Decision Making

Place one or two ice cubes in a beaker of water. Carefully pour water into the beaker until the water is Òheaped upÓ due to the surface tension of the water. Ask the students whether or not the water will overflow and run onto the table top when the ice cubes melt. As the ice cubes melt, lead a discussion of ÒWhat?Ó the students think will happen and ÒWhy?Ó The answer will become self-evident as the cubes melt.

Assessment

1. List the two condensed states of matter and describe each state.

The two condensed states of matter are the solid state and the liquid state. Solids and liquids, unlike gases, are not compressible since the particles in solids and liquids are drawn closely together by quite strong, short range attractive forces.

The solid state is that phase of matter with the lowest total energy. Solid particles are arranged in a regular pattern called a crystal lattice, which allows the precise prediction of the position of nearly all other particles in the solid by knowing the positions of only a few particles. Because of the strong attractive forces in solids, solids have both definite shapes and definite volumes.

When enough energy is added to the solid state, the very strong attractive forces areovercome andthecrystallattice isdisrupted,causingthe materialtomelt.The liquid state is the result of this process. In the liquid state, the particles possess more potential energy and more rotational and translational kinetic energy. The particles in a liquid are thought to be in close contact with one another, with nearest neighbors arranged around one another in fairly regular ways. The particles can be envisioned as moving around in Òclumps.Ó The arrangement is certainly not as regular as in a solid and the attractive forces in a liquid are not as strong. This accounts for the fact that liquids do not have a definite shape.

2. Four samples of the same material were measured.Usethefollowing data (Figure 18) to determine the density of a material.



Figure 18. Sample data for density determination.

According toArchimedesÕprinciple,a bodyimmersedina fluidlosesanamount of weight equal to the weight of the fluid displaced. Water has a density of 1.00 g/cm 3 . Thus,everygramof reductionintheweight duetothisArchimedesÕ buoyancyeffectrepresents avolumeof1.00cm 3 for the immersed object.Below is an extended data chart revealing the densities of the four test objects.



Figure 19. Density of objects.

NOTE: The density of the samples was calculated by dividing the mass of the object in air by the volume of the sample as determined by water displacement. For example, since D = m/V, the density of the first sample would be D = 17.78 g/ 6.47 cm 3 or 2.75 g/cm 3 . The average density of these four test samples is 2.70 g/cm 3 . These data represent trials conducted on aluminum metal. A valuable addition to this test question would be to ask the students to determine the average density, and to predict what the unknown metal is by using a table of densities from a textbook or suitable handbook.

3. Describe the physical and energy changes that take place as water vapor condenses and then freezes.

The particles in water vapor (the gaseous state) are in random motion. As heat energy is removed from the water vapor molecules, their velocities decrease and they collide less violently. At a certain point, the energies in the collisions are reduced enough that the attractive forces exerted on one another by the vapor molecules cause those molecules begin to ÒclumpÓ together. This is the condensation point. During the condensation process, the vapor molecules move closer together and a great deal of potential energy is released. This potential energy is known a latent heat of vaporization. During the condensation process, the kinetic energy of the system is not reduced, thus the temperature remains constant. The flat portion of the characteristic cooling curve for water is formed during this process.

Once all of the water vapor has condensed into liquid water, the molecules continue to lose heat energy as the clumps of molecules slow down, thus decreasing the amount of translational and rotational kinetic energy. Since the average kinetic energy is decreased during this process, the temperature will fall. The ÒslantedÓ portion of the characteristic cooling curve for water is formed during this process.

When the ÒclumpsÓ of water molecules slow sufficiently, the attractive forces cause the water molecules to be arranged in a regular crystalline lattice. The average kinetic energy will not decrease. The latent heat of fusion released at this point comes from a decrease in potential energy representing the change of the water molecules from the liquid state to the regular arrangement in the ice crystal lattice. The second flat portion of the characteristic cooling curve for water is formed during this energy release.

Once the water is a solid, it has very little potential energy remaining. It is in a very regular crystal structure. The major portion of its remaining energy is in the form of kinetic energy which the water would lose as it is cooled toward absolute zero. The final slanted portion of the characteristic cooling curve for water would be formed during this energy release.

4. 5-mL water at 25 °C are injected into an evacuated, sealed, insulated 1-L container. Describe the changes that are expected. Provide suitable explanations for these changes.

A portion of the water evaporates until the pressure of water vapor inside the container reaches the equilibrium vapor pressure. Once this pressure is reached, a dynamic equilibrium is maintained. Some water molecules evaporate while others condense; the total amount in the vapor state remains constant, and the total amount in the liquid state remains constant.

5. Provide a semiquantitative prediction of the boiling temperature of water at a pressure of 1.05 atmospheres. Explain your prediction.

At 1.05 atmospheres, the boiling temperature of water will be higher than 100°C. As the external pressure is increased, the temperature at which boiling takes place (vigorous evaporation throughout the body of the liquid) is increased.

6. Predict whether water or methanol has a higher vapor pressure at 25 °C. Provide an explanation for your prediction.

Check your prediction using a handbook. If your prediction was incorrect, try to see if you can provide an alternative explanation for the effect. Explain what might have been incorrect with your first explanation.

Methanol has a lower normal boiling point than water and is expected to have a higher vapor pressure at room temperature. From a phenomenological perspective, methanol is closer to boiling at room temperature than is water, so its vapor pressure is higher. The underlying explanation is usually given in terms of the more extensive hydrogen bonding available to water as compared to methanol, thus making evaporation (overcoming attractive forces) from methanol an inherently easier process than evaporation from water.

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