Heterogeneous mixtures separating, 68 suspensions

A suspension is a type of heterogeneous mixture. Suspension particles settle out over time Q, and can be separated from the mixture by filtration... 

Particles in a suspension are much larger than atoms. In contrast, particles in a solution are atomic-scale in size. A heterogeneous mixture of intermediate size particles (between the size of solution particles and suspension particles) is a colloid. Colloid particles are between 1 nm and 1000 nm in diameter. The most abundant substance in the mixture is the dispersion medium. Milk is a colloid. Although homogenized milk resembles the cornstarch mixture in Figure 15-19a, you cannot separate its components by settling or by filtration. 

To a stirred suspension of Zn(OTf)2 (0.3 mmol) and cyclohexanone (1 mmol) in CH2CI2 (5 mL) was added TMSCl (3 mmol) at 0 °C under argon. The heterogeneous mixture was stirred for 5 min at 0 °C, then 2-morpholinoethyl isonitrile (70) (1.05 mmol) was added. The reaction mixture was warmed to rt and stirred for 24 h. Saturated aqueous NaHCOs (5 mL) was added and the mixture was stirred for another 20 min. The layers were separated, and the aqueous phase was extracted twice with CH2CI2 (10 mL). The combined organic layers were washed with brine, dried over MgS04, and filtered. The solvent was evaporated and the residue purified by silica gel flash column chromatography to afford 69 (57%). 

If the particles in a soivent are so iarge that they settie out unless the mixture is constancy stirred or agitated, the mixture is caiied a suspension. Think of a jar of muddy water. If left undisturbed, particles of soil collect on the bottom of the jar. The soil particles are denser than the solvent, water. Gravity pulls them to the bottom of the container. Particles over 1000 nm in diameter—1000 times as large as atoms, molecules, or ions—form suspensions. The particles in suspension can be separated from heterogeneous mixtures by passing the mixture through a filter. 

Suspensions are heterogeneous, nonuniform mixtures containing very large particles that are trapped by filters and do not pass through semipermeable membranes. If you stir muddy water, it mixes but then quickly separates as the suspension particles settle to the bottom. Before you use suspensions such as Kaopectate, calamine lotion, antacid mixtures, and liquid penicillin, it is important that you shake well before using to suspend all the particles they contain. 

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