For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Therefore, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column.

    As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component.　In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description.

    More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.

    许多情况下，混合物中各组分的沸点十分接近，以致于必须将拉乌尔定律考虑在内。因此，为了很好地分离各组分，必须通过在一个填充分馏柱内不断重复汽化－冷凝周期的方法进行分馏。

    当要被提纯的液体受热时，它的蒸汽上升到分馏柱。蒸汽上升的同时，冷却并凝结在冷凝器壁和填充物的表面。此时，上升的热蒸气使冷凝物继续受热，并再一次汽化。然而，新生成的蒸汽的成分再一次取决于拉乌尔定律。每一个汽化冷凝周（称为一个理论塔板）都会产生一种易挥发组分含量更高的液体。实际上，一定温度下的每一周期并不是在分馏柱内的同一位置进行的，因而，理论塔板只是一个概念而不是精准描述。

    塔板越多分离越好。旋转带蒸馏系统使用一条转动的特氟纶带或金属带迫使上升的蒸汽与下降的冷凝物紧密接触，而提高理论塔板数。