In
chemistry, a solution is a special type of
homogeneous mixture
In chemistry, a mixture is a material made up of two or more different chemical substances which are not chemically bonded. A mixture is the physical combination of two or more substances in which the identities are retained and are mixed in the ...
composed of two or more substances. In such a mixture, a solute is a substance
dissolved in another substance, known as a
solvent. If the attractive forces between the solvent and solute particles are greater than the attractive forces holding the solute particles together, the solvent particles pull the solute particles apart and surround them. These surrounded solute particles then move away from the solid solute and out into the solution. The mixing process of a solution happens at a scale where the effects of
chemical polarity
In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end.
Polar molecules must contain one or more pola ...
are involved, resulting in interactions that are specific to
solvation. The solution usually has the
state of the solvent when the solvent is the larger fraction of the mixture, as is commonly the case. One important parameter of a solution is the
concentration, which is a measure of the amount of solute in a given amount of solution or solvent. The term "
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be ...
" is used when one of the solvents is
water.
Characteristics
* A solution is a
homogeneous mixture
In chemistry, a mixture is a material made up of two or more different chemical substances which are not chemically bonded. A mixture is the physical combination of two or more substances in which the identities are retained and are mixed in the ...
of two or more substances.
* The particles of solute in a solution cannot be seen by the naked eye. By contrast, particles may be visible in a
suspension.
* A solution does not cause beams of
light to
scatter. By contrast, the particles in a suspension can cause
Tyndall scattering
The Tyndall effect is light scattering by particles in a colloid or in a very fine suspension. Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to the ...
or
Rayleigh scattering.
* A solution is stable; solutes will not precipitate unless added in excess of the mixture's
solubility, at which point the excess would remain in its solid phase, referred to as
hypersaturation
In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a liq ...
.
* The solute from a solution cannot be separated by
filtration
Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter m ...
(or mechanically).
* It is composed of only one
phase.
Types
''Homogeneous'' means that the components of the mixture form a single phase. ''Heterogeneous'' means that the components of the mixture are of different phase. The properties of the mixture (such as concentration, temperature, and density) can be uniformly distributed through the volume but only in absence of diffusion phenomena or after their completion. Usually, the substance present in the greatest amount is considered the solvent. Solvents can be gases, liquids, or solids. One or more components present in the solution other than the solvent are called solutes. The solution has the same
physical state as the solvent.
Gaseous mixtures
If the solvent is a
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
, only gases (non-condensable) or vapors (condensable) are dissolved under a given set of conditions. An example of a gaseous solution is
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
(oxygen and other gases dissolved in nitrogen). Since interactions between gaseous molecules play almost no role, non-condensable gases form rather trivial solutions. In the literature, they are not even classified as solutions, but simply addressed as homogeneous
mixtures of gases. The
Brownian motion
Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended in a medium (a liquid or a gas).
This pattern of motion typically consists of random fluctuations in a particle's position ins ...
and the permanent molecular agitation of gas molecules guarantee the homogeneity of the gaseous systems. Non-condensable gases mixtures (''e.g.'', air/CO
2, or air/xenon) do not spontaneously demix, nor sediment, as distinctly stratified and separate gas layers as a function of their
relative density
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
.
Diffusion forces efficiently counteract
gravitation forces under normal conditions prevailing on Earth. The case of condensable vapors is different: once the
saturation vapor pressure at a given temperature is reached, vapor excess condenses into the
liquid state.
Liquid solutions
If the solvent is a
liquid, then almost all gases, liquids, and solids can be dissolved. Here are some examples:
* Gas in liquid:
**
Oxygen in water
**
Carbon dioxide in water – a less simple example, because the solution is accompanied by a chemical reaction (formation of ions). The visible bubbles in
carbonated water
Carbonated water (also known as soda water, sparkling water, fizzy water, club soda, water with gas, in many places as mineral water, or especially in the United States as seltzer or seltzer water) is water containing dissolved carbon dioxide gas, ...
are not the dissolved gas, but only an
effervescence of carbon dioxide that has come out of solution; 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.
* Solid in liquid:
**
Sucrose (table
sugar) in water
**
Sodium chloride (NaCl) (table
salt) or any other
salt in water, which forms an
electrolyte: When dissolving, salt dissociates into
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
s.
*
Solutions in water are especially common, and are called
aqueous solutions
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be r ...
.
* Non-aqueous solutions are when the liquid solvent involved is not water.
Counter examples are provided by liquid mixtures that are not
homogeneous:
colloid
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
s,
suspensions,
emulsions are not considered solutions.
Body fluid
Body fluids, bodily fluids, or biofluids, sometimes body liquids, are liquids within the human body. In lean healthy adult men, the total body water is about 60% (60–67%) of the total body weight; it is usually slightly lower in women (52-55%). ...
s are examples of complex liquid solutions, containing many solutes. Many of these are electrolytes since they contain solute ions, such as
potassium. Furthermore, they contain solute molecules like sugar and
urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important ...
. Oxygen and carbon dioxide are also essential components of
blood chemistry
Clinical chemistry (also known as chemical pathology, clinical biochemistry or medical biochemistry) is the area of chemistry that is generally concerned with analysis of bodily fluids for diagnostic and therapeutic purposes. It is an applie ...
