Lab / experiment Grade 4 Science
Ice Wedging Under Gattock Road: A Freeze-and-Thaw Lab
A grade 4 lab running three freeze-and-thaw cycles on wet and dry chalk samples to show why a fictional town's roads and pipes crack most in late winter.
Objectives
- Students will run three freeze-and-thaw cycles while keeping every condition the same except the water.
- Students will count loose pieces and measure crack length in millimeters after each cycle.
- Students will explain how freezing water widens cracks in rock, roads, and buried pipes.
Vocabulary
Key words: weathering, freeze-thaw, cycle, millimeter, crack
Teacher overview
Crews in the fictional town of Gattock patch far more road cracks and broken pipes in February and March than in July, and the reason is water that seeps into small openings and then freezes. Water is unusual because it takes up more room as ice than it did as liquid, so a crack that fills with water in the evening can be pried a little wider by morning. This lab lets students watch that process on a small scale over three days, using wet and dry chalk samples treated exactly alike except for the water. The investigation question is whether water inside a sample makes it break apart faster than cold alone does.
Procedure
Step 1. Label the samples. Take six sticks of chalk. Mark three of them A1, A2, and A3. Mark the other three B1, B2, and B3. The A sticks are the wet group. The B sticks are the dry group.
Step 2. Soak the wet group. Put the three A sticks in a tray of water. Leave them there for ten minutes. The B sticks stay dry on a paper towel.
Step 3. Bag every sample. Put each A stick in its own bag. Add two tablespoons of water to each A bag. Put each B stick in its own bag. Add no water to the B bags. Press out the air and seal all six bags.
Step 4. Freeze the bags. Lay the six bags flat on a tray. Ask your teacher to place the tray in the freezer. Leave the bags frozen overnight, for at least eight hours. Only an adult opens the freezer.
Step 5. Thaw the bags. Take the tray out and let the bags sit for two hours. Put on your safety goggles before you open a bag. Chalk chips can be sharp, so do not rub your eyes.
Step 6. Observe and record. Open one bag at a time over the tray. Count every loose piece and crumb that came off the stick. Use the millimeter ruler to measure the longest crack on the stick. Write both numbers in the table. Never taste the water or the chalk, and wash your hands when you finish.
Step 7. Repeat the cycle two more times. Seal each bag again with the same water in it. Freeze the bags overnight and thaw them again. Record your counts and measurements after Cycle 2 and again after Cycle 3.
Data & Analysis
Record the totals for all three sticks in each group. Write each crack length twice, first in millimeters and then in centimeters.
| Cycle | Loose pieces, wet group | Loose pieces, dry group | Longest crack, wet (mm) | Same crack (cm) |
|---|---|---|---|---|
| 1 | ||||
| 2 | ||||
| 3 |
How many loose pieces did your wet group have after Cycle 1?
How many loose pieces did your wet group have after Cycle 3?
How many more pieces did the wet group have after Cycle 3 than after Cycle 1?
Write the longest wet-group crack from Cycle 3 in millimeters. Then write that same length in centimeters.
What happened to the dry group over the three cycles?
Which group changed more? Use two numbers from your table as your reason.
Explain how water that freezes inside a crack can make the crack wider.
Crews in the town of Gattock fix more cracked roads and broken pipes in late winter than in summer. Use your results to explain why.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: Accept the count the group recorded for Cycle 1.
Most groups report a small number after one cycle, often between zero and five pieces. Check that the number written matches the pieces still in the tray.Answer: Accept the count the group recorded for Cycle 3.
Most groups report a larger count than Cycle 1, since damage adds up cycle after cycle. A group whose count did not grow should check whether the bag stayed sealed and whether the water was still inside.Answer: Accept the difference between the group's own two counts.
Check the subtraction against the table. A group with 3 pieces after Cycle 1 and 11 after Cycle 3 should write 8 pieces.Answer: Accept the group's own measurement written in both units.
The centimeter value must be the millimeter value divided by ten. A crack of 40 millimeters is 4 centimeters, and a crack of 25 millimeters is 2.5 centimeters.Answer: The dry group changed very little.
Accept any answer reporting few or no loose pieces and no growing crack. Some chipping from handling is normal and should be noted rather than counted as freezing damage.Answer: The wet group changed more.
A complete answer names the wet group and cites two numbers, such as the Cycle 3 piece counts for both groups or the wet group's growing crack length next to the dry group's unchanged stick.Answer: Water takes up more room when it freezes, so the ice pushes the crack open.
Accept any answer stating that liquid water seeps into the crack, expands as it turns to ice, and presses outward on both sides, leaving the crack slightly wider each time it thaws.Answer: Late winter has had many freeze-and-thaw cycles.
Accept any answer connecting repeated cycles to added damage, such as noting that the samples broke apart more after three cycles than after one, and that a road or a buried pipe goes through many such cycles between December and March.
Differentiation
Support
Give these students a printed tray outline with numbered spots so they place each loose piece in a spot before counting, which turns counting into one-to-one matching. Provide a ruler with the millimeter marks enlarged on a printed strip, and fill in the Cycle 1 row of the table together as a class.
Extension
Have students add a third group of three chalk sticks soaked in salt water and run the same three cycles, then compare all three groups and explain what the salt appears to change. Students can also predict the Cycle 6 piece count from the pattern in their table and write the reasoning behind the prediction.
ELL supports
Pre-teach freeze, thaw, cycle, and crack with a four-picture sequence card showing water in a crack, ice in a crack, a wider crack, and a broken piece. Provide a labeled photo of a millimeter ruler beside a centimeter ruler for item 4. Give sentence frames for items 5 through 8: "The dry group ___" and "Water gets bigger when it ___, so the crack ___."
Extensions
With adult supervision, at a time when no cars are moving, walk the class around the school parking lot in early spring and tally cracks that show loose crumbs at their edges, then compare that tally with a wall or path that stays under cover. Students can also interview a school custodian or a public works crew member about which repairs cluster at the end of winter.