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Rocks are broken down in all sorts of ways - by wind, rain, rivers, and rock slides. But they are also broken apart by freezing and thawing. When water freezes, it expands. Water that has seeped into a rock will expand when frozen, causing cracks in the rock. After it freezes and thaws several times, bits of rock will begin to split off entirely.
Do this experiment to see how it works and find out what kinds of rocks break down the most when frozen.
Look at each rock carefully. Which do you think will break down the most when you freeze and thaw them several times? Why? Write down your prediction, then place the rocks in the plastic bottle and cover them with water. Put the bottle in the freezer. When the water is frozen take it out and let it thaw. After the water melts, put the bottle back in the freezer. Repeat the process 3-5 times.
Take out the rock samples and look at them carefully again. Which one has changed the most? Do you see where small particles have been split off the rock by the freezing water? Was your prediction correct?
Over time whole mountains can be worn down by this freezing/thawing process!
You may be wondering how water gets in rocks in the first place. Aren't rocks solid? Actually, they have tiny pores or pockets that can be filled with water or air (or sometimes oil and natural gas) like a sponge. No rock can soak up as much water as a sponge, but some rocks absorb more water than others - these are called porous rocks.
This experiment will show you how a piece of chalk can absorb a great deal of water in a short time. Chalk is composed of the mineral calcium carbonate. (Different types of rocks are made of different pure minerals; rocks such as limestone have lots of calcium carbonate.) Have older students try this with various types of porous rock, such as pumice - they may need to wait up to a day for the rock to absorb the total amount of water it can hold.
1. Weigh the chalk with the spring scale to determine how heavy it is when dry. To do this, hang a small plastic bag from the hook and place the chalk in the bag. Record how much it weighs.
2. Place the chalk in a cup of water. In five minutes take the chalk out of the water, shake it off and weigh it again. Record the weight. Repeat at five-minute intervals until the chalk no longer increases in weight.
The chalk got heavier because it was absorbing water. After about 10-15 minutes, it absorbed all that it could hold. You can figure out how much water it absorbed, because 1 gram of water = 1 ml of water = 1 cubic centimeter (cc). If the chalk gained 2 grams in water, that means it soaked up 2 ml or 2 cc.
Older students can figure out what percentage of its volume the chalk absorbed. If the piece of chalk is a cylinder, they can find out its volume using the formula Volume = Π r ²(h) where r is the radius and h is the height. (For example, if you have a piece of chalk 2 cm in diameter and 5 cm long, the formula would be 3.14 x 1 ²(5) = 15.7 cc.) If it is a different shape, or they're trying this with other rocks, they can use the displacement method: pour 100 ml of water into a graduated cylinder and add the rock. The water level will rise, and the amount it rises is the rock's volume. If the water level rises to 125 ml, for example, then the volume of the rock is 25 ml, or 25 cc. Once you know the volume of the water absorbed and the volume of the rock, you can find what percentage of its volume the rock absorbed. Just divided the volume of water by the volume of the rock (e.g. 2cc/25cc = .08 = 8%).
Try this experiment with several different kinds of rock. Calculate the percentage of its volume that each rock can absorb. Which absorbs the most? Based on what you learned in the previous experiment, how do you think this affects the erosion of that type of rock?
Certain rocks with carbonate compounds erode or dissolve when they come in contact with acidic chemicals. Carbon dioxide in the atmosphere sometimes can produce rain that is slightly acidic. Over time, this acid rain erodes rocks. You can see it happen quickly if you use vinegar, which is a much stronger acid than acid rain.
Use a piece of limestone, which is made of calcium carbonate. Put the limestone in a bowl, pour a little vinegar on top of it and watch what happens. It will fizz and form bubbles, because the vinegar reacts with the carbonate ions. As it reacts, it dissolves the limestone. When you're done watching it fizz, take the limestone out and look in the bottom of the bowl. You should see a layer of sediment made of small particles of calcium acetate, a chemical made when the acid and carbonates react.
Try it again, but this time use chalk, which is also made of calcium carbonate. Set one piece of chalk in a cup of vinegar and one in water. The chalk will immediately start reacting with the vinegar, making quite a show! Pour off the liquid after about an hour and compare the chalk that was in the water to the chalk in the vinegar. Is there sediment in the bottom of the vinegar cup? Is there any sediment in the water cup?
You can try this experiment again with different strengths of acid. Try lemon juice, diluted lemon juice, coke, or anything else you can think of!