- Online Text
- 1. Introduction
- 2. What Is a Solution?
- 3. Solutions and Solubility
- 4. Solution Concentrations
- 5. Analyzing Solutions
- 6. Raoult's Law
- 7. Henry's Law
- 8. Colligative Properties—Vapor Pressure and Osmosis
- 9. Colligative Properties—Freezing and Boiling
- 10. Separation and Purification
- 11. Conclusion
- 12. Further Reading
- Unit Guide (PDF)
Section 3: Solutions and Solubility
Solutions can be made of liquids, gases, or even solids. However, when we speak of solutions, we commonly think of the solution being in the liquid phase, such as coffee, tea, juice, household bleach, shampoo, gasoline, and ink. Some solutions are also in the gas phase; air is a gaseous solution consisting mainly of nitrogen and oxygen, plus much smaller amounts of argon, carbon dioxide, and a few other gases, all mixed evenly together at the molecular level. There can even be solutions that are in the solid phase; a good example of these are metal alloys, like dental fillings, and these will be revisited in Unit 13.
Figure 8-3. Solutes and Solvents
Left: In tea, water is the solvent, and the molecules released by the tea leaves are the solutes (they dissolve in the water). The tea ball holds the tea inside a mesh so that the leaves don't escape, making your tea into a heterogeneous mixture. Right: Solutes enter the solvent and eventually mix evenly throughout, making a homogeneous solution.
© Left: Flickr.com, Creative Commons License 2.0. Author: Kessop, 17 February 2011. Right: Science Media Group.
The component that represents the largest fraction of a solution is called the "solvent," and its other components are called the "solutes." The solvent is the substance in which other materials (the solutes) dissolve. For example, the solvent in tea is water, and the solutes are all of the various molecules released by the tea leaves. (Note that the tea leaves themselves do not dissolve and become part of the solution—they are steeped to release flavor, then filtered out.) (Figure 8-3)
FIgure 8-4. Solubility
Water can only dissolve so much salt. Beyond this quantity, additional salt will remain in solid form and not become part of the solution.
© Science Media Group.
A solvent cannot always dissolve a given amount of a solute. When table salt (sodium chloride, NaCl) is added to water, it makes a salty solution. But if we keep adding salt, it doesn't keep getting saltier beyond a certain point. The salt that was initially added to the water dissolves, but eventually additional salt just sits in a pile on the bottom of the container. At this point when no more solute can dissolve, we say the solution is saturated. At a given temperature, a particular solvent can dissolve a set amount of a particular solute, and then no more. This amount, usually measured in grams per volume of solvent, is said to be the solubility of the solute in the solvent at that temperature.
For example, table salt has a solubility of 36 grams per 100 mL of water at room temperature. This means that we can dissolve 36 grams of salt in every 100 mL of water we have. If more salt is added to the solution, it will just sit on the bottom of the container. (Figure 8-4) Solubilities are always given for a particular temperature of solvent. For table salt, if the temperature is raised, the 100 mL of water can dissolve more salt; if it is lowered, the water will hold less salt. This is not true for all solutes. Some of them are actually more soluble at lower temperatures.
Figure 8-5. Precipitation
When molecules of a solid begin to disperse throughout a solvent, the substance is said to be "dissolving." When the opposite process occurs, and molecules in a solution collect themselves back into a solid state, they are said to be "precipitating." In this picture, sugar crystals are precipitating out of solution to form rock candy.
© Science Media Group.
If we prepared a saturated solution of salt at 25°C and then cooled the solution, we would see little salt crystals begin to appear in the liquid. This process of molecules coming together to form crystals is called "precipitation." (Figure 8-5) By cooling the solution, we have made the solution supersaturated, which means there is more solute than the solution normally can dissolve. The process of precipitation creates a solid that is called the "precipitate." Precipitation is the opposite of dissolution. When a substance dissolves, its molecules spread themselves out throughout the solvent. When it precipitates, the scattered molecules come together to form solid particles or, in some cases, crystals.