Under pressure and on the move

1. Under pressure and on the move Running hot and cold

2. Running hot and cold What you need: • A balloon • An empty plastic pop bottle without the cap • A sink filled with hot water • A refrigerator

3. Running hot and cold What to do: • Put the empty pop bottle in the freezer. • Blow up the balloon and then let the air out again. This loosens it up. • Fill the sink with hot water. • Take the bottle out of the freezer. Stretch the open end of the balloon over the mouth of the bottle right away to keep the cold air in. • Hypothesis: Predict what will happen to the balloon when you put the bottle in the in the hot water. • Lower the bottle into the water (with the balloon still above the wter) Hold it down for one minute. Observe what happens. • After observing what happened, take the bottle out of the water and put in the freezer. What do you think will happen? • After one minute, remove the bottle and observe what happens/

4. What did you observe happening to the balloon when the bottle was placed in hot water? What happened to the balloon when the bottle was in the freezer?

5. When air molecules are heated, they move faster and faster. They collide with each other and bounce off the side of their container with greater force. These harder collision cause the molecules to bounce farther away from each other. This is what makes air expand, or take up more space, and why the balloon inflated as the air expanded inside. When the bottle was cooled, the molecules slowed down and caused the air to contract, to take up less space. This is why the balloon shrunk in size – the air molecules went back inside the bottle. At room temperature, an air molecule in a small volume of air collides with other air molecules several billion times per second.

6. When air is heated in an open container, the molecules speed up and simply move outside the container, moving further and further apart. But in a closed container, the molecules have nowhere to go. Imagine if you had kept the cap on the bottle when you put it in the hot water. As it warmed, the air would have tried to expand, but the bottle would have kept it contained. This would have increased the pressure of air in the bottle. When air heats and expands in a closed container, the air pressure increases. When it cools and contracts, the pressure decreases. Heating and cooling are not the only ways to change air pressure though…