Master AP Chemistry Topic 3.9 with this free practice quiz. Test your skills on chromatography, distillation, retention factor, and boiling points to prepare for your AP exam.
Report a question
Introduction to Separation of Solutions and Mixtures
A mixture contains two or more substances physically combined without forming a new chemical substance. Its components retain their chemical identities and can often be separated using physical methods based on differences in their properties.
A homogeneous mixture has a uniform composition throughout. A solution is a homogeneous mixture in which one or more solutes are dissolved in a solvent. For example, salt water contains dissolved NaCl distributed throughout water.
A heterogeneous mixture has a nonuniform composition, often with distinguishable phases. Sand mixed with water is an example because the sand does not dissolve and can be separated from the liquid by filtration.
The choice of separation technique depends on differences in physical properties, such as particle size, boiling point, volatility, solubility, polarity, or attraction to a stationary phase. These differences allow the components of a mixture to be separated without changing their chemical identities.
Filtration and Decantation
Filtration
Filtration separates an insoluble solid from a fluid by passing the mixture through a porous material. The filter allows the fluid and sufficiently small dissolved particles to pass through while retaining larger solid particles.
For example, a mixture of sand and water can be separated using filter paper. Sand remains on the filter as the residue, while water passes through as the filtrate.
Filtration cannot separate dissolved NaCl from water because dissolved ions are much smaller than the pores of ordinary filter paper and travel with the solvent. Evaporation or distillation is needed to separate dissolved salt from water.
Decantation
Decantation separates a liquid from a settled solid by carefully pouring off the liquid without disturbing the solid sediment. It can also separate two immiscible liquids, such as oil and water, by allowing them to form distinct layers.
A separatory funnel is particularly useful for separating immiscible liquids. The denser liquid generally forms the lower layer and can be drained through the stopcock.
The effectiveness of decantation depends on the formation of separate phases. It is generally less complete than filtration when fine solid particles remain suspended in the liquid.
Evaporation, Crystallization, and Distillation
Evaporation and Crystallization
Evaporation separates a dissolved, nonvolatile solute from a volatile solvent. When salt water is heated, water molecules escape into the gas phase, leaving dissolved salt behind.
This technique is useful when recovering the solid is the main objective and the solvent does not need to be collected.
Crystallization separates a dissolved solid by forming crystals from a solution. A solution can be concentrated by evaporating some solvent and then cooled to reduce the solubility of the solute. If the solution becomes supersaturated, excess solute may crystallize.
Crystallization can produce a purer solid because many dissolved impurities remain in the liquid, although the effectiveness depends on their solubilities and the experimental conditions.
Distillation
Distillation separates substances based on differences in volatility, which is related to vapor pressure and boiling point. The mixture is heated, the more volatile component enters the vapor phase more readily, and the vapor is cooled in a condenser to form a liquid called the distillate.
Simple distillation is effective for separating a volatile solvent from a dissolved, nonvolatile solute. For example, distilling salt water produces water vapor that can be condensed and collected, while most dissolved salt remains in the distillation flask.
Distillation can also separate miscible liquids with sufficiently different boiling points. However, the vapor usually contains some of each volatile component, so the separation may not be complete in a single step.
Fractional distillation uses a fractionating column to provide repeated vaporization and condensation. The vapor becomes enriched in the more volatile component as it rises through the column. This method is useful for separating miscible liquids with closer boiling points.
For example, ethanol and water can be separated more effectively by fractional distillation than by simple distillation, although ordinary fractional distillation cannot produce completely pure ethanol because the mixture forms an azeotrope.
Chromatography
Chromatography separates mixture components through their different interactions with two phases: a stationary phase, which remains fixed, and a mobile phase, which moves through or across it.
Components that interact more strongly with the stationary phase generally move more slowly. Components that are more strongly carried by the mobile phase generally move farther in the same amount of time.
In paper chromatography, the paper acts as the stationary support, while a solvent moves through it. The solvent carries dissolved substances at different rates, producing separate spots. This method can separate pigments in ink or plant extracts.
A component’s movement depends on its attraction to both phases, including properties such as polarity and solubility. The outcome cannot be predicted from polarity alone without knowing the stationary phase and mobile phase.
