A beaker of water at room temperature is placed in an enclosure, and the air pressure in the enclosure is slowly reduced. When the air pressure is reduced sufficiently, the water begins to boil. The temperature of the water does not rise when it boils; in fact, the temperature ?drops? slightly. Explain these phenomena.
Solution 28DQ Step 1 of 5: The boiling point is the temperature at which a liquid’s vapor pressure is equal to the atmospheric pressure pushing down on it. For water on Earth, the standard boiling point of water is given to be 100 °C. This temperature assumes that the water has one atmosphere of pressure pushing down on it. When this pressure is decreased, the temperature at which water can boil will decrease. Step 2 of 5: To look at is another way, we can at water at the (simplified) molecular level. Water, or any liquid, is a collection of molecules bound by weak intermolecular forces. As long as a molecule is not at absolute zero, it will have some kinetic energy. This means it will be constantly bouncing around. Inside the liquid’s volume, the molecules will constantly bump into one another and exchange energy. Due to conservation of energy, all of the molecules will have about the same average kinetic energy. This average kinetic energy is a good description of the temperature of a material; the higher the average kinetic energy, the higher the temperature. As we know from the Ideal gas law, PV=nRT Where P, V and T are the pressure , volume and temperature of the gas respectively, R is the universal gas constant and n is the number of moles of gas. Step 3 of 5: From above equation, Temperature and pressure are directly related. Once the molecules of water have enough energy to break free of the intermolecular forces they can escape the liquid as a whole or form bubbles inside the liquid as shown in the figure below. The pressure they have to overcome from these forces and any atmospheric pressure is the vapor pressure of the liquid.