# Chapter 13 - Temperature and Kinetic Theory - Questions: 20

a. A pressure cooker is sealed at a set volume and contents. The pressure inside allows higher temperatures to be reached by the liquid contents (i.e. water) before vaporizing. b. At higher altitudes the lower pressure also lowers the boiling point of water, meaning the pasta or rice is actually being cooked at a lower temperature. This requires it to be cooked longer. c. No, the lower boiling point makes it easier to boil water.

#### Work Step by Step

a. The sealed pressure cooker, with fixed volume and contents, experiences a rise in pressure as the temperature increases (see the ideal gas law). It is a good assumption that there is water present. From Figure 13-22, we see that the boiling point of water increases at higher pressure. So, the water boils at a temperature higher than the normal $100^{\circ}C$, which cooks the contents faster. b. At high altitudes, the atmospheric pressure is less than the standard value at sea level. From Figure 13-22, we see that the boiling point of water decreases at lower pressure. So, the water boils at a temperature lower than the normal $100^{\circ}C$, which cooks the pasta more slowly. c. No, it is actually easier to boil water at high altitude. From Figure 13-22, we see that the boiling point of water decreases at lower pressure. It takes less heat to bring water to the (lower) boiling temperature.

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