Making a saline water solution by dissolving table salt (NaCl) in water

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.

[edit] Types of solutions

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

[edit] Gas

If the solvent is a gas, only gases can be dissolved. An example for a gaseous solution is air (oxygen and other gases dissolved in nitrogen). However, in dilute gases interaction between molecules play almost no role. Therefore, gaseous solutions are rather trivial solutions. In part of the literature, they are not even classified as solutions, but addressed as mixtures.

[edit] Liquid

If the solvent is a liquid, 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:
  • 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.

[edit] Solid

If the solvent is a solid, 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.

[edit] Solubility

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.

[edit] Properties

Please help improve this section by expanding it. Further information might be found on the talk page.

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).

[edit] Liquid solutions

See also: Solvent#Solvent classifications

[edit] Polar

When electrons in a covalent bond are not equally shared, the molecule is said to be polar. A polar molecule contains an electrical dipole, meaning that it has a (+) and (-) end. In the depicted case of the hydrogen fluoride molecule, covalently bond electrons are displaced towards the more electronegative fluorine atom.

Hydrogen fluoride as a molecular dipole. Red represents partially negatively charged areas.

In a polar covalent bond, the atom with the stronger affinity for electrons may be shown with a partial negative charge. The atom with the lower affinity for electrons is farther from the electron pair and is shown with a partial positive charge. The illustration depicts the electrical dipole resulting from the formation of a hydrogen fluoride (HF) molecule. The dipole is symbolized by a cross (+ end) pointing in the direction of the negative end with an arrowhead. Note the larger size of the molecule on the negative end. This is because the unequally shared electrons are spending more time there. This is known as a polar covalent bond -- a compromise between ionic and covalent bonding.

[edit] Aqueous

A water molecule, a commonly-used example of polarity. The two charges are present with a negative charge in the middle (red shade), and a positive charge at the ends (blue shade).

It is the polar structure of the water molecule that is responsible for many of the unique physical properties of water. In the water molecule, the oxygen atom, with its eight (+) protons, has a much greater attraction for the shared valence electrons than do either of the hydrogen atoms with a single proton. Therefore, the shared electrons spend more time around the oxygen part of the molecule than they do around the hydrogen part. This results in the oxygen end being more negative than the hydrogen end.

Polar molecules of any substance have attractions between the positive end of one molecule and the negative end of another molecule. When the polar molecule has hydrogen at one end and fluorine, oxygen or nitrogen at the other end, the attractions are strong enough to qualify as type of chemical bonding called hydrogen bonding. Hydrogen bonds are weaker than either covalent bonds or ionic bonds.

Model of hydrogen bonds between molecules of water

A salt such as sodium chloride will form a solution with water because the positively charged sodium cations and the negatively charged chlorine anions are attracted to the positive and negative ends of the water molecule, respectively. The sodium and chlorine ions thus become dissolved (or hydrated) as part of the aqueous saltwater solution. Hydration occurs when the charged solute ions become surrounded by the polar solvent, or water molecules.

Thus, sodium chloride (NaCl) will form a solution with water because the Na+ and Cl- ions in the salt are attracted to the positive and negative parts of water molecules. When ionic compounds dissolve, the resulting solution contains separated ions. The conduction of electricity provides evidence for these ions in solution. Charged ions in solution act as mobile charge carriers (like electrons in metals). Sodium chloride and water together form a strong electrolyte aqueous solution.

Ethanol is one example of a non-ionic solute that is very soluble in water. All alcoholic beverages are aqueous solutions of ethanol. Why is ethanol so soluble in water? The answer lies in the structure of the ethanol molecule. The molecule contains a polar O-H bond like those found in water, which makes it very compatible with water. Just as hydrogen bonds form among water molecules in pure water, ethanol molecules can form hydrogen bonds with water molecules. Table sugar can also be dissolved in water because the glucose molecule has many polar OH groups which will attract the polar water molecules.

[edit] Non-polar

Whether or not two given liquids form solutions depends to some degree on the similarity (or lack thereof) between their respective molecular structures. In general, the forces of attraction between molecules or ions of the solvent and solute will determine the limits of solubility. The water molecule, for example, is a polar molecule with a negative end and a positive end. Polar molecules (such as water) therefore require polar solvents.

Methane, the bonds are arranged symmetrically so there is no overall dipole

On the other hand, covalently bonded substances such as elemental gases, long chain hydrocarbon fuels, oil or grease do not dissolve in water or other polar solvents. Most grease and oil molecules have no polarity. They are virtually electrically neutral, and are therefore non-polar compounds. In general, “like dissolves like” and thus water will not dissolve oil and grease. Non-polar solutes require non-polar solvents.

A molecule may be polar either as a result of polar bonds due to differences in electronegativity as described above, or as a result of an asymmetric arrangement of non-polar covalent bonds and non-bonding pairs of electrons known as a full molecular orbital. In a similar manner, a molecule may be non-polar either because there is (almost) no polarity in the bonds or because of the symmetrical arrangement of polar bonds. For example, in the methane, CH₄ molecule the four C–H bonds are arranged tetrahedrally around the carbon atom. Each bond has polarity (though not very strong). However, the bonds are arranged symmetrically so there is no overall dipole in the molecule.

Many substances do not dissolve in water. When petroleum leaks from a damaged tanker, it does not disperse uniformly in the water (does not dissolve) but rather floats on the surface because its density is less than that of water.

Petroleum is a mixture of molecules composed of long-chain hydrocarbons. Since carbon and hydrogen have similar affinities for electrons, the bonding electrons are shared almost equally and the bonds are essentially non-polar. Covalently bonded substances such as long chain hydrocarbon fuels, oil or grease do not dissolve in water or other polar solvents. Most grease and oil molecules have no polarity. They are virtually electrically neutral, and are therefore non-polar compounds.

In general, “like dissolves like” and thus water will not dissolve oil and grease. Similarly, non-polar solutes require non-polar solvents.

[edit] See also

Look up solution or solute in Wiktionary, the free dictionary.

Wikibooks has a book on the topic of Transwiki:Creating chemical solutions

• Concentration
• Molar solution
• 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.

[edit] References

International Union of Pure and Applied Chemistry. "solution". Compendium of Chemical Terminology Internet edition.