Life on earth depends upon the fact that certain substances dissolve in other substances. We constantly deal with solutions. We enjoy cola drinks because dissolved carbon dioxide gives them an effervescence. Fish can live because oxygen gas is dissolved in lakes and oceans. We grow and develop because nutrients dissolved in our blood can reach cells and tissues.
The process of dissolving has always been fascinating. Early alchemists searched for the “universal solvent”—that mythical liquid that would dissolve everything. Perhaps it never occurred to them that, if they found it, it would be difficult to contain! Of course, there is no universal solvent. Water is perhaps the closest thing we have to a universal solvent; its truly unique properties make it one of our most important chemical substances.
We now know much more about the process of dissolving than did early chemists. We now understand that particles (ions and molecules) are held together within solids and liquids by certain attractive forces. Some of these are weak and some are relatively strong. For a solution to form, bonds must be broken and reformed, involving changes in energy. Solution formation is most likely to happen when the solute and solvent have certain similar chemical properties. We often generalize this fact by saying that “like dissolves like.”
When a solute dissolves in a solvent in a closed system, the process continues until a certain amount of solute enters solution. After that point, particles return to the solute at the same rate they leave the solute and dissolve. Thus, dissolving is a dynamic equilibrium process. It has often been stated that “everything is somewhat soluble in everything else,” and every substance has a characteristic solubility. When the solubility of a solid is exceeded, the substance comes out of solution in the form of a precipitate. The word precipitate comes from the Latin and means “cast down.” Interestingly enough, chemists a hundred years ago knew little about ions and atoms and how they recombine in solution. When a precipitate formed in solution, they would say that a substance was “thrown down.”
Some substances show very limited solubility in solvents like water. We can express this small solubility quantitatively as the “solubility product constant” (K sp ). This is a useful expression since it allows a chemist to predict if and when a precipitate may occur when solutions are mixed.
The solubility of a substance can be changed by altering the conditions that affect equilibrium. Temperature and pressure affect the solubility of gases in liquids. The solubility of solids in liquids is affected by temperature.
If we collect enough data regarding the solubility of certain substances, especially ionic compounds, we can formulate a set of rules or generalizations that help us predict whether or not certain compounds will form solutions.
Students can understand and appreciate the ideas of solubility and precipitation with even greater comprehension after they have studied the concepts of bonding and equilibrium as part of class discussion on the solution process. There are many important and practical applications of these concepts.
- Certain substances have the ability to dissolve other substances.
- The extent of solubility depends upon interparticle solute-solute, solvent-solvent, and solute-solvent attractions
- When the solute is only slightly soluble, the extent of solubility can be quantitatively expressed as the solubility product constant.
- The solubility of a substance can be altered by the common ion effect, pH\par change, temperature change, etc.
- Accumulated data allow one to predict the degree of solubility of many substances (solubility rules).
- Bonding
- Energy
- Entropy
- Polar and nonpolar molecules
- Ionization/dissociation
- Equilibrium
- Kinetic molecular theory
- Exponents
- Constants
- Logarithms
- Working with small-scale volumes
- Measuring volumes using a graduated cylindeer or other volumetric apparatus
After completing their study of solubility and precipitation, students should be able to:
- explain phrase “like dissolves like” as it relates to polar and nonpolar materials.
- discuss the effect that entropy and enthalpy have on the dissolving process.
- explain why a precipitate forms when solutions of two ionic compounds are mixed.
- explain why gases become less soluble at higher temperatures, whereas most solids become more soluble.
- predict the solubility of an ionic compound by applying generalizations (rules) determined by laboratory experiments.
- give practical, personal, and everyday examples of situations that involve solubility and precipitation.
- explain the relationship between equilibrium and solubility.
- explain how solubilities of solids and gases are changed by changing temperature and pressure.
- discuss effect of temperature on endothermic and exothermic solution processes.
- describe how increase in disorder favors the solution process.
- discuss energy required for bond breaking vs. energy released when new bonds are formed.
- calculate the Ksp for an ionic compound if they are given the solubility of the compound.
- calculate solubility of an ionic compound if they are given Ksp for that compound.
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