Observing Energy Changes during Phase Changes



Any homogeneous material can be considered to be a phase. Water can have three phases: solid, liquid, gas. Salt water or seawater; are also considered to be phases. A phase change occurs when a homogeneous material develops a new, uniform series of properties. Ice (solid water) melting to form liquid water and solid sodium chloride dissolving in liquid water to form a saltwater solution are examples of a phase change. In the following experiment we will observe a phase change for a pure substance. Energy changes during the phase change will be observed.

The chemist uses the Kinetic Molecular Theory to explain the properties of matter and how mater undergoes chemical and physical changes. The following statements make up the central ideas of this theory.

  1. All matter is composed of tiny particles (molecules, ions or atoms).
  2. These particles are in constant random motion and travel in straight-lines.
    Particles in solids have very strong attractive forces, which hold the particles in place. These strong forces lead to vibratory motion in solids, ↔ the particles are very limited in motion. Solids have definite volume and shape.
    In liquids the intermolecular forces are weaker and allow particles to slide by one another think of marbles being poured. The forces are strong enough that the particles cannot escape from the other particles. There is a greater degree of freedom of movement liquids do not have definite shape but have definite volume.
    Gases consist of small particles that are far apart in comparison to their own size. There are no attractive forces between gas particles or between particles and the sides of the container with which they collide. Gases will expand to fill their container. Because of the great degree of freedom gases do not have definite shape or volume.
  3. Due to the constant motion, particles collide with one another.
  4. Energy is conserved in these collisions. One particle may gain energy and another particle would lose energy but the total energy is constant. This means that energy can be transferred during collisions among particles. The average kinetic energy of all the molecules is proportional to the temperature. The higher the temperature the faster the particles move.

When the temperature of a liquid goes up, the collisions between the molecules are more numerous because they acquire kinetic energy. When the absorbed energy is enough to break free of the Van der Waals' forces, the particles escape into the atmosphere (become a gas). Because the intermolecular forces are different for all liquids, their phase changes will occur at different temperatures. Similarly, when there is a loss of kinetic energy (lower temperature), it becomes more difficult for the molecules to overcome the Van der Waals' forces, and they go through a phase change (liquid to solid). Therefore all physical changes are accompanied by a gain or a loss of energy.

Temperature is a measure of the average kinetic energy of the molecules. The temperature of the pure substance will be closely monitored during the experiment. Temperature changes of the immediate environment will also be noted. To aid in the monitoring of the environment, a water bath will be used.


The experiment is divided into two parts: Part I is the warming of the solid and Part II is the cooling of the liquid substance. The substance to be used is paradichlorobenzene. The chemical formula for paradichlorobenzene is C6H4Cl2.


Procedure: Part I Warming the solid paradichlorobenzene

Prepare the warming bath for the solid paradichlorobenzene. Place 300 mL of water into a 400 mL beaker. Slowly heat the water in the beaker with a bunsen burner. Stop heating the water when the temperature of the water in the warming bath is 65 1 'C. NOTE: When finding the temperature of the water bath, use the thermometer correctly. Immerse a test tube containing 10 grams of paradichlorobenzene and thermometer into the warm water bath. A second thermometer should be in the water bath (see the diagram below).







solid PDCB


hot water


Diagram for the warming of solid paradichlorobenzene.


Every 30 seconds, record the temperature of the paradichlorobenzene. Each minute record the temperature of the water bath. Once the paradichlorobenzene has started to melt, stir vigorously.

Continue recording temperatures and stirring vigorously until the water bath and the paradichlorobenzene are the same temperature.


Part II - Cooling the liquid paradichlorobenzene

Prepare the cooling water bath for the liquid paradichlorobenzene. Place 300 ml, of water at 34 1C, into a 400 mL beaker.

Obtain a test tube containing approximately 10 grams of paradichlorobenzene. The paradichlorobenzene must be a liquid at 60 2C. (When melting the solid paradichlorobenzene, use a bunsen burner. Heat slowly and stir vigorously.) Immerse the test tube containing the liquid paradichlorobenzene and thermometer into the cool water bath, which already contains another thermometer (see the diagram below).








liquid PDCB

cool water


Every 30 seconds, record the temperature of the paradichlorobdhizene. Each minute, record the temperature of the water in the bath. Caution: While stirring the paradichlorobenzene, do not hit the bottom of the test tube. Hitting the bottom could result in a broken thermometer or test tube.

Continue stirring and recording temperatures until the temperatures of the water and paradichlorobenzene are the same.




1.            To aid in the interpretation of the data, graphs will be drawn. Using the data collected in Part I, draw a graph of temperature versus time. (Draw the two curves ‑ one for the water bath and another for the paradichlorobenzene ‑ on one graph.)

2.            Repeat question 1 using the data collected in Part II.

3.            Interpret the graphs for Part I in terms of energy changes.

4.            Repeat question 3 for the graphs of Part II.

5.            In Part 1, relate the energy changes in the water bath to the energy changes in the paradichlorobenzene.

6.            Repeat question 5 for Part II.

7.            What is the melting temperature of paradichlorobenzene?

8.            What is the freezing temperature of paradichlorobenzene?



1)            Does temperature give a true indication of the energy content of a substance?

2)            Describe, on the molecular level, what happens when paradichlorobenzene melts? ..freezes?