Solvation of Na+ with water

The hydration shell of an ion in water.

A solution is a homogeneous mixture of two or more substances, distinguished from non-homogeneous mixtures such as colloids and suspensions. There may be solid, liquid, or gaseous solutions. For certain types of liquid solutions, it is useful to describe the solution as comprised of one or more solutes dissolved within a solvent. The solubility of a solute describes its ability to dissolve in a certain solvent. A variety of concentration expressions are useful within different contexts to describe the composition of a solution including mole fraction, percent by weight, molarity, and molality. The colligative properties refer to the changes that occur in the physical properties of a solvent when a solute is added to it such as boiling point elevation, freezing point depression, and vapor pressure lowering.

For certain types of aqueous solutions of sparingly soluble electrolytes, an equilibrium will be established at low concentration between the dissolved ions and undissolved solute or precipitate which can be quantatively described using the solubility product corresponding to particular ion pairs.

If you do not know this material backwards and fowards, you will suffer on the MCAT. The topic of Solutions is one of the most important for the MCAT. Nearly every exam will require you predict solubility. You can also expect to apply what you understand about the colligative properties and puzzle through something involving heterogeneous solution equilibria. Do not neglect this material. Solutions will be on the test in a direct way.

WikiPremed Resources

Solution Chemistry Practice Items
Problem set for Solution Chemistry in PDF format

Answer Key
Answers and explanations

Solutions Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Solutions
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Be able to clearly distinguish a solution from other types of mixtures such as colloids.

Be able to name a few solid, liquid, and gaseous solutions.

Understand how to work with various concentration expressions including mole fraction, molality, molarity and normality.

Understand the basis of solubility in chemical thermodynamics. Be capable of predicting the solubility in water of an organic compound from its structural formula.

Understand why some electrolytes are less soluble than others. Remember the solubility rules for electrolytes.

Have a clear sense of the structure at the particle level of an aqueous solution of electrolytes.

Gain facility in employing the solubility product to solve problems such as the final concentration of a solution of weak electrolyte.

Be able to describe the common ion effect and become familiar with typical common ion effect problems.

Understand heterogeneous solution equilibria when it is coupled with acid-base equilibria.

Be prepared to solve problems involving colligative properties including boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

Understand the definition of a colloidal suspension. Be able to conceptualize the structure at the particle level of micellar suspensions involving soaps and detergents.

Familiarize yourself with the chemical separation technique known as solvent extraction.

Suggested Assignments

Study the terminology for solution chemistry using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Perform the practice items for solutions. Here is the answer key for the practice items.

Read pp. 179-189 in ExamKrackers Chemistry. Perform practice items 121-128 on pg 190.

Review the web resources for solutions.

