V.

Solutions and Solubility

A solution is a homogeneous mixture of two or more substances. Solutions form because even electrically neutral molecules have weak attractions for one another. Much of this attraction comes from the polarity, or slight unevenness of the electrical charge distribution within the molecules—a local region of slight negative charge in one molecule attracting a region of slight positive charge in another. These weak opposite charges hold molecules together in a liquid and also account for the ability of a liquid to dissolve other substances.

When a substance (called the solute) dissolves in a liquid (called the solvent), the molecules of the solvent must force their way between molecules of the solute. This occurs, for example, when water dissolves crystals of sugar. The solvent can only dissolve the solute if the solvent and solute molecules have similar attractive forces, which leads to the rule that like dissolves like. (The word like refers to similar characteristics of polarity.)

In general, polar molecules will be strongly attracted to one another. For example, water and alcohol mix readily. This occurs because the electronegative (electron-attracting) oxygen atom in water has a slight negative charge, giving both hydrogen atoms a slight positive charge. Similarly, the electronegative –OH (hydroxyl) group of alcohol has a slight negative charge, giving the other end of the alcohol molecule a slight positive charge. Thus, when water and alcohol are added together, the oppositely charged regions of the two types of molecules attract each other, allowing the liquids to mix. Other polar molecules that will dissolve in water include sugar, starch, and vitamin C. Ionic compounds that will dissolve in water include baking soda (NaHCO₃) and table salt (NaCl). The following reaction shows table salt dissolving into its constituent ions in water: NaCl (in H₂O_(l))→ Na⁺ + Cl^-_.

Similarly, nonpolar liquids will mix with each other, because such molecules are able to pry apart other nonpolar molecules. For example, gasoline and carbon tetrachloride, which are both composed of nonpolar molecules, mix easily and are good solvents for similarly nonpolar molecules, such as fats, greases, and paraffins.

On the other hand, a liquid composed of polar molecules will not readily mix with a liquid composed of nonpolar molecules, because the nonpolar molecules are not able to pry the polar molecules apart. For instance, water will not readily mix with gasoline or benzene (both of which consist of nonpolar molecules), because the polar water molecules are too strongly held together to allow entry of the nonpolar hydrocarbons.

While some liquids, such as water and alcohol, can dissolve in each other in any proportion, other compounds cannot. For example, salt added to water will dissolve until a threshold is reached, after which new salt added will no longer dissolve. This solution of salt water is then called saturated. A compound’s solubility in a given solvent is measured as the maximum amount of the compound that a solution can dissolve.

A.

Effect of Temperature and Pressure

Raising the temperature usually increases the solubility of liquids and solids. The increase in temperature increases the energy of motion of the molecules (the kinetic energy) and partially overcomes the lack of attraction between polar and nonpolar molecules. Pressure has little effect on the solubility of liquids and solids, because the volumes of these materials change only slightly when they are dissolved.

Pressure has more effect on the solubility of gases in liquid solvents. A gas is more soluble as the pressure increases, because the gas atoms or molecules are crowded together, forcing more of the gas particles into contact with the liquid. Gases, however, become less soluble as the temperature increases, because as the gas molecules gain energy of motion, they are more easily able to escape the solution.

B.

Electrolytes

Solutions of ions conduct an electric current, in much the same manner as a wire does. Ions can move about in a solution and carry a charge, just as electrons moving along a wire conduct a current. Substances that can carry a charge through solution in this way are called electrolytes; those that cannot are called nonelectrolytes.

C.

Concentration

The amount of dissolved material in a solution is called the concentration and can be expressed in units, such as grams per liter or ounces per gallon. Chemists sometimes use percent to indicate concentration, and by convention indicate whether the percent is by weight or volume. A 10 percent solution of alcohol in water would normally be thought of as 10 volumes of alcohol in 90 volumes of water. A 10 percent solution of sodium chloride is thought of as 10 weight units of salt in 90 weight units of water. Scientists often use parts per million (or billion) when the amount of solute is very small. For chemical purposes, expressing concentration in terms of the number of molecules (or ions) in solution is often preferable.

C.1.

Mole

The mole is one of the seven fundamental units in the International System of Units (SI). The mole is the unit used for measuring the amount of a substance and is defined as the amount of a substance containing the same number of atoms, molecules, or ions as the number of atoms in 12 grams of ¹²C. Because there are 6.022 × 10²³ atoms of carbon in 12 grams of ¹²C, this number (6.022 × 10²³), known as Avogadro's number, is the amount of matter containing 6.022 × 10²³ atoms, molecules, or ions. (see Avogadro’s Law).

The mole concept provides a means of calculating how many atoms, ions, or molecules are in a sample by weighing the substance. From the definition of atomic weight, the amount of any element that has a mass (in grams) equal to its atomic weight (available on the periodic table) will contain 6.022 × 10²³ atoms. Thus, 4.0026 grams of helium, 32.0064 grams of sulfur, and 200.59 grams of mercury each contain 6.022 x 10²³ atoms.

Similarly, a mole of a molecular substance (6.022 × 10²³ molecules) is the amount of the substance whose mass (in grams) is equal to its molecular weight. Molecular weight is derived by summing the atomic weights of the atoms composing a molecule. For example, 70.906 grams (2 × 35.453) of Cl₂ contains 6.022 × 10²³ molecules (one mole) of Cl₂.

Chemists use the same principle to measure the number of ions in a compound. For example, one mole of sodium ions (Na⁺) has a mass of 22.9898 grams (atomic weight of Na is 22.9898). One mole of NaCl has a mass of 58.443 grams (22.9898 + 35.453).

C.2.

Molarity

Molarity is the concentration of a substance in solution and is expressed as moles of solute per liter of the solution. Thus, a 0.1 molar (abbreviated 0.1 M) solution of sodium chloride contains 5.8443 (58.443 × 0.1) grams of NaCl per liter of solution.

C.3.

Molality

Molality, a term less frequently used than molarity, is the number of moles of solute in 1000 grams of solvent. Thus, a 0.1 molal solution of sodium chloride in water has 5.8443 grams of NaCl in 1000 grams of H₂O.

C.4.

Normality

Normality is the number of equivalents per liter of solution. For acid-base-salt systems, an equivalent is the amount of the substance that will gain or lose one mole of H⁺ ions. For instance, one mole of sulfuric acid (H₂SO₄), which has a mass of 98.0795 grams, produces two moles of H⁺, or two equivalents. Therefore, a one molar solution of sulfuric acid is a two normal (2 N) solution. A 0.1 N solution (containing 0.1 moles of H⁺) of sulfuric acid contains 4.90397 grams of H₂SO₄ per liter of solution ([98.0795/2] × 0.1).