How is boiling point affected by pressure

Even in the coldest of winter, snow and ice can evaporate by a process called sublimation. They can turn straight into vapor the gaseous stage without melting. The vapor that escaped into the air whether by evaporation or sublimation had sufficient energy to break away from the intermolecular forces of the water. The vapor molecules are in constant motion exerting a force on the container or the air around. This force is termed vapor pressure.

For example, in the Denver, Colorado area of the U. The relationship between vapor pressure and temperature or, looked at in the reverse direction, between boiling temperature and pressure is called the vapor pressure curve. Water's vapor pressure curve is of great importance, since it determines the relationship between temperature and pressure in any process where water is going from a liquid to a vapor state or vice-versa. Such processes are important both in industry for example in steam power generation where water is made into steam in boilers and eventually condensed after running through turbines and in nature for example, in rainfall and evaporation from bodies of water.

It requires energy to change from a liquid to a gas see enthalpy of vaporization. In addition, gas molecules leaving the liquid remove thermal energy from the liquid. Therefore the temperature of the liquid remains constant during boiling. A graph of temperature vs. As long as there is liquid the temperature will remain constant. The pressure of gas above a liquid affects the boiling point. In an open system this is called atmospheric pressure.

The greater the pressure, the more energy required for liquids to boil, and the higher the boiling point. In an open system this can be visualized as air molecules colliding with the surface of the liquid and creating pressure. This pressure is transmitted throughout the liquid and makes it more difficult for bubbles to form and for boiling to take place. If the pressure is reduced, the liquid requires less energy to change to a gaseous phase, and boiling occurs at a lower temperature.

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