Conceptual Vocabulary for Solutions


Each list begins with basic conceptual vocabulary you need to know for MCAT questions and proceeds to advanced terms that might appear in context in MCAT passages. The terms are links to Wikipedia articles.
In chemistry, a mixture is a substance made by combining two or more different materials in such a way that no chemical reaction occurs.
A solution is a homogeneous mixture composed of two or more substances.
Precipitation is the formation of a solid in a solution during a chemical reaction.
Hydrophobicity refers to the physical property of a molecule that is repelled from a mass of water
A hydrocarbon is an organic compound consisting entirely of hydrogen and carbon.
Hydrophilicity refers to a physical property of a molecule that can transiently bond with water through hydrogen bonding.
Concentration is the measure of how much of a given substance there is mixed with another substance.
Solubility is a physical property referring to the ability for a given substance, the solute, to dissolve in a solvent.
Aqueous solution
A aqueous solution is a solution in which the solvent is water.
Miscibility is a term in chemistry that refers to the property of liquids to mix, forming a homogeneous solution.
Lipophilicity refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene.
Crystallization is the natural or artificial process of formation of solid crystals from a uniform solution.
Boiling-point elevation
Boiling-point elevation is a colligative property that states that a solution will have a higher boiling point than that of a pure solvent after the addition of a dissolved solute.
Solvation or dissolution is the process of attraction and association of molecules of a solvent with molecules or ions of a solute.
Solubility equilibrium
Solubility equilibrium is any chemical equilibrium between solid and dissolved states of a compound at saturation.
Colligative properties
Colligative properties are properties of solutions that depend on the number of particles in a given volume of solvent and not on the mass of the particles.
Freezing-point depression
Freezing-point depression is the difference between the freezing points of a pure solvent and a solution mixed with a solute.
Molar solution
A molar solution is one that contains one mole of solute per liter.
Aliphatic compound
Aliphatic compounds are organic compounds in which carbon atoms are joined together in straight or branched chains or in rings, that can be either saturated or unsaturated, but not aromatic.
Common-ion effect
The common-ion effect is a term used to describe the effect on a solution of two dissolved solutes that contain the same ion.
Dalton's law
Dalton's law states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture.
Carbonation occurs when carbon dioxide is dissolved in water or an aqueous solution.
Carbonated water
Carbonated water is plain water into which carbon dioxide gas has been dissolved.
Dissociation is a general process in which ionic compounds separate or split into smaller molecules, ions, or radicals, usually in a reversible manner.
Mole fraction
The mole fraction of a component in a mixture is the relative proportion of molecules belonging to the component to those in the mixture, by number of molecules.
An electrolyte is a substance containing free ions that behaves as an electrically conductive medium.
Paraffin is a common name for the group of alkane hydrocarbons.
Volatility is a measure of the speed at which a substance turns into a vapor from a solid or liquid state.
Volatile organic compound
Volatile organic compounds (VOCs) are organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere.
Amphiphile is a term describing a chemical compound possessing both hydrophilic and hydrophobic properties.
Raoult's law
Raoult's law states that the vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution.
Henry's law
Henry's law states that at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
A colloid, emulsion or dispersion is a type of heterogeneous mixture consisting of a dispersed phase made of tiny particles or droplets distributed evenly throughout a continuous phase.
The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances.
Percentage solution
Percentage solution is a form of concentration expression often preferred to molarity within the biological sciences in which a 1 percent solution would have 1 g of solute dissolved in a final volume of 100 ml of solution.
An emulsion, dispersion or colloid is a mixture of two immiscible substances in which a dispersed phase made of tiny particles or droplets is distributed evenly throughout a continuous phase.
Solvation shell
Solvation shell is a shell of any chemical species acting as a solvent, surrounding a solute species.
Partition coefficient
A partition or distribution coefficient is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium.
Protic solvent
A protic solvent is a solvent that carries a hydrogen bond between an oxygen as in a hydroxyl group or a nitrogen as in an amine group.
A suspension is a heterogenous fluid containing solid particles that are sufficiently large for sedimentation.
Partition coefficient
A partition or distribution coefficient is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium.
Van't Hoff factor
The van't Hoff factor is the number of moles of solute actually in solution per mole of solid solute added.
Enthalpy change of solution
The enthalpy change of solution is the enthalpy change when one mole of a substance is dissolved completely in a large volume of a solvent at constant pressure.
An alloy is a homogeneous hybrid of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties.
Liquid-liquid extraction
Liquid-liquid or solvent extraction, also known as partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent.
Hygroscopy is the ability of a substance to attract water molecules from the surrounding environment through either absorption or adsorption.
An azeotrope is a mixture of two or more pure compounds in such a ratio that its composition cannot be changed by simple distillation.
Multiphasic liquid
A multiphasic liquid is a mixture consisting of more than two immiscible liquid phases.
Ideal solution
An ideal solution or ideal mixture is a solution in which the enthalpy of solution is zero.
Multiphasic liquid
A multiphasic liquid is a mixture consisting of more than two immiscible liquid phases.
Relative volatility
Relative volatility is a measure of the vapor pressure differences of the components in a liquid mixture of chemicals.
Surfactants, also known as tensides, are wetting agents that lower the surface tension of a liquid and lower the interfacial tension between two liquids.
Regular solution
A regular solution is a solution that diverges from the behavior of an ideal solution only moderately.
Margules function
A Margules function is a function added to the Raoult's law description of a liquid solution to account for deviations from ideality.
Bolaamphiphiles are amphiphilic molecules that have hydrophilic groups at both ends of a sufficiently long hydrophobic hydrocarbon chain.
Lipophobicity is a term which literally means fat rejection and describes compounds which are not soluble in lipids or other non-polar solvents.
Cryoscopic constant
The cryoscopic constant allows one to relate molality to freezing point depression.
Ebullioscopic constant
The ebullioscopic constant allows one to relate molality to boiling point elevation.
Tyndall effect
The Tyndall effect is the effect of light scattering on particles in colloid systems, such as suspensions or emulsions.
Eutectic point
An eutectic mixture is a mixture at such proportions that the melting point is as low as possible, and that furthermore all the constituents crystallize simultaneously at this temperature from molten liquid solution.
Flory-Huggins solution theory
Flory-Huggins solution theory is a mathematical model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecular sizes in adapting the usual expression for the entropy of mixing.
A material is called lyotropic if it forms liquid crystal phases because of the addition of a solvent.
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