Explain the following on the basis of the behavior of atoms...

Explain the following on the basis of the behavior of atoms and/or ions. a. Cooking with water is faster in a pressure cooker than in an open pan. b. Salt is used on icy roads. c. Melted sea ice from the Arctic Ocean produces fresh water. d. $\mathrm{CO}_{2}(s)$ (dry ice) does not have a normal boiling point under normal atmospheric conditions, even though $\mathrm{CO}_{2}$ is a liquid in fire extinguishers. e. Adding a solute to a solvent extends the liquid phase over a larger temperature range.

Explain the following on the basis of the behavior of atoms and/or ions.
a. Cooking with water is faster in a pressure cooker than in an open pan.
b. Salt is used on icy roads.
c. Melted sea ice from the Arctic Ocean produces fresh water.
d. $\mathrm{CO}_{2}(s)$ (dry ice) does not have a normal boiling point under normal atmospheric conditions, even though $\mathrm{CO}_{2}$ is a liquid in fire extinguishers.
e. Adding a solute to a solvent extends the liquid phase over a larger temperature range.

Key Concepts

Pressure and Boiling Point

An increase in pressure in a closed system, such as in a pressure cooker, raises the temperature at which a liquid boils. At a molecular level, higher pressure forces molecules closer together, making it necessary for the water molecules to reach a higher kinetic energy (temperature) to overcome the atmospheric pressure and enter the vapor phase. This results in faster cooking as the water (and steam) is at a higher temperature, speeding up chemical reactions involved in cooking.

Freezing Point Depression

The addition of a solute such as salt interferes with the ability of solvent molecules (water in this context) to form an orderly crystal lattice, thereby lowering the freezing point of the solution. This molecular disruption arises because the dissolved ions interfere with the interactions required for the formation of a solid phase, allowing ice to melt at lower temperatures. This explains why salt is spread on icy roads to reduce ice formation.

Selective Crystallization and Brine Rejection

During the freezing process of sea water, the arrangement of water molecules into a solid lattice excludes the dissolved salt ions, which remain in the liquid phase. This selective crystallization means that the ice formed is essentially fresh water, as the impurities are left behind. This process highlights how ionic impurities are not incorporated into the crystal lattice of ice, leading to a natural separation of pure water from saline water.

Sublimation and Phase Transitions in CO2

Under standard atmospheric conditions, solid carbon dioxide (dry ice) does not pass through a traditional liquid phase because the conditions required for liquid CO2 (higher pressures) are not met. Instead, it undergoes sublimation, directly transitioning from solid to gas. This behavior is a result of molecular interactions and phase stability as described by the phase diagram of CO2, which shows that a liquid phase exists only under elevated pressures.

Colligative Properties

When a solute is added to a solvent, it induces colligative effects, such as boiling point elevation and freezing point depression, which depend on the number of solute particles rather than their identity. On the molecular level, the presence of solute particles distorts the regular interactions among solvent molecules, thereby altering the energy required to change phases. This results in an extended liquid phase over a larger temperature range, as more energy is needed to overcome the disruption caused by the solute.

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