A solution is another name for a homogeneous mixture. A mixture as a material composed of two or more substances. In a solution, the combination is so intimate that the different substances cannot be differentiated by sight, even with a microscope. Compare, for example, a mixture of salt and pepper and another mixture consisting of salt and water. In the first mixture, we can readily see individual grains of salt and the flecks of pepper. A mixture of salt and pepper is not a solution. However, in the second mixture, no matter how carefully we look, we cannot see two different substances. Salt dissolved in water is a solution.
The major component of a solution, called the solvent, is typically the same phase as the solution itself. Each minor component of a solution (and there may be more than one) is called the solute. In most of the solutions we will describe in this textbook, there will be no ambiguity about whether a component is the solvent or the solute. For example, in a solution of salt in water, the solute is salt, and solvent is water.
Solutions come in all phases, and the solvent and the solute do not have to be in the same phase to form a solution (such as salt and water). For example, air is a gaseous solution of about 80% nitrogen and about 20% oxygen, with some other gases present in much smaller amounts. An alloy is a solid solution consisting of a metal (like iron) with some other metals or nonmetals dissolved in it. Steel, an alloy of iron and carbon and small amounts of other metals, is an example of a solid solution. Table \(\PageIndex{1}\) lists some common types of solutions, with examples of each.
Table \(\PageIndex{1}\): Types of Solutions Solvent Phase Solute Phase Example gas gas air liquid gas carbonated beverages liquid liquid ethanol (C2H5OH) in H2O (alcoholic beverages) liquid solid saltwater solid gas H2 gas absorbed by Pd metal solid liquid Hg(ℓ) in dental fillings solid solid steel alloysWhat causes a solution to form? The simple answer is that the solvent and the solute must have similar intermolecular interactions. When this is the case, the individual particles of solvent and solute can easily mix so intimately that each particle of solute is surrounded by particles of solute, forming a solution. However, if two substances have very different intermolecular interactions, large amounts of energy are required to force their individual particles to mix intimately, so a solution does not form. Thus two alkanes like n-heptane, C7H16, and n-hexane, C6H14, are completely miscible in all proportions. The C7H16 and C6H14 molecules are so similar (recall Section 4.6) that there are only negligible differences in intermolecular forces.
For a similar reason, methanol, CH3OH, is completely miscible with water. In this case both molecules are polar and can form hydrogen bonds among themselves, and so there are strong intermolecular attractions within each liquid. However, CH3OH dipoles can align with H2O dipoles, and CH3OH molecules can hydrogen bond to H2O molecules, and so the attractions among unlike molecules in the solution are similar to those among like molecules in each pure liquid.
This process leads to a simple rule of thumb: like dissolves like. Solvents that are very polar will dissolve solutes that are very polar or even ionic. Solvents that are nonpolar will dissolve nonpolar solutes. Thus water, being polar, is a good solvent for ionic compounds and polar solutes like ethanol (C2H5OH). However, water does not dissolve nonpolar solutes, such as many oils and greases (Figure \(\PageIndex{1}\)).
Figure \(\PageIndex{1}\): A beaker holds water with blue food dye (upper liquid layer) and a much more dense perfluoroheptane (a fluorocarbon) lower liquid layer. The two fluids cannot mix and the dye cannot dissolve in fluorocarbon. A goldfish and a crab have been introduced into the water. The goldfish cannot penetrate the dense fluorocarbon. The crab floats at the liquid boundary with only parts of his legs penetrating the fluorocarbon fluid, unable to sink to the bottom of the beaker. Quarter coins rest on the bottom of the beaker. Animals were rescued from their predicament after the photo was taken. Figure used with permission from Wikipedia (Sbharris (Steven B. Harris)).We use the word soluble to describe a solute that dissolves in a particular solvent, and the word insoluble for a solute that does not dissolve in a solvent. Thus, we say that sodium chloride is soluble in water but insoluble in hexane (C6H14). If the solute and the solvent are both liquids and soluble in any proportion, we use the word miscible, and the word immiscible if they are not.
