SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Liquids, Solids, and Phase Changes
These skills are usually tested on the SAT Subject Test in Chemistry. You should be able to …
• Explain, using a graph, the distribution of the kinetic energy of molecules of a liquid at different temperatures.
• Describe the states of matter and what occurs when a substance changes state.
• Define critical temperature and pressure.
• Analyze a phase diagram and the triple point.
• Solve water calorimetry problems that include changes of state.
• Explain the polarity of the water molecule and hydrogen bonding.
• Solve solubility problems, concentration problems, and changes in boiling point/freezing point of water problems.
• Describe the continuum of water mixtures including solutions, colloids, and suspensions.
This chapter will review and strengthen these skills. Be sure to do the Practice Exercises at the end of the chapter.
Liquids and solids each have their own properties, including intermolecular interactions, surface tension, and more. In fact, one very important compound— water—has distinct properties necessary for life to exist on this planet.
Importance of Intermolecular Interaction
A liquid can be described as a form of matter that has a definite volume and takes the shape of its container. In a liquid, the volume of the molecules and the intermolecular forces between them are much more important than in a gas. When you consider that in a gas the molecules constitute far less than 1% of the total volume, while in the liquid state the molecules constitute 70% of the total volume, it is clear that in a liquid the forces between molecules are more important. Because of this
Figure 22. Distribution of the Kinetic Energy of Molecules
decreased volume and increased intermolecular interaction, a liquid expands and contracts only very slightly with a change in temperature and lacks the compressibility typical of gases.
An increase in temperature increases the average kinetic energy of the molecules.
Kinetics of Liquids
Even though the volume of space between molecules has decreased in a liquid and the mutual attraction forces between neighboring molecules can have great effects on the molecules, they are still in motion. This motion can be verified under a microscope when colloidal particles are suspended in a liquid. The particles’ zigzag path, called Brownian movement, indicates molecular motion and supports the Kinetic-Molecular Theory.
Increases in temperature increase the average kinetic energy of molecules and the rapidity of their movement. This is shown graphically in Figure 22. The molecules in the sample of cold liquid have, on the average, less kinetic energy than those in the warmer sample. Hence the temperature reading T1 will be less than the temperature reading T2. If a particular molecule gains enough kinetic energy when it is near the surface of a liquid, it can overcome the attractive forces of the liquid phase and escape into the gaseous phase. This is called a change of phase. When fast-moving molecules with high kinetic energy escape, the average energy of the remaining molecules is lower; hence the temperature is lowered.
Viscosity is the friction or resistance to motion that exists between the molecules of a liquid when they move past each other. It is logical that the stronger the attraction between the molecules of a liquid, the greater its resistance to flow—and thus the greater its viscosity. The viscosity of a liquid depends on its intermolecular forces. Because hydrogen bonds are such strong intermolecular forces, liquids with hydrogen bonds tend to have high viscosities. Water, for example, is strongly hydrogen bonded and has a relatively high viscosity. You may have noticed how fast liquids with low viscosity, such as alcohol and gasoline, flow.
More viscous liquids move more slowly.
Molecules at the surface of a liquid experience attractive forces downward, toward the inside of the liquid, and sideways, along the surface of the liquid. On the other hand, molecules in the center of the liquid experience uniformly distributed attractive forces. This imbalance of forces at the surface of a liquid results in a property called surface tension. The uneven forces make the surface behave as if it had a tight film stretched across it. Depending on the magnitude of the surface tension of the liquid, the film is able to support the weight of a small object, such as a razor blade or a needle. Surface tension also explains the beading of raindrops on the shiny surface of a car.
Capillary action, the attraction of the surface of a liquid to the surface of a solid, is a property closely related to surface tension. A liquid will rise quite high in a very narrow tube if a strong attraction exists between the liquid molecules and the molecules that make up the surface of the tube. This attraction tends to pull the liquid molecules upward along the surface against the pull of gravity. This process continues until the weight of the liquid balances the gravitational force. Capillary action can occur between water molecules and paper fiber, causing the water molecules to rise up the paper. When a water soluble ink is placed on the paper, the ink moves up the paper and separates into its various colored components. This separation occurs because the water and the paper attract the molecules of the ink components differently. These phenomena are used in the separation process of paper chromatography, as shown in the paper chromatography experiment on. Capillary action is at least partly responsible for the transportation of water from the roots of a plant to its leaves. The same process is responsible for the concave liquid surface, called a meniscus, that forms in a test tube or graduated cylinder.