing the measured mass of the rock by the calculated mass of water. Specific gravity tells you how many times more massive a material is than an equal volume of water. Most rocks have a specific gravity greater than 1, meaning that they are heavier than water.

### For Further Investigation

A larger rock would make more water pour out of the bottle. Does that mean a larger rock has a greater specific gravity? A project question might be, How does size affect the specific gravity of a substance?

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