Water is considered a polar solvent. Which substances should dissolve in water?
Because water is polar, substances that are polar or ionic will dissolve in it.
Toluene (C6H5CH3) is widely used in industry as a nonpolar solvent. Which substances should dissolve in toluene?
Octane only.
Predict which of the following compounds will be most soluble in water:
Since ethanol contains an OH group, it can hydrogen bond to water. Although the same is true of hexanol, the OH group is found only at one end of a fairly large molecule. The rest of the molecule can be expected to behave much as though it were a nonpolar alkane. This substance should thus be much less soluble than the first. Experimentally we find that ethanol is completely miscible with water, while only 0.6 g hexanol dissolves in 100 g water.
Would I2 be more soluble in CCl4 or H2O?
I2 is nonpolar. Of the two solvents, CCl4 is nonpolar and H2O is polar, so I2 would be expected to be more soluble in CCl4.
Solutions are all around us, e.g., air, seawater, body fluids, metal alloys are solutions. Fig. 5.1.1 illustrates that air is a mixture of nitrogen, oxygen, carbon dioxide, and some other gases; Fig. 5.1.2 illustrates that seawater is a mixture of water, chloride, sodium, sulfate, magnesium, and some other ions, and Fig. 5.1.3. illustrates that about 60% of the human body is composed of solutions called body fluids.
Figure \(\PageIndex{1}\): Air is a solution of nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and some other gases . Source: RoRo / CC0 Figure \(\PageIndex{2}\): Seawater is a solution of water and several ions, expressed in wt/wt%. Source: derivative work: Tcncv (talk)Sea_salt-e_hg.svg: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany; SVG version by Stefan Majewsky / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5) Figure \(\PageIndex{3}\): This schematic shows the relative volumes of the different fluid compartments in an adult male. Source: Alan Sved and David Walsh / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)What is a solution?
For example, the air is a solution in which nitrogen is the solvent, and water is the solvent in seawater and body fluids. Oxygen, carbon dioxide, and water vapors are solutes in the air; and sodium, chloride, sulfate, magnesium, and some other ions are solutes in seawater.
The solutions are generally classified in two ways: i) based on the physical state of the solution and the solute, and ii) based on the particle size of the solute.
The solutions can be classified based on the physical state of the solution, solvent, and solute. For example, the air is gas in a gas solution; carbonated water is a gas in a liquid solution; vinegar is a liquid in a liquid solution; metal alloys are solid in solid solutions. Table 5.1.1 lists the major types of solutions, solvents, and primary solutes in them.
Table 1: Examples of main types of solutions and solvent and major solute in them. Type Example solvent Primary solute Gas in gas Air Nitrogen Oxygen Gas in liquid Carbonated water Water Carbon dioxide Liquid in liquid Vinegar Water Acetic acid Solid in liquid Seawater Water Sodium chloride Solid in solid Brass Copper ZincA solution is a homogeneous mixture comprising smaller component/s called solute/s of small molecules or ions comparable in size to the molecules of a larger component called the solvent.
For example, NaCl dissolved in water is a solution. The solute is almost uniformly distributed in the solvent, making a homogeneous mixture. The solute does not separate by filtration or by a semipermeable membrane but can be separated by some other physical process. For example, the distillation process separates a solid in a liquid or a liquid into a liquid solution. The solution is transparent, though it may be colored. A light passing through a solution is not visible, as shown in Fig. 5.1.4.
Figure \(\PageIndex{4}\): Demonstration of Tyndall effect: a red-laser bean passing through a solution is not visible (left), but visible when passing through a colloid (right). Source: https://youtu.be/8Xcpq6e8pBY, Creative Commons Attribution license (reuse allowed)A suspension is a heterogeneous mixture of solvent and solute particles of larger than 10,000 Å.
