Expansion and Contraction of Matter

Heat does to matter is changes it state. There is something more subtle though that can cause big problems. Look at this devise. When you heat both this ball and the ring the ring expands like a long bar of metal. The ball expands less so when they are heated the ball fits through the ring. You may want to look for these and try this demo as many of you probably have these. Another neat tool to show the expansion of metals is this bimetal bar.

It is made with one metal on one side and another metal on the other side. One metal expands more rapidly so the bar twists when heated. This affects things in the real world drastically. If this is not considered when building something we can end up with a road buckling. Engineers then plan for the expansion and contraction due to heat. In a pipe we may see special parts like this or this so that the pipe can expand in length without breaking. Behaviour of matter - Expansion and contractioncontraction Substances expand or get bigger when they are heated up. They contractor get smaller when they are cooled down.

This property can be useful.  Thermometers work because the liquid inside them expands and rises up the tube when it gets hotter.  Metal parts can be fitted together without welding using shrink fitting. The animation shows how this works. Expansion and contraction in metal The rod is too big to fit through the hole. The rod is cooled, causing it to contract. The rod fits in the hole. When warmed, it expands to fit tightly. All matter is affected by heating and cooling. With a very few exceptions, when any matter is heated, it will expand. When it  is cooled, it will contract.

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Observing a Gas Expand When Heated

Place the neck of the balloon over the mouth of the bottle. Put about 6 cm (2 in) of water in the bottom of the pot and place the bottle and balloon in the pot. Heat the water slowly over the stove. You do not need to heat the water to boiling. What happens to the balloon? Remove the bottle and allow it to cool. What happens to the balloon now?  When the air inside the bottle was heated by the hot water, it expanded. As the air expanded, some was pushed into the balloon causing it to expand slightly. When the air inside the bottle cooled, it contracted and the balloon shrank.

Observing a Gas Contract When Cooled

Just as you saw a gas expand when heated, you can see how it contracts when cooled using the same material as in the last experiment. Place a couple of inches of water in the bottom of the pot and place the bottle in the pot. Heat the water to almost boiling. Then, using the oven mitts, remove the bottle and quickly place the balloon over the neck. Allow the bottle to cool and observe the balloon as the bottle cools.

As the air inside the bottle cooled, it contracted. This caused the balloon to be drawn into the bottle. Put a little food coloring or a pinch of coffee or fruit drink mix in the bottle. Fill the bottle completely with cool tap water. Place a couple of inches of water in the bottom of the pot and carefully place the bottle in the pot, being careful not to spill any of the water. Slowly heat the water in the pot almost to boiling and observe what happens to the water in the bottle.

As the water inside the bottle was warmed, it began to expand. The bottle could no longer hold all of the water and the water began to “bulge” from the top.

Some may have even spilled out. Fill the bottle completely to the top with hot tap water. Try to get as few bubbles in the water as possible. Allow the bottle to cool where it will not be disturbed. Be careful not to spill any of the water. After the bottle has cooled to room temperature, observe the level of the water in the bottle. What Happened: The water level was slightly below full. As the water cooled, it contracted causing the water level to drop. However, there may also be another effect here as well.

If you used water with a lot of bubbles, those bubbles also took up a part of the volume inside the bottles. As the bubbles eventually floated to the top, they would have decreased the volume slightly. Can you think of some way to insure that what you are seeing is not a result of the bubbles?

A Major Exception to the Rule

The general rule that has already been stated is that matter expands when heated and contracts when cooled, but there are a few exceptions. The most important exception is water when it freezes. Fill the bottle to the top with water and replace the cap.

Wrap the bottle in several layers of newspaper and place the bottle and paper in the bag. Put the bag in the freezer and leave it there until the water freezes. Remove the bag and paper and examine the bottle. What do you see? What Happened: The bottle was shattered or very swollen. When water is cooled, whether as a gas (water vapor), a liquid, or a solid, it will contract. The one major exception to this is when water reaches the freezing point and changes from liquid water to ice. At that point, the water expands, rather than contracts. This expansion caused the bottle to break.

The reason water behaves this way has to do with the shape of it’s molecules. When water freezes into ice, it’s molecules line up in a certain way, and when they do, they take up more space than they did as a liquid. It is almost as if the molecules elbow each other out of the way, and this causes the ice to take up more space than the liquid water. Once the water freezes and gets colder than the freezing point ( 0? C or 32? F), it begins to contract again. The only time water expands when cooled is at the point where it freezes. However, the fact that water expands when it freezes is very important in nature.

