Tell the class, “It’s time to pick up where we left off, talking about molecular and ionic solutes, whether they were strong or weak electrolytes, or whether they were nonelectrolytes. Today, we are going to learn about solubility.” Ask the class if anyone can explain what the solubility of a substance is. Give the class a few seconds to think, and then ask for a show of hands. After listening to answers, say, “The solubility is the amount of a certain substance that will dissolve in a certain amount of solvent such as water. Usually the solubility is expressed in grams of solute per 100 grams (or milliliters) of water.” Show students the pictures of examples of solubility in the natural world (S-C-7-3_Solubility Example Pictures.doc). Use a laptop computer or color printouts of the pictures to show the class.
Ask the class to observe and describe the colors of the rock. Students should state that the rocks look red, orange, and black. Next, read the following passage from the handout Mineral and Thermal Springs of the U.S. and Canada from Lesson 1 (S-C-7-1_Mineral and Thermal Springs of the U.S. and Canada.doc).
“It (Cave Spring) is excavated by nature out of a large hill, and is about sixty feet below the surface of the earth. It is overhung, and some places arched over with large flat rock, which are covered with calcareous incrustations, strongly impregnated (or coated) with the sulphate of iron and alum.”
Help the class with the old terminology by saying, “Remember ‘calcareous incrustations’ means calcium carbonate crusts and that ‘sulphate of iron’ and ‘alum’ are old names for iron(II) sulfate and hydrated aluminum potassium sulfate.” Write the names and formulas on the whiteboard with the black marker:
- calcium carbonate (CaCO3)
- iron(II) sulfate (FeSO4)
- alum (KAl(SO4)2.12H2O)
Tell the class, “We know that these minerals come from water deep within the earth. I want you now to develop a hypothesis to explain how these substances ended up staining the rocks around the mineral springs.” Allow students three to four minutes to develop their hypotheses. Write down some of the student hypotheses on the whiteboard. Examples might include:
- The rock around the springs chemically reacted with the minerals in the spring water forming the colorful coatings.
- The solubility of the dissolved minerals in the water decreased as the minerals rose to the surface of the springs, forming a coating on the rocks.
Next, tell students, “As you probably remember, the following substances were reported in the water.” Write or show these on the board:
- dissolved carbon dioxide (CO2) / carbonic acid (H2CO3)
- iron(II) carbonate (FeCO3)
- magnesium carbonate (MgCO3)
- hydrogen sulfide (H2S)
- sodium chloride (NaCl)
- bitumen (163 mg Ca2+ and 76 mg Mg2+ / L H2O)
Say, “The most likely hypothesis is that the hydrogen sulfide reacted with some of the dissolved minerals in the water such as the iron forming iron sulfide. This iron sulfide then precipitated from the water as its solubility dropped when it emerged from the ground at Earth’s surface. The precipitated iron sulfide then coated the rocks surrounding the springs. Over time, air and water oxidized the coating to produce iron(II) sulfate and iron oxides, which are responsible for the bright red and orange colors on the rocks.”
Make the transition to a discussion of the factors that affect solubility by handing out the Learning Activity Worksheet 1–Factors that Affect Solubility (S-C-7-3_Solubility Worksheet and KEY.doc). Say, “Now class, I would like for you to break up into groups of three or four and examine the solubility graphs for some of the substances found in Frankfort Mineral Springs. After each set of graphs, answer the questions and then after 20 minutes, we will reconvene.” After the 20 minutes have passed, have spokespersons from each group present their answers to the questions. Appropriate answers are given in the key of the learning activity.
Next, have students view the following HowStuffWorks video entitled, “Factors Affecting Solubility”: https://www.youtube.com/watch?v=4cr9w23GcTs. You may project the video on a screen using a laptop projector or have students separate into groups to watch the video at different computer stations. After the video has concluded, say to the class, “Let’s summarize what we just learned from the video. First, what factors affect the solubility of a substance?” List or show the following on the board:
- Nature of solute and solvent affects solubility.
- Like dissolves like (polar substances dissolve polar or ionic substances; nonpolar substances dissolve nonpolar substances).
- Increasing temperatures generally increase solubility of solids and decrease solubility of gases and liquids.
- Increasing pressures generally do not affect the solubility of solids but increases solubility of gases.
- Do not confuse dissolving rate with solubility!
- Smaller solute particles dissolve faster than larger solute particles because of larger surface area.
- Stirring (agitation) dissolves solutes faster since solute is exposed to more fresh solvent.
- Increasing the solubility will lead to a corresponding increase in the dissolving rate.
Notes about the video:
- The dipoles on polar molecules are due to partial charges not full charges as shown.
- Carbon tetrachloride is a tetrahedral molecule not a planar molecule.
Tell the class, “You will now take a quiz to test your knowledge.” Hand out the Learning Activity Worksheet 2–Solubility Quiz (S-C-7-3_Solubility Quiz and KEY.doc). After students have completed the quiz, they may hand them in to be graded.
Extension:
- Students who may be going beyond the standards can investigate thermal pollution and how it affects the solubility of oxygen in water. The student may use reputable Web sites to discuss:
- What is thermal pollution?
- Where does thermal pollution come from?
- What effect does it have on streams and other waterways.
- Why do higher temperatures lower the amount of dissolved oxygen (DO)?
- What is the effect of low levels of DO on aquatic wildlife?
- What are some causes of low levels of DO besides thermal pollution?
- How can thermal pollution be minimized?
- How can low levels of DO be increased?
Some suggested Web sites can be found at the following URLs: