QUIZ The Earth’s Interior and Plate Tectonics



Scientific Background on Freezing/Melting PointLiquid water consists of billions of molecules in constant random motion. Each water molecule (H2O) is polar—there is a slight negative charge on the oxygen atom and a slight positive charge on each hydrogen atom. Because opposite charges attract, there is a slight attraction between a hydrogen atom of one molecule and the oxygen atom of another. If molecules pass by each other or collide at low enough velocities, they will tend to stick together. A crystalline structure forms as more molecules join the group; this is freezing. The crystalline structure of water is shown in figure A. The temperature of a substance is a measure of the kinetic energy contained in its molecules. Average molecular speed increases as temperature increases, and vice versa. As water is cooled, the molecules slow down and the rate at which molecules stick together, or the water-to-ice transformation rate, increases. At the same time, the rate at which nascent ice crystals are broken up, or the ice-to-water transformation rate, decreases. The freezing point is the temperature at which these rates are equalized. As room temperature is decreased below the freezing point, the liquid water will gradually freeze into a solid block of ice.When table salt (NaCl) is dissolved in water, the salt crystals dissociate into individual sodium (Na+) and chlorine (Cl–) ions. These ions are represented by the blue and green spheres in figure B. The salt ions collide with the water molecules, but do not join into the ice crystals (shown on the top half of figure B). The greater the number of solute particles dissolved in the water, the greater the number of collisions that do not involve water molecules becoming frozen. Because many of the water-water collisions are replaced by water-solute collisions, the rate at which water molecules are “captured” into the ice decreases, and the water-to-ice rate decreases as well. As a result of the decrease in the water-to-ice rate, the temperature at which the water freezes will decrease as well. This is called freezing point depression. The amount that the freezing point is lowered depends on the concentration of solute. Seawater has a freezing point of approximately -2 °C. Water that is completely saturated with salt has a freezing point of -20 °C. Freezing point depression is an example of a colligative property, a property that depends on the concentration of solute particles. Like other colligative properties, the freezing point depression will be the same regardless of the solute. An equal number of sucrose, KCl, or CaCl2 particles will yield the same freezing point depression as NaCl. (It should be noted that an equivalent amount of sucrose will yield only half as many particles as NaCl because sucrose does not dissociate when dissolved in water.) Adding salt to water also has the effect of raising the boiling point (boiling point elevation). The reason for this is similar to the reason for freezing point depression. In pure water at temperatures below the boiling point (100 °C at sea level), a certain fraction of molecules will be moving fast enough to escape the surface of the liquid and enter the gas phase. The gas produced in this way exerts a certain pressure, called vapor pressure. Boiling occurs when the vapor pressure is equal to atmospheric pressure. If solute particles are added to the water, the presence of solute particles on the surface of the liquid reduces the number of water molecules escaping into the gas phase, and vapor pressure is reduced. This raises the temperature required to initiate boiling. ................
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