Chapter 7 Section 3 Objectives • Explain the meaning of solubility and compare the solubilities of various substances. • Describe dilute, concentrated, saturated, and supersaturated solutions. • Relate changes in temperature and pressure to changes in solubility of solid and gaseous solutes. Express the concentration of a solution as molarity, and calculate the molarity of a solution given the amount of solute and the volume of the solution Solubility in Water • Solubility is the maximum amount of a solute that will dissolve in a given quantity of solvent at a given temperature and pressure. • Some substances, such as oil, are insoluble in water, meaning they never dissolve. • Other substances are said to be soluble in water because they dissolve easily in water. • However, there is often a limit to how much of a substance will dissolve. Different substances have different solubilities. • To express how much of a substance can dissolve in a solvent, you need to use the concentration. • Concentration is the amount of a particular substance in a given volume of solution. • • A solution whose ratio of solute to solvent is relatively high is referred to as concentrated. • A solution whose ratio of solute to solvent is relatively low is referred to dilute. An unsaturated solution contains less than the maximum amount of solute that can dissolve. • A saturated solution is at a point where no more solute can be dissolved under the same conditions. • • If you add more solute to a saturated solution, it will simply fall to the bottom of the container. A supersaturated solution holds more dissolved solute than is required to reach equilibrium at a given temperature. • To make a supersaturated solution, you raise the temperature of a solution, dissolve more solute, then let the solution cool again. • Gases can also dissolve in water. • Unlike solid solutes, gaseous solutes are less soluble in warmer water than they are in colder water. • • Example: Soda goes flat quickly at room temperature. The solubility of gases also depends on pressure. Lowered pressure of gas above a solution leads to dissolved gas bubbling out of the solution. • Example: When a can of soda is opened, carbon dioxide gas that had been dissolved in the soda bubbles out of solution. • Example: If a scuba diver surfaces too quickly, dissolved nitrogen gas in the bloodstream bubbles out of solution, which causes a painful condition called the bends. Concentration of Solutions • There are several ways to express the concentration of solutions, but one of the most useful ways is by using molarity: moles of dissolved solvent per liter of solution. • Note that molarity is moles per liter of solution, not per liter of solvent. • A 1.0 M, which is read as “one molar,” solution of NaCl, contains 1.0 mol of dissolved NaCl in every 1.0 L of solution. • Other measures of solution concentration can be used. • These include: • mass percent (grams of solute per 100 g of solution) • • Ingredients in many food and household products use mass percent. parts per million (grams of solute per 106 g of solution) • Used for very small concentrations, such as for environmental regulations.