1. Solids and liquids occupy roughly 1/1000th the volume of the corresponding gases. Determine the ratio of the distance between solid or liquid particles to the corresponding distance between gaseous particles? [The solid and liquids are about 10 times closer together. Volume is length cubed. Particles moving 10 times closer in all of their linear dimensions actually move into 1/1000th as much volume.]
2. State the physical evidence that supports the idea that particles in both solids and liquids are very close together. [Neither solids nor liquids are very compressible, hence there is little space between their particles. Gases, on the other hand, are relatively easily compressed.]
3. Criticize the statement ÒWater boils at 100 °C.Ó [Water only boils at its ÒnormalÓ boiling point if the sample is pure water under one atmosphere of pressure. Atmospheric pressure variation and/or impurities in the water cause the boiling point to differ from the norm.]
4. Describe similarities and differences among the warming curves of different crystalline substances. [All warming curves of crystalline materials have portions sloped upward to the right, indicating the speeding up of solid, liquid, or gaseous particles (increasing kinetic energy). All have flat portions, indicating change of phase of the particular substances (increasing potential energy). The slopes for individual materials will vary, indicating differences in energy necessary to speed up the particles (specific heat). The lengths of the flat portions will vary, indicating differences in the amount of energy necessary to overcome attractive forces within the substances. The flat portions will occur at different temperatures, indicating that substances have different melting and boiling points.]
5. State the temperature effect observed during the addition of heat to a substance at its melting point. [Until all solid melts, the temperature remains the same. At the melting point, added heat is utilized to separate particles and increase potential energy rather than to speed up the particles of the substance.]
6. Many scientists believe that earthÕs inner core is solid while its outer core is liquid. State conditions that might make this possible even though the inner core is thought to be at a higher temperature than the outer core? [The inner core is under tremendous pressure. The particles in the inner core are pressed so tightly together that they are bonded into a solid crystal lattice even though the kinetic energy is such that the material would be liquid at lower pressure.]
7. Suppose an insulated container held a solid/liquid mixture of a certain substance at its melting point so that heat neither entered nor left the container. Predict what would be observed. [Melting and freezing would continue at equal rates. A dynamic equilibrium would be established. The heat needed to melt the substance would be released as an equal amount of the substance would freeze. The amount of solid and liquid would remain constant even though melting and freezing would continue unabated. (Although the size of the solid ÒchunkÓ would not change, its shape would change over time.)]
8. Explain the loss of solid and liquid lubricants in devices sent into outer space. [Even though liquid and solid greases have extremely low vapor pressures, the prevailing pressure in outer space is below this pressure, and thus the lubricant will boil away violently.]
9. Compare the distance between solid and liquid particles. [The distances between liquid particles and solid particles are nearly the same: both liquids and solids are essentially incompressible, and there is little expansion as substances go from the solid to liquid phase.]
10. Why are liquids, but not solids or gases, utilized in hydraulic systems such as automobile brakes? [Solids arenÕt fluid and cannot flow throughout the hydraulic system. Gases are fluid and can flow throughout the hydraulic system, but since gases are extremely compressible, they cannot transfer the pressure.]
11. At the melting point of a substance, it consists of a mixture of solid and liquid portions. State the molecular differences expected between the solid and liquid portions. [The solid particles must be distinguished from liquid particles by observing the differences in their motions. Since both the solid and liquid particles are at the same temperature, they have the same average kinetic energy but may be distinguished by differences in the amount of translational, rotational, and lattice vibrational kinetic energy. The liquid particles possess potential energy because they are farther apart when compared to particles in the solid phase.]
12. Motor oil has a low vapor pressure compared to water. Compare the intermolecular forces in these materials. Predict which liquid has the higher boiling point. [The fact that motor oil has a low vapor pressure indicates that the attractive forces between the relatively large oil molecules are quite great. Few of these molecules escape easily into the gaseous state, thus the low vapor pressure. Since the vapor pressure is low for motor oil, one would have to heat the oil to a high temperature to increase its vapor pressure to atmospheric pressure. Oil boils at a higher temperature than water.]
13. It is possible to place warm water in a syringe and to cause the water to boil by pulling out on the plunger of the syringe. Explain the physical changes that make this possible. [As the plunger is drawn out, pressure in the syringe drops low enough that it is below the vapor pressure of the warm water. The water begins to boil.]
14. Which is more dense, milk or cream? Support your answer. [Milk is more dense than cream; cream floats on milk. (Students frequently confuse creamÕs higher viscosity with density.)]
15. Nurseryworkers ÒmistÓ their bedding plants with water on cold spring nights when frost is predicted. Describe the process that protects the plants. [As the misting water cools and freezes, a great deal of heat is released into the surrounding atmosphere. This heat keeps the ambient temperature higher than it would be without the cooling/freezing water, thus protecting the tender plants.]
16. Describe the molecular changes that cause the potential energy of a substance to increase when that substance is melted or boiled? [The increase in potential energy at the melting or boiling point is caused by overcoming attractive forces and moving particles farther apart. Gases have the potential for attracting one another and moving closer together thus decreasing the amount of potential energy between the particles as the substance liquifies. This decrease in potential energy manifests itself as an increase in heat flow out of the system.]
17. Describe the molecular cause of the expansion characteristics of water near its freezing point? [Hydrogen bonding in the polar water molecules produce an open hexagonal crystal lattice causing water to expand during the phase change.
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