S ECTION 8–2: S OLUBILITY AND C ONCENTRATION Physical Science Coach Kelsoe Pages 235–239

O BJECTIVES Define solubility and describe factors affecting solubility. Classify solutions as unsaturated, saturated, or supersaturated. Calculate and compare and contrast solution concentrations expressed as percent by volume, percent by mass, and molarity.

S OLUBILITY The maximum amount of a solute that dissolves in a given amount of solvent at a constant temperature is called solubility. Solubilities are usually expressed in grams of solute per 100 grams of solvent at a specified temperature. As a comparison, here are the solubilities in 100 grams of water at 20 ° C of common substances: Table Salt (NaCl) 36.0 g Baking Soda (NaHCO 3 )9.6 g Least Table Sugar (C 12 H 22 O 11 )203.9 g Most

S OLUBILITY The maximum amount of a solute that will dissolve in a certain amount of solvent at a given temperature is that solute’s ___. A: solubility. Knowing the solubility of a substance can help you classify solutions based on how much solute they contain. Solutions are described as saturated, unsaturated, or supersaturated, depending on the amount of solute in solution. The statement “203.9 g of table sugar will form a saturated solution in 100 g of water” is not complete because temperature is not mentioned.

S ATURATED S OLUTIONS Table sugar is very soluble in water. At 20 ° C, grams of table sugar will dissolve in 100 grams of water. If you tried to dissolve more than this amount of sugar in the same amount of water, the extra sugar will not go into the solution, because it is already saturated. A saturated solution is one that contains as much solute as the solvent can hold at a given temperature. No more solute will dissolve into the solution. One way to determine the degree of saturation of a solid-liquid solution is to drop a crystal of the solute into the solution. If the crystal sits at the bottom of the container, the solution is __. A: saturated.

U NSATURATED S OLUTION A solution that has less than the maximum amount of solute that can be dissolved is called an unsaturated solution. For example, many of the beverages you drink are unsaturated solutions of sugar in water. If you sweeten your lemonade with a spoonful of sugar, and the sugar dissolves, you know that the solution is unsaturated. As long as the amount of solute is less than the solubility at that temperature, the solution is unsaturated. If a saturated solution of sugar water is heated, the solution will become unsaturated.

S UPERSATURATED S OLUTION Sometimes a solvent can dissolve more solute than you might expect, based on its solubility. Solubility is given at a specific temperature, such as 20 ° C. If you heat a solvent above that temperature, more solute may dissolve. If you then carefully cool the solvent back to 20 ° C without jarring it, you may be able to keep the extra solute in the solution. A supersaturated solution is one that contains more solute than it can normally hold at a given temperature. Supersaturated solutions are very unstable. A solution that contains more solute than it would normally hold at that temperature is said to be _____. A: supersaturated.

F ACTORS A FFECTING S OLUBILITY Solubility varies not only with the solvent used, but also with the conditions of the solution process. Three factors that affect the solubility of a solute are: The polarity of the solvent Temperature Pressure The solubility of carbon dioxide gas in water can be increased by ___. A: increasing the pressure. Table salt is less soluble in cold water than it is in hot water.

P OLAR AND N ONPOLAR S OLVENTS Oil does not dissolve in water because oil molecules are nonpolar and water molecules are polar. A common guideline for predicting solubility is “like dissolves like.” Solution formation is more likely to happen when the solute and solvent are either both polar or both nonpolar. A soap molecule has a polar end, which attracts water molecules, and a nonpolar end, which attracts oil. The soap molecules break up the oil into small droplets that are soluble in water.

T EMPERATURE AND P RESSURE In general, the solubility of solids increases as the solvent temperature increases. When you add sugar to cold tea, very little will dissolve. When you add sugar to hot tea, more will dissolve. Unlike most solids, gases usually become less soluble as the temperature of the solvent increases. Increasing the pressure on a gas increases its solubility in a liquid. Manufacturers make carbonated drinks by using pressure to force carbon dioxide to dissolve in the liquid.

C ONCENTRATION OF S OLUTIONS The concentration of a solution is the amount of solute dissolved in a specified amount of solution. Concentration can be expressed as percent by volume, percent by mass, and molarity.

P ERCENT BY V OLUME We can calculate the percent by volume using the following equation: Percent by volume = Vol. of solute x 100% Vol. of solution For example, if a 100 milliliter solution of saltwater contains 32 milliliters of salt, its concentration would be 32%.

P ERCENT BY M ASS We can calculate the percent by mass using the following equation: Percent by mass = Mass of solute x 100% Mass of solution For example, if a 100 gram solution of saltwater contains 28 grams of salt, its concentration would be 28%. A 20-g sample of sugar is dissolved in 59 g of water. The concentration of the solution is _____. A: 25 percent by mass.

M OLARITY To compare the number of solute particles in solutions, chemists often use moles to measure concentration. Remember, a mole is the amount of substance that contains approximately 6.02 x particles. Molarity is the number of moles of a solute dissolved per liter of solution. You can calculate molarity using the following equation: Molarity = Moles of Solute Liters of Solution

M OLARITY To make a 1-molar (1M) solution of sodium chloride in water, first calculate the molar mass of the solute. Sodium chloride, NaCl, has a molar mass of 58.5 grams If 58.5 grams of sodium chloride is mixed with enough water to make one liter of solution, the resulting solution is 1-molar. Table sugar, C 12 H 22 O 11, has a molar mass of 342 grams. To make a 1-molar solution of table sugar in water, 342 grams of table sugar must be added to enough water to make one liter of solution. To calculate the molarity of a solution, you need to know the moles of solute and the ___. A: volume of the solution.

V OCABULARY Solubility Saturated solution Unsaturated solution Supersaturated solution Concentration Molarity

S TUDY G UIDE Q UESTIONS #8-13, should be answered.