, where significant changes in their concentrations may be a sign of severe illness or injury.
Solid solutions
If the solvent is a
solid, then gases, liquids, and solids can be dissolved.
* Gas in solids:
**
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
** Water in solid salt or
sugar, forming moist solids
**
Hexane
Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14.
It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relatively ...
in
paraffin wax
**
Polymers containing
plasticizers such as
phthalate
Phthalates (, ), or phthalate esters, are esters of phthalic acid. They are mainly used as plasticizers, i.e., substances added to plastics to increase their flexibility, transparency, durability, and longevity. They are used primarily to soften ...
(liquid) in
PVC (solid)
* Solid in solid:
**
Steel, basically a solution of carbon atoms in a crystalline matrix of iron atoms
**
Alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
s like
bronze
Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (including aluminium, manganese, nickel, or zinc) and sometimes non-metals, such as phosphorus, or metalloids such ...
and many others
**
Radium sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
dissolved in
barium sulfate
Barium sulfate (or sulphate) is the inorganic compound with the chemical formula Ba SO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and ...
: a true solid solution of Ra in BaSO
4
Solubility
The ability of one
compound
Compound may refer to:
Architecture and built environments
* Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall
** Compound (fortification), a version of the above fortified with defensive struc ...
to dissolve in another compound is called
solubility. When a liquid can completely dissolve in another liquid the two liquids are ''miscible''. Two substances that can never mix to form a solution are said to be ''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
Saturation, saturated, unsaturation or unsaturated may refer to:
Chemistry
* Saturation, a property of organic compounds referring to carbon-carbon bonds
**Saturated and unsaturated compounds
**Degree of unsaturation
**Saturated fat or fatty acid ...
. 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
The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.
The boiling point of a liquid varies depending upon the surrounding envi ...
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
Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of sol ...
,
volume fraction, and
mole fraction.
The properties of
ideal solution
In chemistry, an ideal solution or ideal mixture is a solution that exhibits thermodynamic properties analogous to those of a mixture of ideal gases. The enthalpy of mixing is zero as is the volume change on mixing by definition; the closer to zer ...
s 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 solution characteristics
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 can form
hydrogen bonds (
protic
In chemistry, a protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group ), a nitrogen (as in an amine group or ), or fluoride (as in hydrogen fluoride). In general terms, any solvent that contains a labil ...
and aprotic solvents).
Water, the most commonly used solvent, is both polar and sustains 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 Hydration may refer to:
* Hydrate, a substance that contains water
* Hydration enthalpy, energy released through hydrating a substance
* Hydration reaction, a chemical addition reaction where a hydroxyl group and proton are added to a compound
* ...
occurs when the charged solute ions become surrounded by water molecules. A standard example is aqueous saltwater. Such solutions are called
electrolytes. Whenever salt dissolves in water
ion association has to be taken into account.
Polar solutes dissolve in polar solvents, forming polar bonds or hydrogen bonds. As an example, all alcoholic beverages are
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be ...
s of
ethanol. On the other hand, non-polar solutes dissolve better in non-polar solvents. Examples are hydrocarbons such as
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
and
grease that easily mix, while being incompatible with water.
An example of 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.
Preparation from constituent ingredients
It is common practice in laboratories to make a solution directly from its constituent ingredients. There are three cases in practical calculation:
* Case 1: amount of solvent volume is given.
* Case 2: amount of solute mass is given.
* Case 3: amount of final solution volume is given.
In the following equations, A is solvent, B is solute, and C is concentration. Solute volume contribution is considered through the ideal solution model.
* Case 1: amount (mL) of solvent volume V
A is given. Solute mass m
B = C V
A d
A /(100-C/d
B)
* Case 2: amount of solute mass m
B is given. Solvent volume V
A = m
B (100/C-1/ d
B )
* Case 3: amount (mL) of final solution volume Vt is given. Solute mass m
B = C Vt /100; Solvent volume V
A=(100/C-1/ d
B) m
B
* Case 2: solute mass is known, V
A = m
B 100/C
* Case 3: total solution volume is known, same equation as case 1. V
A=Vt; m
B = C V
A /100
Example: Make 2 g/100mL of NaCl solution with 1 L
water. The density of the resulting solution is considered to be equal to that of water, statement holding especially for dilute solutions, so the density information is not required.
:m
B = C V
A = ( 2 / 100 ) g/mL × 1000 mL = 20 g
Chemists often make concentrated stock solutions that may then be diluted as needed for laboratory applications.
Standard solution In analytical chemistry, a standard solution is a solution containing a precisely known concentration of an element or a substance. A known mass of solute is dissolved to make a specific volume. It is prepared using a standard substance, such as a ...
s are those where concentrations of solutes are accurately and precisely known.
See also
*
*
*
* 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 the evaporation of water from a sample.
*
*
*
References
*
External links
*
{{Authority control
Homogeneous chemical mixtures
Alchemical processes
Physical chemistry
Colloidal chemistry
Drug delivery devices
Dosage forms