Frost Wedging Lab

Purpose:
To determine the percentage by which water expands as it freezes. NOTE: Answers will vary because some samples will freeze more quickly than other. The expansion of water as it freezes is a major source of mechanical weathering throughout much of the world.

In a nutshell . . .
Students will determine the height of water in a test tube. They will cause the water to freeze, measure the new height, and then determine how much it has expanded. Click on the Hot Link below to print out the student handout. It is fairly self explanatory.

Free advise, etc. . . .
1. Save this activity for a day when you have fresh, fairly wet snow. For those of you who don't have snow, crushed ice will work also.

2. Collect the snow in a large cooler and then use a smaller container to mix the slush yourself. The mixture should inlcude snow, salt, and water as needed. The amount of water that you will need to add depends on the moisture content of the snow. You want the consistency of the slush to be similar to that of the slush-like drinks that can be purchased at convenience stores. Use a scooper to distribute the mixture to the students. I used to have the students mix the slush, but it was too messy and took too much time. Use plastic beakers (450 mL is best) or clear plastic cups. Once everyone gets started, move around the room and add more slush to their beakers as needed.

3. The small rock seems to serve as a starting place for the formation of ice crystals. This seems to help the formation of ice proceed slowly, allowing for bigger crystals and greater expansion from the bottom up. Optional: Have students put metal disks (washers) at the bottom of the beaker before you put the slush in. This will help conduct heat away from the bottom of the test tube. On rare occasions the expansion does cause a test tube to crack.

4. Another thing that sometimes happens is that the water becomes supercooled and then freezes instantly. When this happens the ice crystals don' get much of a chance to form, so the amount of expansion is minimal. But that's OK! There will be some expansion and this provides another discussion point. In fact, if the water hasn't frozen and its getting late in the class period, drop a tiny piece of snow into the water to trigger the freezing of the super-cooled water.

5. Provide a tub of warm water for students to place the tubes in when they are finished.

Answers to questions . . .

Pre-Lab: Do these together. They will help students understand the lab.
1. Ice floats because it is less dense than liquid water. It is less dense because water molecules arranged in crystals take up more space than water molecules that are in the liquid phase.
2. To determine this first subtract 1650 from 1700. Next take this answer (50) and divide it by 1650 to get 3%.

Procedures (example in photos below):
height of water in test tube; 7.3 cm . . . . . . . height of ice in test tube: 7.8 cm cm

Follow-Up Questions:
1. 7 % (answers will vary)
2. It will expand more when it freezes slowly. The slow rate of freezing allows for larger crytstals to form.
3. The answer is "a".
4. The answer is "c".
5. Water enters cracks in rocks and then expands as it freezes. This helps to break the rock into smaller pieces.
6. It is a type of mechanical weathering.
7. Frost wedging simply breaks the rock into smaller pieces. It does not change the chemical composition.
8. Breaking the rock into smaller pieces provides more surface area for chemical weathering.

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