BACKGROUND:
Rock is the solid state of matter, while lava (outside
the Earth) or magma (inside the Earth) is the liquid state of matter.
Cooling of the lava will change the state of matter to solid.
Students at this grade are familiar with water, honey,
vinegar, and syrup, but not molten rock. Molten rock is a liquid. The
thickness of a fluid is called its viscosity. Viscosity is technically
defined as the amount of internal resistance to flow in a liquid, which
depends of how "sticky" the molecules and atoms in the fluid are. Water, a
fluid we are all used to has relatively low viscosity. Molten rock has a
high viscosity; it is much thicker, more like wet concrete or cold
honey. Lava (molten rock that flows on the surface of the Earth), actually
has many different viscosities, which are largely controlled by the
temperature of the lava and its composition.
Students are familiar with how water flows, but they
sometimes don't realize that other substances like oil or sand can also
flow. This activity familiarizes the students with the concept of viscosity.
The class should first watch parts of a video which show real viscous lava
flows.
PROCEDURE:
- Set up 5 stations. At each station have 2 cups and a different
material: red water, blue water, oil, syrup, or sand. In the lab, the
students will pour the contents of one cup into another. You may want to
put trays, buckets, wax paper, or newspaper under the experiment in case
a spill occurs.
- Show the class portions of the Kilauea video tape that illustrate
flowing lava. A good section to use is the point at which the lava flow
engulfs the schoolhouse. Emphasize the movement of the lava and point
out the different speeds of the lava flow. Describe the movement of the
lava as "viscous flow". Although this is an unfamiliar word
for the students, it correctly labels the lava flow as having a slower
rate of flow than water.
- Divide the class into groups. Have the groups move from station to
station. They should pour the liquid at each back and forth, mentally
noting its viscosity. When they have completed all 5 stations, have them
return to their seats and discuss which liquid most closely resembled
the lava seen in the video and why. Remind them not to think of color,
but how the liquid moves (how viscous a substance is).
- Take a poll of the students to see which liquid they observed to be
most similar to the lava flow. They might choose the water with red food
coloring because of the red color. Try to illustrate that the motor oil or
syrup is more viscous, and therefore similar to the lava. The point is not
to convince the students of the right answer, but to get them thinking.
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