For example, muddy water is a suspension. If the suspension is allowed to stand, the suspended particles settle down and separate. The suspended particles can be filtered out. Some medicines, e.g., milk of magnesia, are suspensions. It is instructed to shake just before administering medicine to re-suspend the settled suspension.
A colloid falls between a solution and a suspension. The colloidal particles are larger molecules like proteins or groups of molecules or ions.
Unlike a suspension, the colloids usually do not settle if allowed to stand. The colloidal particles can not be filtered but can be separated by a semipermeable membrane. When a light beam passes through a colloid, it scatters by the colloid particles, called the Tyndall effect, and becomes visible, as shown in Fig. 5.1.4.
Examples of colloids include:
Water (H2O) is an essential substance for life. It covers more than 70% of the earth’s surface (Fig. 5.1.5), and it comprises more than 60% of the human body (5.1.3). In addition to being the most abundant solvent, water is a universal solvent because it is a polar molecule with a partial negative charge on oxygen and a partial positive charge on hydrogen atoms as shown in Fig. 5.1.6. The polarity of water molecules allows them to interact with other water molecules as well as with other polar compounds through dipole-dipole interactions and with other ions through ion-dipole interactions. These interactions help to dissolve a lot of polar and ionic compounds that are in and around us.
Figure \(\PageIndex{5}\): the image of the earth showing saltwater in the sea, fresh water on the Antarctic ice sheet, and clouds. Source: by NASA mission AS17. Public domain. Figure \(\PageIndex{6}\): A model of water molecule showing oxygen in red and hydrogen in white color balls overplayed with an electrostatic potential map showing the partial positive (+d) region in red and the partial negative (-d) region in blue, drawn using a free web app https://molview.orgHow does water dissolves polar and ionic compounds?
Water molecules establish electrostatic interaction, called hydrogen bonding, through the partial +ve end of one molecule with a partial –ve end of a neighboring molecule. These interactions impart unique properties to water, like its relatively higher boiling point and melting point compared to other substances of similar molecular weight. Other polar substances have similar interactions, e.g., ethanol has hydrogen bonding similar to water as illustrated in Fig. 5.1.7.
When water mixes with other polar substances, like ethanol, some of the hydrogen bonding between water molecules replace with similar hydrogen bonding with ethanol molecules. Since the electrostatic potential energy is similar, the natural tendency to go towards more dispersion drives the dispersion of ethanol molecules uniformly in water resulting in the solution.
Figure \(\PageIndex{7}\): Models showing hydrogen bonding in ethanol (left), water (middle), and ethanol water solution (right). Source: modified from Benjah-bmm27, and snek01. Public domainIonic compounds are held together by electrostatic forces between opposite ions, i.e., ionic bonds. When an ionic compound is added to water, water molecules surround the cation and establish ion-dipole interaction by orienting their partial -ve end to the cation. Similarly, water molecules establish ion-dipole interaction with anions by orienting their partial +ev end towards the anion, as illustrated in Fig. 5.1.8
The ion-dipole interactions, along with nature’s tendency to disperse the particles, are usually strong enough to overcome the ionic bonds, dissociate the compounds into ions, and disperse them almost uniformly in the water.
Figure \(\PageIndex{8}\): Dissolution of sodium chloride in water through dissociation into ions and dispersion in water driven by ion-dipole interactions and nature’s tendency to disperse particles. Source: Andy Schmitz / CC BY (https://creativecommons.org/licenses/by/3.0)The separation of the cations from the anions of the ionic compound is called dissociation.
The formation of a layer of water molecules around ions, driven by ion-dipole interactions, is called hydration.
Non-polar substances, like vegetable oil or gasoline, do not dissolve in water. The molecules in non-polar substances have only London dispersion forces. They easily dissolve in non-polar solvents like hexane or carbon tetrachloride that have similar London dispersion forces among their molecules.
The fact that ionic and polar substances dissolve in polar solvents and non-polar substances dissolve in non-polar solvents of similar intermolecular interactions is called “like dissolves like.”