SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 7

Liquids, Solids, and Phase Changes

SOLUBILITY

Water is often referred to as “the universal solvent” because of the number of common substances that dissolve in water. When substances are dissolved in water to the extent that no more will dissolve at that temperature, the solution is said to be saturated. The substance dissolved is called thesolute and the dissolving medium is called a solvent. To give an accurate statement of a substance’s solubility, three conditions are mentioned: the amount of solute, the amount of solvent, and the temperature of the solution. Since the solubility varies for each substance and for different temperatures, a student must be acquainted with the use of solubility curves such as those shown in Figure 31.

Figure 31. Solubility Curves of Some Common Salts

These curves show the number of grams of solute that will dissolve in 100 grams (milliliters) of water over a temperature range of 0°C to 100°C. Take, for example, the very lowest curve at 0°C. This curve shows the number of grams of KClO3 that will dissolve in 100 grams of water over a temperature range of 0°C to 100°C. To find the solubility at any particular temperature, for example at 50°C, you follow the vertical line up from 50°C until it crosses the curve. At that point you place a ruler horizontally across the page and take the reading on the vertical axis. This point happens to be slightly below the 20 gram mark, or 18 grams. This means that 18 grams of KClO3 will dissolve in 100 grams (milliliters) of water at 50°C.

As a soluble solute is added to water at a given temperature, the solute will continue to go into solution until the water cannot dissolve anymore. This is shown graphically in the figure below. It is the sloped portion of the graph. From that point on, the solute will fall to the bottom and does not dissolve, because an equilibrium has been established between molecules leaving and entering the solid phase. This is the condition when the line on the graph becomes horizontal. The solution is holding the maximum amount of dissolved solute in this amount of water and at 20°C. This is asaturated solution. If more water is added to the saturated solution, then more sodium acetate will dissolve in it. A solution that contains less solute than a saturated solution under the existing conditions is described as an unsaturated solution.

TYPE PROBLEM USING THE SOLUBILITY CURVE:

A solution contains 20 grams of KClO3 in 200 grams of H2O at 80°C. How many more grams of KClO3 can be dissolved to saturate the solution at 90°C?

Reading the graph at 90° and up to the graph line for KClO3, you find that 100 grams of H2O can dissolve 48 grams. Then 200 grams can hold (2 × 48) grams or 96 grams. Therefore 96 g − 20 g = 76 g KClO3 can be added to the solution.

General Rules of Solubility

All nitrates, acetates, and chlorates are soluble.

All common compounds of sodium, potassium, and ammonium are soluble.

All chlorides are soluble except those of silver, mercury(I), and lead. (Lead chloride is noticeably soluble in hot water.)

All sulfates are soluble except those of lead, barium, strontium, and calcium. (Calcium sulfate is slightly soluble.)

The normal carbonates, phosphates, silicates, and sulfides are insoluble except those of sodium, potassium, and ammonium.

All hydroxides are insoluble except those of sodium, potassium, ammonium, calcium, barium, and strontium.

TIP 

You should be familiar with these general rules of the solubility of solids.

Some general trends of solubility are shown in the chart below.

 

Temperature Effect

Pressure Effect


Solid

Solubility usually increases with temperature increase.

Little effect

Gas

Solubility usually decreases with temperature increase.

Solubility varies in direct proportion to the pressure applied to it: Henry’s Law.

Factors That Affect Rate of Solubility (How Fast They Go Into Solution)

The following procedures increase the rate of solubility

Pulverizing

increases surface exposed to solvent.

Stirring

brings more solvent that is unsaturated into contact with solute.

Heating

increases molecular action and gives rise to mixing by convection currents. (This heating affects the solubility as well as the rate of solubility.)

Summary of Types of Solutes and Relationships of Type to Solubility

Generally speaking, solutes are most likely to dissolve in solvents with similar characteristics; that is, ionic and polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents.

It should also be mentioned that polar molecules that do not ionize in aqueous solution (e.g., sugar, alcohol, glycerol) have molecules as solute particles; polar molecules that partially ionize in aqueous solution (e.g., ammonia, acetic acid) have a mixture of molecules and ions as solute particles; and polar molecules that completely ionize in aqueous solution (e.g., hydrogen chloride, hydrogen iodide) have ions as solute particles.