Vapor pressure and boiling point?

Question

Explain the following observations:

water evaporates more quickly on a hot dry day, than on a hot humid day

it takes longer to cook hard boiled eggs at high altitudes than at lower altitudes

Answer 1

Liquids evaporate or boil at a rate determined by the pressure of whatever gases are in the air above it. Boiling and evaporation are simply the process of individual atoms or molecules of the liquid escaping the liquid phase into the air, and the rate at which this happens is dictated by three things: 1) the vapor pressure of the liquid itself (how much of the liquid is already present in the air, pressing back against the liquid), 2) how much energy is available in the system (a hotter liquid will want to escape into the gaseous phase more readily than if it is cooler, generally speaking), and 3) the atmospheric pressure (how much the air is pressing down upon the liquid, preventing the molecules of liquid from escaping)

So on a hot, dry day, there is very little water in the air and a lot of energy available -- so the vapor pressure of the water is low and the liquid becomes heated by the sun and becomes more energetic. On a hot, humid day (one presumes the same temperature and time of day), the difference is in the vapor pressure of the water -- there is already a lot of water in the air, so not as much can escape the liquid phase as on a dry day.

As far as cooking eggs at high altitudes, there is less atmosphere pressing down on top of your pot of water, so the water molecules can escape more readily into the air. Therefore, you don't have to input as much energy into the water to get it to boil, and it boils at a lower temperature, which means it takes longer for your eggs to cook because you're not pumping as much heat into them as you would down at sea level, when your water boils at 212F (100C, if you prefer)

Answer 2

At higher altitudes water has a lower temperature boiling point due to the lower pressure. Therefore the heat required to make the water boil will not be as great therefore taking longer to cook the egg.

If the temperature is the same then the water would be less likely to evaporate into air that is already saturated with water vapor. Much of the water vapor would probably be condensating back onto the surface of the water at the same time it would be evaporating.

Answer 3

well u have to understand that drying is a mass tranfer process (water transfer from "whatever to air")...in fact, in this process the greater is the temperature of the air, the greater is the capacity of the air to carry water...it is a form easy to explain this

The second question is easy to explain with the concept of pressure...when u are at high altitudes the pressure is lower, then the molecules are united with a lower force, the boiling point of water decreases. This process produce that the water to cook is to a smaller temperature and then, u need more time to cook eggs or something

Answer 4

Vapour pressure is dependent on temperature. All substances have a vapour pressure. Vapour pressure is present at all temperatures. The vapour pressure of a substance depends upon its temperature. As temperature increases, vapour pressure increases and vice-versa. At any SPECIFIC temperature, the substance and its vapour are in 'Equilibrium' ...i.e. the number of molecules entering the vapour space is equal to the number returning to the substance. When the VAPOUR PRESSURE of a liquid is equal to the surrounding pressure, the liquid is at its BOILING TEMPERATURE. e.g. at 100°C, water has a vapour pressure equal to the atmospheric pressure of 1atm acting upon it.