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# Slow Mover: How Does the Viscosity of Lava Affect its Flow Rate?

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Source:
Author: Janice VanCleave

### PROBLEM

How does the viscosity of lava affect its flow rate?

### Materials

• scissors
• clear plastic dish-detergent bottle with pull top
• ruler
• marking pen
• modeling clay
• jar with a mouth slightly smaller than the base of the dish-detergent bottle
• pitcher
• cold tap water
• timer

### Procedure

1. Prepare a viscometer (meter used to measure the flow rate of a fluid) by following these steps:
2. Hold the bottle upside down. The bottom edge of the detergent bottle will now be referred to as the top. Draw two straight, horizontal lines on the bottle: one about 1 inch (2.5 cm) below the open top, and the other 4 inches (10 cm) below the first line.
3. Label the top line START and the bottom line STOP.
4. Make sure the pull top of the bottle is closed.
5. Place a role of clay around the mouth of the jar.
6. Rest the detergent bottle upside down on the mouth of the jar. Mold the clay roll so that the bottle stands upright, but do not secure the bottle with the clay.
2. Fill the pitcher with cold tap water.
3. Pour the water into the open end of the detergent bottle until the water is about 1/2 inch (1.3 cm) above the START line.
4. lift the bottle and pull the top open.
5. Immediately return the bottle to the mouth of the jar and start the timer when the water level reaches the START line.
6. Stop the timer when the water level reaches the STOP line.
7. Repeat the procedure three times.
8. Average the flow time of cold tap water by adding the three flow-time results together and dividing the sum by three. The following example shows the author's flow-time results and the calculated average flow time.
9. Sum of flow-time results (seconds)

= 39.2 + 39.4 + 39.3
= 117.9 seconds

Average flow time = 117.9 seconds ÷ 3

= 39.3 seconds

### Results

The flow time will vary with the shape of the bottle used. The author's average flow time for cold water from the faucet was 39.3 seconds.

### Why?

The amount of time it takes a liquid to flow out of a container depends on its viscosity. The viscosity of a liquid is the resistance of the liquid to flowing; or its stickiness. Water has a low viscosity and flows quickly out of the open viscometer.

Different types of lava have different viscosities. Some are so thick and sticky and have such a high viscosity that they creep along, moving only a few yards (m) per day. Others have a low viscosity, and flow a few miles (km) per hour.

### LET'S EXPLORE

1. How does the viscosity of other liquids compare to the viscosity of water? Repeat the experiment using liquids such as oil, dishwashing liquid, honey, and/or syrup. Wipe out the viscometer after each test with a paper towel or prepare separate instruments for each liquid tested. Compare the flow times of each liquid.
2. Does the temperature of the liquid affect its viscosity? Repeat this experiment twice: first by reducing the temperature of the liquids by chilling them in the freezer, and then by heating the liquids by placing a cup of each in a bowl of warm tap water. Science Fair Hint: Prepare bar graphs to display the results of each testing liquid. Label the results from the room-temperature testing as medium temperature.

### SHOW TIME!

What happens to the flow rate of lava when it contains particles of a solid? Build two volcanoes out of clay. Make each volcano about 6 inches (15 cm) tall with a small bowl-shaped indention at the top, and put each in the center of a plate. Pour 1 cup (250 ml) of dish washing liquid into the indention on the top of one volcano. Observe the flow rate of the liquid as it overflows and runs down the side of the clay mountain. Mix 3/4 cup (188 ml) dishwashing liquid with 1/4 cup (63 ml) of sand. Pour this mixture in the top of the second volcano. Again, observe the flow rate of the liquid as it flows down the volcano's sides. Take photographs of the experiments and use them as part of a project display.

### CHECK IT OUT!

Hot, thin lava flows freely and forms smooth layers when cooled. Cool, thick lava breaks and tumbles forward rather than flowing. It produces a rough texture when cooled. Find out more about the movement of lava and its appearance when cooled. Describe pahoehoe and aa, Hawaiian names for two different types of lava. (See Experiment 17 for more on this subject.)