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  3. When the temperature of a liquid is raised enough, the liquid boils. This means that bubbles of vapor, containing millions of mo

When the temperature of a liquid is raised enough, the liquid boils. This means that bubbles of vapor, containing millions of mo

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问题     When the temperature of a liquid is raised enough, the liquid boils. This means that bubbles of vapor, containing millions of molecules, form below the surface. In order for such bubbles to be produced, the pressure of the vapor inside them must be equal to the pressure of the air upon the surface of the liquid. If the air pressure is greater, the bubble will collapse. The boiling point of a liquid, then, is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure upon the liquid at the surface.
    The pressure of the atmosphere at sea level is about one kilogram per square centimeter. We call this pressure one atmosphere. The boiling point of a liquid, therefore, is the temperature at which its vapor pressure equals one atmosphere. The boiling point of water is 100 Celsius; of ethyl alcohol 78.5 Celsius; of mercury, 356.9 Celsius.
    If the pressure on the surface of the water is reduced, the boiling point will be lower. Now the higher up we go from sea level, the lower the atmospheric pressure becomes, since there is less air to press upon us. Hence, while water boils at sea level at a temperature of 100 Celsius, its boiling point on a mountain three kilometers high is only 90 Celsius. Since the temperature of water does not rise as a result of boiling, it would take you longer to cook potatoes or carrots in boiling water on the mountain than it would at sea level.
The liquid boils only when ______.
选项 A、the temperature is raised
B、the bubbles of vapor form below the surface
C、the temperature of the liquid is raised enough
D、the temperature of the liquid is near 100 Celsius
答案C
解析 本题为细节辨认题。文章一开始就点明了这一点。另外，根据常识，也能判断出液体沸腾的条件是液体的温度必须达到它的沸点。
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