Age: grades 3-8
Subject: Water action and processes
Skills: observation, measurement, inference
Duration: two or three 30 minute sessions
Group size: any
Setting: indoors and outdoors
Key vocabulary: physical weathering, expansion
Materials:
rocks
several pieces of cloth
Mason's hammer
plastic milk carton
plaster of Paris
matchbox (large)
pencil
freezer
Objectives: The student will be able to:
1. Describe how Freezing water breaks rocks.
2. Infer where this process is most likely to occur.
Method:
The students will observe the effect on various objects of the expansion of water as it turns to ice.
Background
When water freezes it occupies a larger volume. As freezing water expands, it exerts a force. Such a force may widen cracks in rocks and eventually break the rocks apart. Such changes are physical changes, just as freezing water is a physical change.
Freezing water splits rocks from mountains. These rocks may be split again and again, and end up as small pebbles carried away by streams. The action of freezing water is a type of physical weathering, one of several ways rocks are broken down.
Procedure
1. Wrap several rocks in several pieces of cloth. Place them on a sidewalk and have the students hit them several times each with the hammer. Unwrap and examine the particles; compare them with sand.
What is sand?
2. Fill an empty milk carton to the top with water. Close the top securely and. place the carton in a freezer until the next day.
What do you observe?
What caused it?
What might break rocks off a mountain?
3. Fill a large matchbox with plaster of Paris paste. With a pencil, make a one-inch deep groove in the block before it dries. The groove should not extend to the ends of the block. Allow the block to dry.
4. Measure and record the width and length of the groove. Fill the groove with water and place the block in the Freezer. After the water has frozen, remeasure the groove and record.
Are there any changes in the size of the groove?
Allow the ice to melt and refreeze.
How many times do you have to repeat the freezing until the block cracks?
How does freezing water break rocks from mountains?
Extension
Soak a soft red brick or a piece of sandstone in water overnight. Place it in a pan and put it in a freezer overnight. What changes are observed? Repeat the thawing and freezing several times and observe.
Evaluation
Have the student describe how freezing water breaks rocks.
Source
Sund, R., Tillery, B., & Trowbridge, L. (1973) Elementary Science Discovery Lessons: The Earth Sciences. Boston, MA: Allyn and Bacon, Inc.