(a) Two pans of water are on different burners of a stove. One pan of water is boiling vigorously, while the other is boiling gently. What can be said about the temperature of the water in the two pans?

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Understand that boiling occurs when a liquid's vapor pressure equals the atmospheric pressure.

Recognize that the boiling point of water at standard atmospheric pressure is 100°C (212°F).

Realize that once water reaches its boiling point, additional heat does not increase the temperature but instead converts more water into steam.

Acknowledge that both pans of water, whether boiling vigorously or gently, are at the same temperature, which is the boiling point of water.

Conclude that the difference in the intensity of boiling is due to the rate of heat being supplied, not a difference in temperature.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure, causing it to change from liquid to gas. For water, this occurs at 100°C (212°F) at standard atmospheric pressure. Regardless of the intensity of boiling, once water reaches its boiling point, its temperature remains constant until all the liquid has vaporized.

Heat Transfer

Heat transfer is the process of thermal energy moving from one object to another due to a temperature difference. In the context of boiling water, the burner provides heat to the pan, causing the water to absorb energy. The vigorous boiling indicates a higher rate of heat transfer, but the temperature of the water remains at the boiling point, regardless of the boiling intensity.

Phase Change

A phase change occurs when a substance transitions between solid, liquid, and gas states. During boiling, water undergoes a phase change from liquid to gas. This process requires energy, known as the heat of vaporization, and occurs at a constant temperature, meaning that both pans of water, despite differing boiling intensities, are at the same temperature when boiling.

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Textbook Question

(a) Place the following substances in order of increasing volatility: CH₄, CBr₄, CH₂Cl₂, CH₃Cl, CHBr₃, and CH₂Br₂. (b) How do the boiling points vary through this series? (c) Explain your answer to part (b) in terms of intermolecular forces.

Textbook Question

You are high up in the mountains and boil water to make some tea. However, when you drink your tea, it is not as hot as it should be. You try again and again, but the water is just not hot enough to make a hot cup of tea. Which is the best explanation for this result? (a) High in the mountains, it is probably very dry, and so the water is rapidly evaporating from your cup and cooling it. (b) High in the mountains, it is probably very windy, and so the water is rapidly evaporating from your cup and cooling it. (c) High in the mountains, the air pressure is significantly less than 1 atm, so the boiling point of water is much lower than at sea level. (d) High in the mountains, the air pressure is significantly less than 1 atm, so the boiling point of water is much higher than at sea level.

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Textbook Question

Using the vapor-pressure curves in Figure 11.24, (d) estimate the external pressure at which diethyl ether will boil at 40 °C.

Textbook Question

Appendix B lists the vapor pressure of water at various external pressures. (c) A city at an altitude of 5000 ft above sea level has a barometric pressure of 633 torr. To what temperature would you have to heat water to boil it in this city?

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