Question

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.

Short answer

Cooking with water is faster in a pressure cooker because the increased pressure raises the boiling point of water, allowing for faster heat transfer to the food. Salt is used on icy roads because it lowers the freezing point of water, melting the ice and creating safer driving conditions. Melted sea ice from the Arctic Ocean produces fresh water because the ice crystals trap only water molecules, excluding most of the salt ions. Dry ice does not have a normal boiling point under normal atmospheric conditions, but CO₂ exists in liquid form in fire extinguishers due to high pressure; when the pressure is released, it becomes gaseous to extinguish the fire. Adding a solute to a solvent extends the liquid phase over a larger temperature range by lowering the freezing point and increasing the boiling point of the solvent.

Step-by-step solution

Understanding Pressure Cookers

Pressure cookers work by increasing the pressure inside the pot, which in turn raises the boiling point of water. At higher pressure, water boils at a higher temperature than at normal atmospheric pressure.

Effects of Temperature on Cooking

Higher temperatures facilitate faster heat transfer to the food being cooked. This means the food will cook more quickly at the higher temperatures inside a pressure cooker compared to an open pan, where water boils at its normal boiling point (100°C or 212°F). #b. Salt is used on icy roads.#

Freezing Point Depression

When salt is spread on icy roads, it mixes with the ice and lowers the freezing point of water. This process is called freezing point depression. As a result, the ice starts to melt even though the temperature remains constant.

Effects on Road Safety

The melting of ice reduces the risk of accidents by creating safer driving conditions, as it provides better grip for vehicle tires and increases road visibility. #c. Melted sea ice from the Arctic Ocean produces fresh water.#

Sea Ice and Saltwater

Sea ice forms from the freezing of saltwater, but as it freezes, the ice crystals trap only water molecules, excluding most of the salt ions. Therefore, the sea ice has a much lower salt concentration than the surrounding saltwater.

Melting of Sea Ice

When this ice melts, the resulting water has much less salt concentration than the surrounding seawater, making it effectively 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.#

Sublimation of Dry Ice

Dry ice is the solid form of carbon dioxide. Under normal atmospheric pressure, dry ice doesn't have a liquid phase; instead, it undergoes sublimation, which means that it directly transitions from a solid to a gas.

Carbon Dioxide in Fire Extinguishers

In fire extinguishers, \(\mathrm{CO}_2\) is under high pressure, which allows it to exist in a liquid form. When the pressure is released, the liquid carbon dioxide rapidly expands and cools, creating a cloud of gaseous CO₂ that can put off the fire. #e. Adding a solute to a solvent extends the liquid phase over a larger temperature range.#

Freezing Point Depression and Boiling Point Elevation

Adding a solute to a solvent will lower the freezing point and increase the boiling point of the solvent. This process is called freezing point depression and boiling point elevation, respectively. This means that the temperature range for the liquid phase is broader when a solute is added to the solvent.

Examples of Everyday Applications

These concepts apply in various situations, such as adding antifreeze to car radiator water to avoid freezing in winter temperatures, or adding salt to water when cooking pasta to increase the boiling temperature and cook the pasta faster.