Water vapor and latent heat

Here comes seemingly unrelated thread number three. Water molecules can exist together in any of three phases: gas, liquid, or solid. A transformation from liquid or solid to gas requires an input of a considerable amount of energy. One could write a chemical reaction for water as

Vapor = Liquid + Heat

If you have ever burned your skin with steam from a teapot, this reaction will have meaning for you. Steam from a teapot is probably at the boiling temperature of water, 373 K. This is a temperature you might set for your oven to warm bread; you can easily blow the air from the oven on your hand when you reach in for the bread without burning yourself. Steam burns, not because it is so hot, but because it deposits its heat when it condenses on your skin. The heat that the steam is carrying is called latent heat. You charge up an air parcel with latent heat when you evaporate water into it, and you get the heat back again when the water condenses. A thermometer doesn't measure the latent heat content of an air parcel unless the heat is released by condensation.

If you set a tray of water or ice in a closed box with some extra space around it, some of the water molecules are going to evaporate, becoming a gas. Here's an interesting tidbit; it makes almost no difference whether there are other gases in that space or just vacuum. The same number of water molecules will make the jump into the gas phase regardless; that is to say, the pressure arising from water vapor doesn't care how much oxygen or nitrogen is there. The pressure from water vapor depends primarily on the temperature. If the temperature is high, many molecules will have the energy required to jump into the vapor phase, and the vapor pressure of water will be high. If it is cold, the vapor pressure will be lower (Fig. 5.6).

At any given temperature, the water vapor pressure will drift toward some equilibrium pressure. If the amount of water vapor is lower than equilibrium, that is to say undersaturated, water will tend to evaporate. If water vapor is supersaturated, meaning the pressure is higher than equilibrium, then water will tend to condense, perhaps into raindrops or snowflakes. Equilibrium is the state of lowest energy; a dead

Water Vapor And Temperature Chart

270 275 280 285 290 295 300 305 310 Temperature (K)

Fig. 5.6 Water has a greater tendency to evaporate at higher temperature than at a cooler temperature. The graph shows what a chemist would call the equilibrium vapor pressure of water, and a meteorologist would call 100% relative humidity. Air in the real world can have less or sometimes slightly more water vapor than this, but if the water vapor concentration gets too much higher than this curve, the water will condense into water droplets or snow.

270 275 280 285 290 295 300 305 310 Temperature (K)

Fig. 5.6 Water has a greater tendency to evaporate at higher temperature than at a cooler temperature. The graph shows what a chemist would call the equilibrium vapor pressure of water, and a meteorologist would call 100% relative humidity. Air in the real world can have less or sometimes slightly more water vapor than this, but if the water vapor concentration gets too much higher than this curve, the water will condense into water droplets or snow.

battery is in equilibrium. However, a chemical system can be out of equilibrium for a long time if the reaction rates are very slow. In the clean remote marine atmosphere, forming droplets can be very slow, and water vapor 25% supersatured has been observed. The relative humidity is the water vapor pressure divided by the saturation value; from the last example, 25% supersaturated would be 125% relative humidity.

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Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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Responses

  • mariam daniel
    Why latent heat of water is related with climate?
    9 years ago
  • PRINCIPIO
    What will happen if too much water vapor is released?
    9 years ago
  • louis
    Is the equilibrium vapor pressure of water unrelated to temperture?
    8 years ago

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