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!)

*47/8 =
5.875 mi p h*

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

*Temperature
(average kinetic energy)*

All of the
molecules really want to leave the surface. But in this case they need to reach
a speed of 10 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: ** 10** DONNA:

BILL: ** 7**
EDGAR:

CURT: ** 11**
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?

*Curt,
Alice and Helena*

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?

*Helena
becomes a water vapor bubble stuck inside the liquid*

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

* NAME
SPEED
(from question # 3) *

*Donna 7*

*Grace 3*

*Bill 7*

*Edgar 2*

*Helena 10*

*Fred 4*

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

divide by
6!)

*33/6 =
5.5 mi p hr*

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?

*It is
lower than 5.875 , even though we heated up! That’s because it is an AVERAGE,
and some (the hot molecules) left, leaving the cold ones behind.*

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

* The temp is lower*

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

*Evaporation is a cooling process because the hot
molecules leave, and the cooler ones are left behind.*