Solution (chemistry)

This article is about chemical solutions. For other uses, see Solution (chemistry) (disambiguation).

Making a saline water solution by dissolving table salt (NaCl) in water. The salt is the solute and the water the solvent.

In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent.

Types of solutions

Usually, the substance present in a greatest amount is considered the solvent. Solvents can be gases, liquids, or solids. The solution has the same physical state as the solvent.

Gas

If the solvent is a gas, only gases are dissolved under all given set of conditions. An example for a gaseous solution is air (oxygen and other gases dissolved in nitrogen). Since interactions between molecules play almost no role, dilute gases form rather trivial solutions. In part of the literature, they are not even classified as solutions, but addressed as mixtures.

Liquid

If the solvent is a liquid, then gases, liquids, and solids can be dissolved. Examples are:

• Gas in liquid:
  • Oxygen in water.
  • Carbon dioxide in water is a less simple example, because the solution is accompanied by a chemical reaction (formation of ions). Note also that the visible bubbles in carbonated water are not the dissolved gas, but only an effervescence; the dissolved gas itself is not visible since it is dissolved on a molecular level.
• Liquid in liquid:
  • The mixing of two or more substances of the same chemistry but different concentrations to form a constant.(Homogenization of solutions)
  • Alcoholic beverages are basically solutions of ethanol in water.
  • Petroleum is a solution of various hydrocarbons.
• Solid in liquid:
  • Sucrose (table sugar) in water
  • Sodium chloride or any other salt in water forms an electrolyte: When dissolving, salt dissociates into ions.

Counterexamples are provided by liquid mixtures that are not homogeneous: colloids, suspensions, emulsions are not considered solutions.

Body fluids are examples for complex liquid solutions, containing many different solutes. They are electrolytes since they contain solute ions (e.g. potassium and sodium). Furthermore, they contain solute molecules like sugar and urea. Oxygen and carbon dioxide are also essential components of blood chemistry, where significant changes in their concentrations can be a sign of illness or injury.

Solid

If the solvent is a solid, then gases, liquids, and solids can be dissolved.

• Gas in solid:
  • Hydrogen dissolves rather well in metals, especially in palladium; this is studied as a means of hydrogen storage.
• Liquid in solid:
  • mercury in gold, forming an amalgam
  • Hexane in paraffin wax
• Solid in solid
  • Steel, basically a solution of carbon atoms in a crystalline matrix of iron atoms.
  • Alloys like bronze and many others.
  • Polymers containing plasticizers.

Solubility

Main article: Solubility

Main article: Solvation

The ability of one compound to dissolve in another compound is called solubility. When a liquid is able to completely dissolve in another liquid the two liquids are miscible. Two substances that can never mix to form a solution are called immiscible.

All solutions have a positive entropy of mixing. The interactions between different molecules or ions may be energetically favored or not. If interactions are unfavorable, then the free energy decreases with increasing solute concentration. At some point the energy loss outweighs the entropy gain, and no more solute particles can be dissolved; the solution is said to be saturated. However, the point at which a solution can become saturated can change significantly with different environmental factors, such as temperature, pressure, and contamination. For some solute-solvent combinations a supersaturated solution can be prepared by raising the solubility (for example by increasing the temperature) to dissolve more solute, and then lowering it (for example by cooling).

Usually, the greater the temperature of the solvent, the more of a given solid solute it can dissolve. However, most gases and some compounds exhibit solubilities that decrease with increased temperature. Such behavior is a result of an exothermic enthalpy of solution. Some surfactants exhibit this behaviour. The solubility of liquids in liquids is generally less temperature-sensitive than that of solids or gases.

Properties

The physical properties of compounds such as melting point and boiling point change when other compounds are added. Together they are called colligative properties. There are several ways to quantify the amount of one compound dissolved in the other compounds collectively called concentration. Examples include molarity, mole fraction, and parts per million (PPM).

The properties of ideal solutions can be calculated by the linear combination of the properties of its components. If both solute and solvent exist in equal quantities (such as in a 50% ethanol, 50% water solution), the concepts of "solute" and "solvent" become less relevant, but the substance that is more often used as a solvent is normally designated as the solvent (in this example, water).

Liquid solutions

See also: Solvent § Solvent classifications

In principle, all types of liquids can behave as solvents: liquid noble gases, molten metals, molten salts, molten covalent networks, and molecular liquids. In the practice of chemistry and biochemistry, most solvents are molecular liquids. They can be classified into polar and non-polar, according to whether their molecules possess a permanent electric dipole moment. Another distinction is whether their molecules are able to form hydrogen bonds (protic and aprotic solvents). Water, the most commonly used solvent, is both polar and sustains hydrogen bonds.

Water is a good solvent because the molecules are polar and capable of forming hydrogen bonds.

Salts dissolve in polar solvents, forming positive and negative ions that are attracted to the negative and positive ends of the solvent molecule, respectively. If the solvent is water, hydration occurs when the charged solute ions become surrounded by water molecules. A standard example is aqueous saltwater. Such solutions are called electrolytes.

For non-ionic solutes, the general rule is: like dissolves like.

Polar solutes dissolve in polar solvents, forming polar bonds or hydrogen bonds. As an example, all alcoholic beverages are aqueous solutions of ethanol. On the other hand, non-polar solutes dissolve better in non-polar solvents. Examples are hydrocarbons such as oil and grease that easily mix with each other, while being incompatible with water.

An example for the immiscibility of oil and water is a leak of petroleum from a damaged tanker, that does not dissolve in the ocean water but rather floats on the surface.

See also

• Concentration
• Molar solution
• Phase separation
• Percentage solution
• Solubility equilibrium
• Stock solution
• Total dissolved solids is a common term in a range of disciplines, and can have different meanings depending on the analytical method used. In water quality, it refers to the amount of residue remaining after evaporation of water from a sample.
• Critical Point
• Upper critical solution temperature
• Lower critical solution temperature
• Coil-globule transition

References

IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "solution". doi:10.1351/goldbook.S05746