When two or more kinds of atoms, molecules, or ions mix and form a homogeneous blend— a mixture that is the same throughout—a solution has been created, whether the blend is a liquid, a solid, or a gas. The air people breathe is a solution of gases—in any given cubic foot or meter of air, the ratio of nitrogen to oxygen, the two main constituents, is the same— roughly 4:1. There are also solid solutions, one example of which is the alloy bronze—a solution of one metal in another. In nature, many varieties of minerals are solid solutions, including a series of common rock-forming minerals called the plagioclase feldspars.
From a chemist’s point of view, however, liquid solutions can be more interesting and versatile than solid or even gaseous solutions. For one thing, liquid solutions are easy to study in the laboratory; for another, many are based on water, which is an excellent solvent-inexpensive and plentiful.
Water forms three different kinds of solutions, depending on the state of the solute (substance) being dissolved, whether it is solid, liquid, or gas. Beer, an aqueous (water-based) solution, demonstrates all three because it contains a liquid solute—alcohol; a gaseous solute—C02; and a solid solute-sugar.
The properties of a solution reflect the homogeneity that is the essence of its definition:
■ The solute remains in solution unless something is added, such as energy or certain other substances, to cause it to precipitate, or separate out.
■ A solution cannot be separated by filtering it, no matter how fine the filter.
■ Solutes are evenly distributed in a solution-gravity has no effect on them, so there is as much solute near the top of the solution as there is near the bottom.
Another property of solutions with many useful applications is the way different solutes affect the freezing or boiling point of the solvent. For instance, salt (NaCI) is often scattered over a frozen sidewalk to lower the melting point of ice and trigger melting. Interestingly, solutes also raise the boiling point of solutions. Salty water has a higher boiling point than pure water.
Molarity
Chemists need a system of measurement that will show the strength of a solution so that they can judge how much is needed to produce a particular chemical reaction. If they want to mix two solutions together and get exactly the reaction they need, they have to know the relative number of molecules of solute in each solution. They express the strength of a solution by calculating what they call its molarityiM) (the number of moles of solute per liter of solution):
M = moles of solute -s- liters of solution
For example, 1 mole of NaCI dissolved in 1 liter of water is a 1 M solution. So how is a mole of NaCI measured?
By adding the atomic weights of all the elements in the molecular formula of a compound, it is possible to calculate the weight of a mole of the compound—its molecular weight; or molar mass. In the case of salt, 23.0 (Na) + 35.5 (Cl) = 58.5 grams, the weight of a mole of salt. A mole of solute dissolved in 1 liter of water is a 1 molar or 1 M solution, so a 1 M solution of salt consists of 58.5 grams of NaCI dissolved in 1 liter of water, and a 2 M solution of 117 grams, and so on. Chemists can rely on the knowledge that a 1 M, or 2 M, or 3 M … or xM solution contains the same number of molecules of solute as an equal volume ofal M, or 2 M, or 3 M … or * M solution of any other compound.
Is it possible to continue to add salt, increasing the molarity of the solution indefinitely?
No. If the temperature of the solution stays the same, a point is reached at which the next salt crystal added sinks to the bottom, instead of dissolving. The solution is then saturated. However, if the temperature is raised, the solubility will increase and the crystal that sank to the bottom will then dissolve.
Ionization
If salt affects the freezing or boiling point of the solvent—lowering the melting point of ice and raising the boiling point of water, then why does 1 liter of 1 M solution of alcohol in water freeze at a higher temperature than 1 liter of 1 M salt solution? Both solutions contain the same number of molecules. The answer is ionization.
Ions are atoms that have lost or gained one or more electrons and have thus become electrically charged, either positively or negatively. Water dissolves salt by breaking the bond that holds NaCI ions together, freeing Na+ ions and Cl- ions to separate from each other. So, in answer to the question, “Why does an aqueous solution of salt freeze at a lower temperature than a solution of alcohol?”—it can be said that it’s because the salt solution ionizes and the alcohol does not. Although the number of molecules is the same in two solutions with the same molarity, there are twice as many particles— ions—in the salt solution as in the alcohol because of ionization, and each particle helps lower the freezing temperature.
Another interesting thing that happens when salt dissolves in water, forming an ionized solution, is that the water becomes a better conductor of electricity. The mobile ions with their + and – charges make it easier for an electric current to flow through the water.
Lavoisier prepared acids in the laboratory by combining the oxides of nonmetallic elements such as carbon, sulfur, and phosphorus with water, which led him to the mistaken conclusion that oxygen from the oxides was the essential ingredient in an acid. But then an acid was discovered that contained no oxygen-hydrogen chloride (HCI). Swedish chemist Svante Arrhenius (1859-1927), who developed the theory of ionization, recognized that what actually constitutes an acid is H+ ions in solution. For this reason, Arrhenius defined an acid as any compound which causes the release of H+ ions when dissolving in water (as do all the non-metallic oxides that Lavoisier experimented with).
In fact, the strength of an acid, measured on the pH scale (acid pH = 0 to 6), depends on how many more H+ ions there are in a solution than OH- ions. Similarly, the strength of a base—a compound that neutralizes acids, changing them to water—depends on how many more OH- ions there are in a solution than H+ ions (basic pH = 8 to 14). (When a water molecule, H20 or HOH, comes apart or dissociates, two ions result: H+ and OH-).
Acid solutions form when oxidized nott-metaiiic elements dissolve in water. Most of the common or alkali bases form from metallic elements, specifically the lightest ones— sodium, potassium, and calcium—and basic solutions form when these dissolve in water. Bases that form from the union of an hydroxide ion with these alkali metals are called alkalis.
Finally, when a base neutralizes an acid, the hydroxide ion (OH-) of the base combines with the hydrogen ion (H+) of the, acid to form water—neutral, pH = 7:
H + + OH—>HOH
At the same time, the positive ion of the base combines with the negative ion of the acid to form a salt
Na + + Cl- -> NaCI
The pH scale measures acidity or basicity by expressing the number of moles of H+ ions in a solution. In a strongly acidic solution such as battery acid, the pH = 0, which means there is 1 X 10°= 1 mole of H+ ions per liter of solution; in a solution that is one-tenth as acidic, pH = 1, there is 1 X 10~’ = 0.1 mole of H+ ions per liter of solution; and so forth. Notice that in each case the pH number is the same number (except for sign) as the exponent, which is why pH is defined as the negative logarithm of the H+ concentration. At a pH of 7, the concentration of H+ ions (1 X 10^7) is exactly equal to the concentration of OH- ions, and the solution is neutral, neither acid nor basic. At a pH of 14, the concentration of H+ ions (1 X 10-14) is much, much smaller than the concentration of OH- ions (1 X 10°), which makes a solution strongly basic—as basic as lye, for example.