Retention Factor
In paper or thin-layer chromatography, the retention factor, Rf, is calculated using:
Rf = distance traveled by the substance / distance traveled by the solvent front
Both distances are measured from the same starting line.
For example, if a substance travels 3.0 cm and the solvent front travels 6.0 cm:
Rf = 3.0 cm / 6.0 cm = 0.50
The Rf value has no units and is normally between 0 and 1 when the spot remains behind the solvent front. Under identical experimental conditions, a substance often produces a characteristic Rf value that can help identify it.
Rf values depend on the solvent, stationary phase, temperature, and experimental conditions. Therefore, values measured under different conditions should not be compared as if they were directly equivalent.
Separation Based on Solubility and Polarity
Solubility describes how much of a substance dissolves in a particular solvent under specified conditions. It depends on the interactions between solute and solvent particles.
A useful principle is that substances tend to dissolve well in solvents with similar types of intermolecular interactions. Polar solvents such as water often dissolve ionic and polar substances, whereas nonpolar solvents such as hexane often dissolve nonpolar substances.
For example, a mixture containing a water-soluble ionic compound and a water-insoluble solid can sometimes be separated by adding water, dissolving the ionic compound, filtering out the insoluble solid, and then evaporating the water to recover the dissolved compound.
This process is called selective dissolution because the solvent dissolves one component more effectively than another.
Solubility can also depend on temperature. For many solid solutes, solubility increases as temperature rises, allowing more solute to dissolve in a hot solvent. Cooling the solution can then cause crystals to form. However, this trend is not universal, so the actual solubility behavior must be considered.
Quantitative Relationships in Separation
Mass and Percent Recovery
The amount of material recovered during a separation can be evaluated using percent recovery:
Percent recovery = (mass recovered / initial mass of the component) × 100%
Suppose a student begins with 5.00 g of a compound and recovers 4.20 g after recrystallization.
Percent recovery = (4.20 g / 5.00 g) × 100% = 84.0%
The recovery is 84.0%. A value below 100% may result from material remaining dissolved in the solvent, losses during transfers, or incomplete collection.
Percent recovery is not the same as percent purity. A high recovery does not necessarily mean the recovered material is pure, and a lower recovery may be acceptable if the separation removes substantial impurities.
Concentration and Dilution During Separation
When a solvent evaporates while the amount of dissolved solute remains constant, the solution becomes more concentrated.
Molarity is defined as:
M = n / V
where M is molarity in mol/L, n is the amount of dissolved solute in moles, and V is the solution volume in liters.
For example, if 0.20 mol of NaCl remains dissolved in 0.50 L of solution, its concentration is:
M = 0.20 mol / 0.50 L = 0.40 M
If evaporation reduces the solution volume to 0.25 L without loss of NaCl, the concentration becomes:
M = 0.20 mol / 0.25 L = 0.80 M
This relationship explains why evaporation can eventually lead to crystallization: as solvent is removed, the solution may reach or exceed the solubility limit.
Choosing a Separation Technique
The appropriate technique depends on the physical properties of the mixture and the material that must be recovered.
Filtration is appropriate for an insoluble solid suspended in a fluid. Evaporation is useful when recovering a nonvolatile dissolved solid, while crystallization is useful when obtaining a solid with improved purity. Distillation is suitable when a volatile liquid must be recovered or when liquids with different volatilities need to be separated.
Chromatography is especially useful for separating and identifying components present in small quantities, such as dyes or pigments. Selective dissolution is useful when the components differ significantly in solubility in a chosen solvent.
Some mixtures require several techniques in sequence. For example, a mixture of sand and NaCl can be separated by adding water to dissolve the NaCl, filtering to remove the sand, and evaporating the filtrate to recover the salt.
Connection to AP Chemistry Concepts
Separation techniques connect to several major AP Chemistry concepts. Intermolecular forces help explain differences in solubility and volatility. Vapor pressure and boiling point explain distillation. Dynamic equilibrium between dissolved solute and undissolved solid helps explain saturation and crystallization.
Separation also provides a practical application of conservation of matter. In a physical separation, the chemical identities of the components remain unchanged, although some material may be lost during the experiment. The total mass of the substances is conserved in an ideal closed system.
When interpreting experimental data, distinguish between the amount recovered, the purity of the recovered substance, and the effectiveness of the separation. These measurements describe different aspects of the process and should not be treated as interchangeable.