升华是物质经历直接从固态到气态或蒸汽的相变而不经过两者之间更常见的液相的术语。这是汽化的具体情况。升华是指转变的物理变化,而不是由于化学反应导致固体转化为气体的情况。因为从固体到气体的物理变化需要向物质中添加能量,所以它是吸热变化的一个例子。如果你看相图,这是一个图表,描绘了各种压力和体积的物质状态。该图中的“三重点”表示物质可以对液相施加的最小压力。低于该压力,当温度降至固相水平以下时,其直接转变为气相。这是化学家用来净化化合物的一种方法,并且是在炼金术的前化学时期开发的,作为产生纯化的元素蒸气的手段。然后这些纯化的气体可以经过缩合过程,最终得到纯化的固体,因为升华的温度或缩合的温度对于杂质而言与对于所需的固体不同。考虑这个问题的一种方法是,如果你想要升华,你需要通过降低压力来获得三重点以下的物质。化学家经常采用的方法是将物质置于真空中并在称为升华装置的装置中施加热量。真空意味着压力非常低,因此即使通常熔化成液体形式的物质现在也会通过加热直接升华为蒸气。关于我上面描述的内容的一个注意事项:冷凝实际上会将气体带入液体中,然后液体会冻结成固体。还可以在保持低压的同时降低温度,使整个系统保持在三相点之下,这将导致直接从气体转变为固体。这个过程叫做沉积。相变取决于所讨论材料的温度和压力。在正常条件下,如通常由动力学理论所描述的,加热导致固体内的原子获得能量并变得彼此不那么紧密地结合。取决于物理结构,这通常导致固体熔化成液体形式。

英国诺丁汉特伦特大学物理学Assignment代写:升华的定义

Sublimation is the term for when matter undergoes a phase transition directly from a solid to gaseous form, or vapor, without passing through the more common liquid phase between the two. It is a specific case of vaporization. Sublimation refers to physical changes of transition, and not to cases where solids convert into gas due to a chemical reaction. Because the physical change from solid into a gas requires the addition of energy into the substance, it is an example of an endothermic change. If you look at the phase diagrams, which is a graph that depicts the states of matter for various pressures and volumes. The “triple point” on this diagram represents the minimum pressure for which the substance can take on the liquid phase. Below that pressure, when the temperature drops below the level of the solid phase, it transitions directly into the gas phase. This is a method used by chemists to purify compounds, and was developed in the pre-chemistry days of alchemy as a means of creating a purified vapors of elements. These purified gases can then go through a process of condensation, with the end result being a purified solid, since either the temperature of sublimation or the temperature of condensation would be different for the impurities than for the desired solid. One way to think about this is that if you want to have sublimation, you need to get the substance beneath the triple point by lowering the pressure. A method that chemists often employ is placing the substance in a vacuum and applying heat, in a device called a sublimation apparatus. The vacuum means that the pressure is very low, so even a substance that usually melts into liquid form will now sublimate directly into vapor with the addition of the heat. One note of consideration on what I described above: Condensation would actually take the gas into a liquid, which would then freeze back into a solid. It would also be possible to reduce the temperature while retaining the low pressure, keeping the whole system beneath the triple point, and this would cause a transition directly from gas into solid. This process is called deposition. Phase transitions are dependent upon the temperature and pressure of the material in question. Under normal conditions, as generally described by kinetic theory, adding heat causes the atoms within a solid to gain energy and become less tightly bound to each other. Depending on the physical structure, this usually causes the solid to melt into liquid form.

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