Question

Explain why the lower vapor pressure for a solution containing a nonvolatile solute results in a higher boiling point and lower melting point compared to the pure solvent.

Short answer

A nonvolatile solute lowers the vapor pressure of a solution which means a higher temperature (higher than the solvent's boiling point) is needed for the vapor pressure to match atmospheric pressure, elevating the boiling point. Similarly, a lower temperature (lower than the solvent's freezing point) is needed to reach equilibrium between the solid and liquid phases, depressing the freezing point.

Step-by-step solution

Understanding Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid (or solid) phase at a given temperature. For a pure solvent, the vapor pressure is determined by the tendency of its molecules to escape from the liquid into the gas phase. When the vapor pressure reaches the external atmospheric pressure, the liquid starts to boil.

Effect of Nonvolatile Solute on Vapor Pressure

When a nonvolatile solute is dissolved in a solvent, it reduces the solvent's vapor pressure. This occurs because the solute particles occupy space at the surface of the liquid and hinder the evaporation of solvent molecules, thus reducing the number of solvent molecules that can escape to the vapor phase at any given temperature.

Boiling Point Elevation

The boiling point of a liquid is the temperature at which its vapor pressure equals the external atmospheric pressure. Since a solution with nonvolatile solute has a lower vapor pressure, it requires a higher temperature to reach the atmospheric pressure. Therefore, the boiling point is elevated compared to that of the pure solvent.

Freezing Point Depression

The presence of a nonvolatile solute disrupts the formation of a solid lattice structure that would be required for freezing (or solidifying). This disruption requires the solution to be cooler than the pure solvent before the vapor pressure of the solid and liquid phases are in equilibrium, term known as freezing point depression.

Key concepts

Vapor Pressure

Imagine the surface of a liquid like a busy dance floor where molecules are constantly moving and, occasionally, some have enough energy to leap into the air, turning into a gas. This escaping act creates what we call vapor pressure. It's a bit like the buzz in the room when the music's just right – the better the tune, the more people are jumping. In pure solvents, there's a perfect rhythm allowing the solvent molecules to vaporize at a predictable rate. But, when nonvolatile solute enters the mix, it's like someone walking through the dance floor – they take up space, making it harder for dancers (solvent molecules) to leap into the gaseous state, thus lowering the vapor pressure.

Dive deeper, and you'll see that these solute particles really like to hog the surface area, diminishing the chance of the solvent molecules to escape. The vapor pressure is now lower because there are fewer molecules of the solvent in the air above the liquid, which changes the dynamic of the whole 'dance' between liquid and gas.

Boiling Point Elevation

Picture boiling as the moment the party gets so lively that everyone is jumping around – that's when a liquid becomes gas. This happens at a temperature where the vapor pressure matches the atmospheric pressure. Now, add our nonvolatile solute to the solvent, which has already been partying a bit too hard and took over the dance floor, making it more difficult for the liquid's molecules to escape. What happens? The temperature has to turn up even more to get the party – or boiling – started. This is why scientists say the boiling point is elevated.

To get technical, the boiling point is like the temperature threshold the liquid needs to overcome to match the pressure of the outside world. When nonvolatile solutes are in the mix, reaching this threshold needs more heat, hence a higher boiling point. It's worth noting that this effect is way more pronounced under a dome of constant atmospheric pressure.

Freezing Point Depression

Now let's switch the analogy: If boiling is like a party, freezing is like trying to get everyone to stand in a perfect grid – it's a structured, orderly affair. The molecules in a liquid lineup to form a solid crystalline structure during freezing. Introducing a nonvolatile solite throws a wrench into this orderly process. It's like having a bunch of people who don't want to follow the rules at a military drill, disrupting the formation. This means the liquid has to get colder before these molecules give up and settle into the solid structure – hence, the freezing point is lower, or depressed.

The technical scoop? Lowering the temperature makes the liquid's vapor pressure drop until it's equal to the vapor pressure of the solid. But with a solute mucking things up, we need to go even colder to achieve this equilibrium vapor pressure between the liquid and solid phases, resulting in freezing point depression.

Nonvolatile Solute

The term 'nonvolatile' might make it sound like these solutes are uninteresting, but they're the life of the chemical party when it comes to changing boiling and freezing points. Nonvolatile refers to substances that don't easily evaporate into a gas under existing conditions. They're like the chaperones at a prom, keeping a low profile while subtly influencing everything around them. In a solution, these nonvolatile solutes are the quiet game-changers. They don't become gas at the same rate, or even at all, compared to the solvent, but they sure know how to throw off the balance, leading to a lower vapor pressure, a higher boiling point, and a lower freezing point.

Why care about these wallflowers? Because the behavior of these nonvolatile solutes is crucial. It's why adding salt to water changes how it boils and freezes, affecting everything from cooking pasta al dente to de-icing roads in winter. They don't fly away like the solvent molecules, but they radically alter the physical properties of the solution they're in, demonstrating the power of being nonvolatile.