For example, one of the ways that rocks are broken down into soil is by water freezing in the cracks of rocks. When the frozen water expands, it has enough force to cause the rock to split or break into smaller and smaller pieces. Using the wire cutters, cut a rod from the long bottom section of a wire coat hanger. Keep this wire as straight as possible. Use the sandpaper to sand the paint off the wire. (You are going to heat this wire, and you need to remove the paint to avoid fumes from burning paint! ) Cut a section of the coffee stirrer or broom straw about 3 inches long.

Push the straight pin through the middle of this section. The straight pin should fit snugly. If it doesn’t, use a small piece of tape to hold it in place. Set up the rod, pin, books and heat source as shown. Make sure that the end of the rod away from the pointer is firmly against a book, and that the rest of the rod is touching only the pin, and not the surface of the books. Heat the rod using your heat source, and observe what happens to the pointer. Remove the heat source without disturbing the rod and watch what happens as the rod cools. What Happened: As the rod was heated, it began to expand.

Since one end of the rod was against the book, it could expand in one direction only. As it expanded, the rod moved over the pin, causing the pin to roll slightly. Although the pin may not have rolled more than a quarter turn, the pointer allowed you to see this motion very clearly. As the rod cooled, it contracted and moved the pin and pointer back to where they started. We have seen that solids expand when heated and contract when cooled. Engineers who design roads, buildings, towers, and other large structures must know how much a substance will expand or contract over the range of temperatures it is expected to encounter.

The engineers then have to design the structure to prevent damage from expansion or contraction. These next experiments will have you to examine some of these structures on a hot day in summer and a cold day in winter. They could become part of a science project on heating and cooling.

Expansion and Contraction of Railroad Tracks Raildroad Track Expansion

Joint Walk along a railroad track until you find a place where two rails are joined together. You should see a small gap between the rails where they are fastened together. This gap is called an “expansion joint”.

Some newer tracks have rails that are continuously welded together and do not use expansion joints. If you walk for some distance and do not see a gap between two rails, the tracks you are looking at are probably of this kind. If possible, you should try to locate an older track, or even one that is no longer in use. Such track will be more likely to have expansion joints. If you are able to locate an expansion joint, measure and write down the outside temperature, along with the date and the time. Also, measure and record the size of the gap. The millimeter scale is probably the best scale to use.

If you have a camera (particularly if this is part of a science project) take a picture of the joint. It is a good idea to take this picture with the ruler in place. Save your notes. If you did this on a hot day, repeat it on a cold day, or vice versa. Can you measure any difference in the size of the gap? What Happened: The gap is slightly narrower on a hot day, because the rail sections on either side of the gap expand with the increased heat. If expansion joints were not put in place and the rails were placed tightly together on a cold day, when they were warmed by the sun, they would buckle and perhaps come loose. If the rails were put down tightly on a hot day, they would pull apart in cooler weather. Either could cause a very serious accident.

Expansion and Contraction of Power Lines

On a very hot or cold day, locate power lines near your home hanging between two poles or towers. Notice how much the lines sag. Measure and record the temperature as well as the date and time. Select a good spot to take a picture of the lines. Carefully note exactly where you make this picture in your notebook, including any zoom setting and the center of your photo.

Make sure that you will be able to return to the exact spot several months from now. Return to the same spot when the weather is much hotter or colder, depending on when you did this the first time. Again, measure and record the temperature, date and time. Take another picture of the wires using the same zoom settings and center point as before. Compare the two pictures. What do you see? What Happened: The wires sagged much less in cold weather. Wire, like all other solids, expands when it is warmed and contracts when it is cooled. In hotter weather, it will expand more, causing the wires to sag more. When power or phone lines are strung, they are always sagged to allow for expansion and contraction. If they were to be strung too tight, they could snap when they contract in colder weather.

An Exception to Expanding and Contracting in Solids

As we have seen, solids usually expand when heated and contract when cooled. However, some solids don’t always behave according to this rule, as this experiment will show. Turn the box on its side as shown. Place the pushpin in the top edge of the box and hang the rubber band over the pushpin. (If you can’t get the pushpin to hold firmly, try taping the rubber band. Tie a small weight to the other end of the rubber band. The weight should be heavy enough to stretch the rubber band, but not enough to break it. Set the dryer on it’s hottest setting and heat the rubber band. Note what happens to the rubber band. Instead of expanding as we would have expected, the rubber band contracted and lifted the weight. Molecules of rubber are long and twisted, something like a loose spring. Rubber molecules compress when they are heated, causing them to draw together like a tighter spring. When all of the molecules do this, the rubber band contracts.

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