Problem 95 - Chemistry An Atoms First Approach

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.

Step-by-Step Solution

Answer

a. Cooking is faster in a pressure cooker because the increased pressure raises water's boiling point, allowing it to heat up more and cook food faster. b. Salt is used on icy roads because it lowers the melting point of ice by disrupting its crystalline structure, making the roads less slippery. c. Melted sea ice produces fresh water because when the ice forms, salt ions are expelled from the ice lattice, resulting in a lower concentration of salt ions in the ice. d. Dry ice does not have a normal boiling point under normal atmospheric conditions because it sublimates, transitioning directly from solid to gas. In fire extinguishers, high pressure forces \(\mathrm{CO}_{2}\) into a liquid state. e. Adding a solute to a solvent extends the liquid phase over a larger temperature range by increasing the boiling point and decreasing the freezing point due to the interference of solute particles with the solvent's cohesive forces.

Step 1: a. Cooking with water is faster in a pressure cooker than in an open pan.

This phenomenon occurs due to the change in boiling point of water under increased pressure. In a pressure cooker, the pressure inside the container increases, causing water molecules to require more energy to escape the liquid phase and turn into steam. The increased pressure results in an increased boiling point, which in turn, allows the water to heat up more and cook food faster.

Step 2: b. Salt is used on icy roads.

Salt is used on icy roads to decrease the melting point of ice, ensuring that the ice melts at a lower temperature than normal. When salt is added to ice, it disrupts the crystalline structure of the ice, making it more difficult for the water molecules to bond together in a solid arrangement. This causes the ice to melt at a lower temperature, which helps to clear the roads and prevent accidents due to slippery conditions.

Step 3: c. Melted sea ice from the Arctic Ocean produces fresh water.

When sea ice forms, salt ions are expelled from the ice lattice and are left behind in the surrounding water, resulting in a lower concentration of salt ions in the ice compared to the water. As a result, when sea ice melts, the water produced is relatively fresh, with a much lower concentration of salt ions compared to the surrounding seawater.

Step 4: 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.

This is because dry ice sublimates, meaning it transitions directly from solid to gas without passing through the liquid phase under normal atmospheric pressure. The behavior of \(\mathrm{CO}_{2}\) in fire extinguishers is due to the high pressure in the container, which forces the gas into a liquid state. When the pressure is released, the liquid \(\mathrm{CO}_{2}\) quickly turns into a gas, with some of it freezing into dry ice due to rapid cooling.

Step 5: e. Adding a solute to a solvent extends the liquid phase over a larger temperature range.

When a solute is added to a solvent, the solute particles interfere with the cohesive forces between the solvent molecules. This means that the solvent molecules need more energy to escape the liquid phase and transition into the gas phase, resulting in an increased boiling point. Similarly, the presence of solute particles makes it more difficult for the solvent molecules to organize themselves into a solid structure, leading to a decreased freezing point. This results in an extended liquid phase over a broader temperature range.