Section outline

  • What causes this toy to move?
    ::是什么原因导致这个玩具移动?

    The drinking duck is a toy that many kids (and adults) enjoy playing with. You can see the drinking duck in action in the video below: 
    ::许多儿童(及成人)都乐于玩耍。

     

     

     

    The motion of the duck illustrates a physical principle called vapor pressure. As the vapor pressure changes, the in the duck moves up and down, causing the duck to move.
    ::鸭子的动作显示了一种叫蒸气压力的物理原理。 随着蒸气压力的变化,鸭子上下移动,鸭子上下移动。

    Vapor Pressure
    ::蒸气压力

    When a partially filled container of liquid is sealed with a stopper, some liquid molecules at the surface evaporate into the vapor phase . However, the vapor molecules cannot escape from the container and so after a certain amount of time, the space above the liquid reaches a point where it cannot hold any more vapor molecules. Now some of the vapor molecules condense back into a liquid. The system reaches the point where the rate of is equal to the rate of (see Figure ). This is called a dynamic equilibrium between the liquid and vapor phases.
    ::当部分装满的液体容器被用阻塞密封时,表面的一些液体分子蒸发到蒸发阶段,然而,蒸发分子无法从容器中解脱出来,因此经过一定时间,液体上方的空间达到一个它无法再持有蒸发分子的点。现在,一些蒸发分子凝聚到液体中。这个系统达到与蒸发阶段的速率相等的点(见图)。这被称为液体和蒸发阶段之间的动态平衡。

    Liquid evaporation and vapor condensation in a sealed container.
    Equilibrium between liquid phase and vapor phase.

    A dynamic equilibrium can be illustrated by an equation with a double arrow, meaning that the reaction is occurring in both directions and at the same rate.
    ::动态平衡可以用双箭方程式来说明,这意味着反应是双向以相同速度发生的。

    H 2 O ( l ) ⇌ H 2 O ( g )

    ::H2O(l) H2O(g)

    The forward direction represents the evaporation process, while the reverse direction represents the condensation process.
    ::前进方向代表蒸发过程,相反方向代表凝结过程。

    Because they cannot escape the container, the vapor molecules above the surface of the liquid exert a pressure on the walls of the container. The vapor pressure is a measure of the pressure (force per unit area) exerted by a above a liquid in a sealed container. Vapor pressure is a property of a liquid based on the strength of its intermolecular forces . A liquid with weak intermolecular forces evaporates more easily and has a high vapor pressure. A liquid with stronger intermolecular forces does not evaporate easily and thus has a lower vapor pressure. For example, diethyl ether is a nonpolar liquid with weak dispersion forces. Its vapor pressure at 20°C is 58.96 kPa. Water is a polar liquid whose molecules are attracted to one another by relatively strong . The vapor pressure of water at 20°C is only 2.33 kPa, far less than that of diethyl ether.
    ::蒸气压力是密封容器内一个以上液体所施加的压力(单位面积的强度)的量度。蒸气压力是一种液体的属性,其内分子力量的强度取决于蒸气压力的强度。具有较弱的中间分子力量的液体较容易蒸发,蒸气压力高。具有较强的中间分子力量的液体不易蒸发,因此蒸气压力较低。例如,二乙醚是一种非极性液体,其分散力量较弱。其20°C的蒸气压力为58.96千帕。水是一种极性液体,其分子因相对强而相互吸引。20°C的水蒸气压力只有2.33千帕,远低于二乙醚。

    Vapor Pressure and Temperature
    ::蒸气压力和温度

    Vapor pressure is dependent upon temperature . When the liquid in a closed container is heated, more molecules escape the liquid phase and evaporate. The greater number of vapor molecules strike the container walls more frequently, resulting in an increase in pressure. Table shows the temperature dependence of the vapor pressure of three liquids.
    ::蒸气压力取决于温度。当封闭容器中的液体变暖时,更多的分子会逃离液体相,蒸发。蒸气分子数量增多,更频繁地撞击容器壁壁,导致压力增加。表显示三种液体蒸气压力的温度依赖性。

    Vapor Pressure (in kPa) of Three Liquids at Different Temperatures
      0°C 20°C 40°C 60°C 80°C 100°C
    Water 0.61 2.33 7.37 19.92 47.34 101.33
    Ethanol 1.63 5.85 18.04 47.02 108.34 225.75
    Diethyl ether 24.70 58.96 122.80 230.65 399.11 647.87

    Notice that the temperature dependence of the vapor pressure is not linear. From 0°C to 80°C, the vapor pressure of water increases by 46.73 kPa, while it increases by 53.99 kPa in only a span of twenty degrees from 80°C to 100°C.
    ::注意蒸气压力的温度依赖性不是线性。从0°C到80°C,水的蒸气压力增加46.73千帕,而水的蒸气压力增加53.99千帕,距离从80°C到100°C只有20度。

     

     

     

     

     

     Ever try to boil water on the top of a mountain? Explore the relationship between altitude, vapor pressure and boiling point in this simulation: 
    ::在模拟中探索高度、蒸气压力和沸点之间的关系:

     

     

    Further Reading
    ::继续阅读

    Summary
    ::摘要

    • Vapor pressure is a measure of the pressure exerted by a gas above a liquid in a sealed container.
      ::蒸气压是气体在密封容器中液体上方施加的压力的量度。
    • Strong intermolecular forces produce a lower rate of evaporation and a lower vapor pressure.
      ::强烈的分子分子力量产生较低的蒸发速度和较低的蒸气压力。
    • Weak intermolecular forces produce a higher rate of evaporation and a higher vapor pressure.
      ::微弱的中间分子力量产生较高的蒸发率和更高的蒸气压力。
    • As the temperature increases, the vapor pressure increases.
      ::随着温度的升高 蒸气压力的升高

    Review
    ::回顾

    1. Define vapor pressure.
      ::定义蒸气压。
    2. How do intermolecular forces affect vapor pressure?
      ::分子间流力如何影响蒸气压力?
    3. How does temperature affect vapor pressure?
      ::温度如何影响蒸气压力?