Now that we have a better understanding of the general phase behavior of pure substances, we can use our knowledge to analyze and discuss how important separation techniques are used in chemical analysis. Not all separation techniques rely on phase properties or phase behavior to separate substances, but some of the most commonly used strategies do. Among them, we find Filtration, crystallization, and distillation.

Filtration: This technique is based on the mechanical separation of substances in the solid-state from substances in a fluid phase (liquid or gas) by using a physical barrier that only the fluid can pass. The separation of the two types of phases is never complete as some solid will pass through the filter, and some fluid will be attached to the solid material. The separation efficiency will largely depend on the filter’s thickness and pore size. Air filters are commonly used to improve air quality in house, building, and car engine ventilation systems.

Crystallization: In this strategy, the formation of a solid phase is induced by changing the temperature or the concentration of the components in a fluid (liquid or gas) mixture or by adding other substances. In a mixture of substances, these changes affect the solubility of one substance in the other, and the components are likely to separate. The solid phase that is formed tends to be a pure crystalline material. Crystallization is commonly used in processes such as the extraction of common salt from seawater and the production of pure silicon for computer chips and other electronic devices.

Distillation: This technique is used to separate substances in a liquid mixture taking advantage of differences in the boiling points of the various components. The method involves a phase change from liquid to gas and subsequent reconversion of the separated substances to the liquid phase. In a simple distillation, the liquid mixture is heated up in a flask. When the boiling point of the most volatile component is approached, the mixture boils and the temperature remains relatively constant until most of the volatile substance becomes a vapor. The vapor produced can be directed to a condenser, where it cools down and transforms back into a liquid that can be collected. By continuously heating the liquid mixture, the same process can be repeated until all components are separated.

**Figure 1.7** Main components of a fractional distillation apparatus.

Separation by distillation is never perfect, and it is common to redistill the different portions or fractions of liquid that are collected. One may use fractional distillation to improve the separation, particularly for substances with similar boiling points (see Figure 1.7). In this case, the evaporating fluid passes through a vertical column with trays or plates placed at different heights. The temperature decreases gradually from the bottom to the top of the column, and substances condense on different plates depending on their boiling points: the most volatile substances will condense at lower temperatures and will be found towards the top of the column; the least volatile substances will condense on the bottom plates.

Lets Think

Now that your done reviewing this section, let's think. Test your understanding by completing the reading review below.

U1M1 D - Reading review