NAME_________________                        DATE_________ PERIOD____________                                      MOVING MOLECULES WORKSHEET

A different way to think about heat energy and the phases of matter is to think about the speeds of the molecules. Answer all the questions as

best you can.

Let's pretend there are 8 molecules of water near the surface. We'll  even give them names:

Each molecule is moving at a different speed. In any group of molecules, you find that they are all moving at different speeds. Let us  assign speeds to our 8 friends:

ALICE:  9 miles an hour   DONNA: 6 miles an hour    GRACE: 3 miles an hour

BILL:   6 miles an hour   EDGAR: 1 miles an hour   HELENA: 9 miles an hour

CURT:  10 miles an hour   FRED:  3 miles an hour

1) What is the average speed of these 8 molecules? (add them all up and

divide by 8!)

2) The average speed of the molecules is like what property that you could  measure?

All of the molecules really want to leave the surface. But in this case they need to reach a speed of 11 miles per hour to change from liquid to gas, and have enough energy to overcome gravity, to push up against air  pressure, and break their bonds with their friends.   One way to give them enough energy is to heat them, and raise all of  their speeds. So let's cook them up!

3) Suppose all the molecules are now heated so that they each increase their speed by 1 miles per hour. What is the speed of       each   of the 8  molecules now?

ALICE:                                           DONNA:                         GRACE:

BILL:                    EDGAR:                           HELENA:

CURT:                    FRED:

4) Obviously the average speed (and the temperature) has gone up. But some molecules now have enough energy to leave the surface. Which molecules now  (have a speed of at least 10 miles per hour?

Well, it turns out that those molecules do leave their friends. But,  Helena happens to be stuck behind Edgar, so she can't leave the surface.

5) What do you think would Helena now become?

6) There are now 6 molecules left, including Helena. Who are they, and  what are their speeds?  :

NAME                                    SPEED (from question # 3)

7) What is the average speed of the 6 molecules left? (add them up and

divide by 6!)

8) Look at the average speed  before it was heated, and any molecules  left. (question # 1). Is it higher or lower now than in the beginning?

9) Would the temperature of the molecules in the liquid be higher or lower  than in the beginning?

10) How does this help explain that evaporation is a cooling process?