AP Chemistry 3.9-3.10: Separation Techniques and Solubility

Solubility

The maximum concentration of a solute that can exist in a particular solvent under specified conditions when dissolution and crystallization are at equilibrium.

How do saturated, unsaturated, and supersaturated solutions differ?

A saturated solution contains solute at its equilibrium solubility, an unsaturated solution contains less than that amount, and a supersaturated solution contains more than the equilibrium solubility. Supersaturated solutions are nonequilibrium states.

Dilute solution versus concentrated solution

A dilute solution has a relatively small amount of solute compared with solvent, whereas a concentrated solution has a relatively large amount. These terms do not by themselves indicate whether the solution is saturated.

What determines whether a solute dissolves readily in a solvent?

Dissolution depends largely on the relative strengths of solute-solute, solvent-solvent, and solute-solvent attractions. Strong solute-solvent attractions favor dissolution when they can compensate for disrupting the original interactions.

Why does the phrase "like dissolves like" help predict solubility?

Polar solutes tend to dissolve in polar solvents, and nonpolar solutes tend to dissolve in nonpolar solvents, because the new solute-solvent attractions are comparable to the original intermolecular attractions.

How does temperature generally affect the solubility of solid solutes in water?

The solubility of many solid solutes increases as temperature rises, although exceptions exist. The actual temperature dependence must be determined from experimental data or a solubility curve.

How can heating and cooling produce a supersaturated solution of a solid?

Dissolve a solute in a hot solvent until the solution is saturated, then cool it carefully. If precipitation does not occur, the cooled solution contains more solute than is normally soluble at the lower temperature.

How can precipitation be initiated in a supersaturated solution?

Adding a seed crystal or mechanically disturbing the solution can initiate crystallization of the excess solute. The system then moves toward the equilibrium solubility.

What is the role of solute-solute attractions when a gas dissolves in a liquid?

Gas particles are far apart and have negligible solute-solute attractions, so those interactions generally do not need to be overcome. Solute-solvent attractions are therefore the primary intermolecular factor affecting gas solubility.

How does temperature affect the solubility of most gases in liquids?

Gas solubility usually decreases as temperature increases. Warming a body of water can therefore reduce dissolved oxygen and harm aquatic organisms.

Why can thermal pollution cause fish kills?

An increase in water temperature generally lowers the equilibrium solubility of oxygen. If dissolved oxygen falls below the level required by aquatic organisms, widespread stress or death can result.

How does the partial pressure of a gas affect its solubility in a liquid?

At constant temperature, increasing the gas's partial pressure above the liquid increases its solubility. Lowering the partial pressure can cause dissolved gas to escape as bubbles.

Henry's law

At constant temperature, the concentration of a dissolved gas is directly proportional to its partial pressure above the solution: $C_g = kP_g$. The constant $k$ depends on the gas, solvent, and temperature.

What are the units of the Henry's law constant when concentration is in mol/L and pressure is in kPa?

From $k = C_g/P_g$, the units are $\mathrm{mol\,L^{-1}\,kPa^{-1}}$. Any consistent concentration and pressure units may be used.

How can Henry's law be used to compare gas solubilities at two pressures?

For the same gas and solvent at constant temperature, use $C_{g,1}/P_{g,1} = C_{g,2}/P_{g,2}$. Thus, $C_{g,2} = C_{g,1}(P_{g,2}/P_{g,1})$.

What happens to dissolved gas when a solution warms or the gas pressure decreases?

The gas solubility decreases, so the solution may become supersaturated. The excess gas can leave the solution until the dissolved concentration reaches its new equilibrium value.

Why does opening a carbonated beverage produce bubbles?

The beverage was equilibrated with carbon dioxide at elevated pressure. Opening the container lowers the carbon dioxide partial pressure, reducing its solubility; the solution becomes supersaturated and excess dissolved $\mathrm{CO_2}$ escapes as bubbles.

Why can ammonia show a deviation from Henry's law in water?

Ammonia reacts with water to form ammonium and hydroxide ions. Because chemical reaction removes dissolved ammonia from the molecular-gas equilibrium, its solubility can increase more rapidly with pressure than Henry's law predicts.

