Section 3 -Solubility and the Dissolving Process

2. Section 3 -Solubility and the Dissolving Process Solubility and Polarity Some pairs of liquids form a solution when they are mixed. Solubility is the ability of one substance to dissolve into another at a given temperature and pressure; expressed in terms of the amount of solute that will dissolve in a given amount of solvent to produce a saturated solution Oils, such as cooking oil, do not mix with water. An oil is nonpolar, and water is polar. However, paint thinner is soluble with the oil in oil-based paints. Both the paint and paint thinner are nonpolar. Polar compounds tend to dissolve in other polar compounds, and nonpolar compounds tend to dissolve in other nonpolar compounds.

3. The Rule Is “Like Dissolves Like” When nonpolar molecules are mixed with other nonpolar molecule, the intermolecular forces of the molecules easily match. (They are both very weak) Thus, nonpolar molecules are generally soluble with each other. This is one part of the rule “like dissolves like”: liquids that are completely soluble with each other are described as being miscible in each other. If molecules are sufficiently polar, there is an additional electrical force pulling them toward each other. The negative partial charge on one side of a polar molecule attracts the positive partial charge on the other side of the next polar molecule. If you add polar molecules to other polar molecules, such as water, the attraction between the two is strong. This is another part of the rule “like dissolves like”: polar molecules dissolve other polar molecules.

4. Paint Thinner - NonPolar Oil Based Paint - NonPolar Cooking Oil – NonPolar Water - Polar Immiscible Miscible “Like Dissolves Like”

5. Solubilities of Solid Compounds Even two polar liquids placed in the same container may not dissolve in each other rapidly. Their strong intermolecular forces can only act on nearby molecules—not between molecules at the top of the container and those at the bottom. The speed of the process can be increased by shaking the mixture. This action breaks the two liquids into small droplets and thereby increases the amount of contact between the surfaces of the liquids. This process works because the only place that dissolving can occur is at the surface between the two liquids, where the different molecules are near each other. Similarly, in considering the solubility of solids in liquids, the only place where dissolution can occur is at the surface of the solid particles. The solid must be broken into smaller particles and then into molecules or ions, which can form a solution with the solvent molecules. To simplify the above statements: Stirring liquids will hasten the dissolving process as will breaking solids into smaller pieces.

6. Solubilities of Solids Generally Increase with Temperature Another way to make most solids dissolve more and faster is to increase the temperature. Increasing the temperature is effective because, in general, solvent molecules with greater kinetic energy can dissolve more solute particles. Gases Can Dissolve in Liquids Gases can dissolve in liquids, however it is usually not in great amounts. Gases are usually considered to be slightly soluble. You can increase the amount of gas that will dissolve by decreasing temperature of the solvent and by increasing the pressure. This is why it is easier to keep dissolve CO2 in soda by keeping the soda cold and keeping the pressure high by keeping the cap on the bottle.

7. Solubilities of Ionic Compounds Solubilities of ionic solids are difficult to predict because of the many factors involved, so they must be measured experimentally. From experimental results of ionic solubilities in water, some patterns emerge, as shown in the chart below from the reference tables.

8. Determine if the following substances will dissolve in water. YES LiCl KNO3 PbS Na2S LiHCO3 CaCO3 NH4OH CaSO4 YES MgCrO4 PbBr2 Sr(ClO3)2 AlPO4 YES NO YES NO NO YES YES YES NO NO

9. How Water Polarity Dissolves Ionic Compounds Water has very strong hydrogen bond poles. As shown below. - O H H + + The poles will attract the ion of the opposite charge into solution as shown below

10. Notice the positive dipole surrounds the negative chloride ion and the negative dipole surrounds the positive sodium ion.

11. Saturation When the maximum amount of solute is dissolved in a solution, the solution is said to be a saturated. If a solution is saturated, any additional solute that is added collects at the bottom of the container. If more solute can be added to a solution and dissolve the solution is considered to be an unsaturated. Under special conditions, supersaturated solutions can also exist. Supersaturated solutions have more solute dissolved than the solubility indicates would normally be possible

12. Study the NaNO3 solubility curve. Any point plotted on the graph below the line will create a unsaturated solution. Any point plotted on the line will create a saturated solution. Any point plotted on the graph Above the line will create a Supersaturated solution. Caution: This graph is based on 100 grams of water.

13. Based onReference your reference tables, which salt solution could contain 42 grams of solute per 100 grams of water at 40°C? 1. a saturated solution of KClO3 2. a saturated solution of KCl 3. an unsaturated solution of NaCl 4. an unsaturated solution of NH4Cl A solution containing 60 grams of NaNO3 completely dissolved in 50. grams of water at 50°C is classified as being 1. saturated 2. supersaturated 3. dilute and unsaturated 4. dilute and saturated One hundred grams of water is saturated with NH4Cl at 50°C. According to Table G, if the temperature is lowered to 10°C, what is the total amount of NH4Cl that will precipitate? 1. 5.0 g 2. 17 g 3. 30. g 4. 50. g

14. According to Reference Table G, how many grams of KNO3 would be needed to saturate 200 grams of water at 70°C? 1. 43 g 2. 86 g 3. 134 g 4. 268 g According to Reference Table G, which of these substances is most soluble at 60°C? 1. NaCl 2. KCl 3. KClO3 4. NH4Cl Which compound forms a saturated solution at 40°C that contains 46 grams per 100 grams of water? 1. KNO3 2. NH4Cl 3. NaNO3 4. KCl