Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Separation of Mixtures


· To learn two methods of separating mixtures.

We have seen that the matter found in nature is typically a mixture of pure substances. For example, seawater is water containing dissolved minerals. We can separate the water from the minerals by boiling, which changes the water to steam (gaseous water) and leaves the minerals behind as solids. If we collect and cool the steam, it condenses to pure water. This separation process, called distillation , is shown in Fig. 3.6.

Figure 3.6.A set of two illustrations are shown. The first illustration shows distillation in progress and the second illustration shows the apparatus after distillation. The figure shows a simple, one-stage distillation apparatus. A stoppered, round-bottomed distilling flask, held in place by a stand, is heated by a Bunsen burner. This boiling flask contains salt water, the mixture that needs to be separated. The flask is equipped with a thermometer and an outlet tube which is connected to a condenser held in place by a stand. The condenser has two tubes of different diameters; the smaller tube in which the vapor condenses is placed within the larger, outer tube in which cool water circulates. The condenser is connected to a source of cool running water and the input hose is placed at the end furthest from the distilling flask. The outlet hose is at the end nearest the flask. The distillate, pure water, is collected in the receiving flask placed at the end of the condenser, and the nonvolatile component of the mixture, salt, remains in the distilling flask.

Distillation of a solution consisting of salt dissolved in water.

When we carry out the distillation of salt water, water is changed from the liquid state to the gaseous state and then back to the liquid state. These changes of state are examples of physical changes. We are separating a mixture of substances, but we are not changing the composition of the individual substances. We can represent this as shown in Fig. 3.7.

Figure 3.7.An illustration depicting the distillation of saltwater shows a beaker containing saltwater solution, a homogenous mixture. An arrow, labeled Distillation (physical method) points from this beaker to the products of distillation, salt and pure water.

No chemical change occurs when salt water is distilled.

Suppose we scooped up some sand with our sample of seawater. This sample is a heterogeneous mixture because it contains an undissolved solid as well as the saltwater solution. We can separate out the sand by simple filtration . We pour the mixture onto a mesh, such as a filter paper, which allows the liquid to pass through and leaves the solid behind (Fig. 3.8). The salt can then be separated from the water by distillation. The total separation process is represented in Fig. 3.9. All the changes involved are physical changes.

Figure 3.8.An illustration demonstrating the process of simple filtration shows a mixture of solid and liquid being poured from a beaker into a funnel that is held in place by a stand. A stirring rod helps the mixture pass through the funnel, and a filter paper placed in the funnel traps the solids in the mixture as the filtrate, which is the liquid component of the mixture, passes through the funnel into a beaker.

Filtration separates a liquid from a solid. The liquid passes through the filter paper, but the solid particles are trapped.

Figure 3.9.An illustration demonstrating the separation of sand-salt water mixture shows sand-salt water mixture in a beaker. An arrow labeled Filtration (physical method) points from the beaker containing the mixture to the products of filtration, sand and salt water solution. A second arrow, labeled Distillation (physical method) points from the salt water solution to the products of distillation, namely salt and pure water.

Separation of a sand—saltwater mixture.

Critical Thinking

· The scanning tunneling microscope allows us to “see” atoms. What if you were sent back in time before the invention of the scanning tunneling microscope? What evidence could you give to support the theory that all matter is made of atoms and molecules?

We can summarize the description of matter given in this chapter with the diagram shown in Fig. 3.10. Note that a given sample of matter can be a pure substance (either an element or a compound) or, more commonly, a mixture (homogeneous or heterogeneous). We have seen that all matter exists as elements or can be broken down into elements, the most fundamental substances we have encountered up to this point. We will have more to say about the nature of elements in the next chapter.

Figure 3.10.A flowchart depicting the organization of matter shows that matter can be classified as homogenous mixtures, heterogeneous mixtures, and pure substances. Both homogenous and heterogeneous mixtures can be converted to pure substances by physical methods. Pure substances can be further classified as elements and compounds, which in turn can be converted to elements by chemical methods.

The organization of matter.