How does decompression sickness relate to Henry's law?

At depth, increased pressure causes more gases from inhaled air to dissolve in a diver's blood. A rapid ascent lowers pressure and gas solubility, allowing bubbles to form in tissues and blood; slow ascents and decompression stops reduce this risk.

Miscible liquids

Liquids that mix in all proportions and have infinite mutual solubility are miscible. Water and ethanol are an example.

Immiscible liquids

Liquids that have very low mutual solubility and form separate layers are immiscible. Water and oil are a common example.

Partially miscible liquids

Liquids with a moderate, limited mutual solubility are partially miscible. They commonly form two layers, with each layer saturated by some of the other liquid.

Why are polar and nonpolar liquids often immiscible?

Strong interactions among polar solvent molecules, such as hydrogen bonding in water, may not be replaced by sufficiently strong interactions with nonpolar molecules. Mixing is therefore unfavorable to a significant extent.

What do the layers of a partially miscible bromine-water mixture contain?

The water-rich layer is saturated with bromine, while the bromine-rich layer is saturated with water. Each layer is a solution, even though the two liquids are not completely miscible.

Chromatography

A separation method in which mixture components distribute differently between a moving mobile phase and a stationary phase. Components with different affinities for the phases travel at different rates.

Mobile phase

The fluid phase that carries a mixture through a chromatographic system. It may be a liquid or a gas.

Stationary phase

The phase fixed in place in a column, capillary, plate, or sheet. Analytes interact with it to different extents, producing differential retention.

Analyte

The substance or component being separated, identified, or measured in a chromatographic sample.

Why does chromatography separate the components of a mixture?

Each component partitions differently between the mobile and stationary phases because of differences in intermolecular interactions or other chemical properties. Components with weaker or less frequent interactions with the stationary phase generally move faster.

Analytical chromatography versus preparative chromatography

Analytical chromatography is used to determine whether components are present and often to measure their relative amounts. Preparative chromatography is used to isolate components for later use or purification.

Chromatogram

The recorded output of a chromatographic separation. In a typical chromatogram, retention time is plotted on the x-axis and detector response is plotted on the y-axis; distinct peaks correspond to separated components.

Retention time

The time required for an analyte to travel through the chromatographic system and reach the detector. Under fixed conditions, different retention times help identify mixture components.

What does the area or height of a chromatographic peak generally indicate?

The detector signal is often related to the amount or concentration of the analyte. Calibration is required for accurate quantitative analysis.

Eluent and eluate

The eluent is the solvent or mobile phase used to carry analytes through a chromatographic column. The eluate is the mixture that exits the column, containing solvent and eluted components.

Column chromatography versus planar chromatography

Column chromatography carries the mobile phase through a packed or coated column. Planar chromatography carries the mobile phase across a flat stationary phase, such as paper or a thin-layer plate.

Thin-layer chromatography (TLC)

A planar chromatographic technique in which analytes move across a thin layer of stationary material on a plate. Components separate because they adsorb to the stationary layer and dissolve in the mobile phase to different extents.

How is the retention factor, $R_f$, calculated in thin-layer chromatography?

The retention factor is $R_f = \frac{\text{distance traveled by solute}}{\text{distance traveled by solvent front}}$. Under identical conditions, different $R_f$ values can help identify substances.

Gas chromatography (GC) and liquid chromatography (LC)

Gas chromatography uses a gas as the mobile phase, whereas liquid chromatography uses a liquid. The choice depends on properties such as volatility, thermal stability, and solubility of the analytes.

Filtration

A separation method that uses a porous barrier to separate an insoluble solid from a liquid or gas. The solid retained by the filter is the residue, and the fluid that passes through is the filtrate.

Distillation

A separation method based on differences in volatility or boiling point. The more volatile component vaporizes preferentially, and the vapor is then condensed and collected.

When is simple distillation effective?

Simple distillation is effective for separating a volatile liquid from a nonvolatile solute or for separating liquids with substantially different boiling points.

Why is fractional distillation used for liquids with similar boiling points?

A fractionating column provides repeated vaporization and condensation steps, enriching the vapor in the more volatile component and allowing liquids with closer boiling points to be separated.