Question

activity b: temperature and molecular motion
get the gizmo ready:

• click reset, and select the micro view.
• set ice volume to 0 cm³.
• set add/remove heat energy to 0 j/s.

question: why do phase changes occur?

1. compare: set the water temperature to 10 °c and click play. observe the water molecules. click reset, set the water temperature to 100 °c, and click play again.

what do you notice?

1. observe: click reset. the average molecular speed of the water molecules is displayed below the container. set the water temperature to 0 °c and add/remove heat energy to 500 j/s. click play.

a. how does the average speed of the water molecules change as they are heated?
b. does the average molecular speed change as much as the temperature as the water heats up? explain.
3 explain: how is temperature related to the motions of molecules?
4 observe: click reset. set the water temperature to -10 °c and add/remove heat energy to 100 j/s. click play. as the ice melts, how do the molecules in the liquid interact with the molecules in the solid?
(activity b continued on next page)

Explanation:

Answer:

1. At 10 °C, water - molecules move relatively slowly and are close together. At 100 °C, water - molecules move much faster and are more spread out.

2.
(A) The average speed of the water molecules increases as they are heated.
(B) The average molecular speed does not change as much as the temperature as the water heats up. This is because temperature is a measure of the average kinetic energy of the molecules (\(KE=\frac{1}{2}mv^{2}\)), and as the temperature increases, the kinetic energy increases, but the mass of the molecules remains constant. The relationship between kinetic energy and speed is non - linear (\(v = \sqrt{\frac{2KE}{m}}\)), so a change in temperature (kinetic energy) does not result in a proportional change in speed.

1. Temperature is a measure of the average kinetic energy of the molecules. As the temperature increases, the average kinetic energy of the molecules increases, and the molecules move faster. As the temperature decreases, the average kinetic energy of the molecules decreases, and the molecules move slower.
2. As the ice melts, the faster - moving liquid water molecules interact with the slower - moving solid ice molecules. The liquid water molecules transfer some of their kinetic energy to the ice molecules, causing the ice molecules to vibrate more and eventually break free from the solid lattice structure.