Chapter 16 A sinkhole forms when the roof of a cave weakens from being dissolved by groundwater and suddenly collapses. One recorded sinkhole swallowed a house, several other buildings, five cars, and a swimming pool 16.1 Solution Formation What factors determine the rate at which a substance dissolves? stirring (agitation) temperature the surface area of the dissolving particles 16.1 A cube of sugar in cold tea dissolves slowly Little surface area, cold temperature and no agitation. Granulated sugar dissolves in cold water more quickly than a sugar cube, especially with stirring More surface area and agitation. Granulated sugar dissolves very quickly in hot tea More surface area and increased temperature. Anything that increases solute to solvent particle contact increases the rate of dissolving. Stirring, heating, pulverizing Solubility How is solubility usually expressed? The solubility of a substance is the maximum amount of solute that can dissolve in a given quantity of a solvent at a specified temperature and pressure. Solubility is often expressed in grams of solute per 100 g of solvent. Reference table G 16.1 A saturated solution contains the maximum amount of solute for a given quantity of solvent at a given temperature and pressure. Points on the curve An unsaturated solution contains less solute than a saturated solution at a given temperature and pressure. Points below the curve 16.1 In a saturated solution, the rate of dissolving equals the rate of crystallization, so the total amount of dissolved solute remains constant. This is a dynamic equilibrium. 16.1 The mineral deposits around hot springs result from the cooling of the hot, saturated solution of minerals emerging from the spring. 16.1 What conditions determine the amount of solute that will dissolve in a given solvent? Temperature affects the solubility of solids, liquids, and gases Pressure also affects the solubility of gases (not liquids or solids) 16.1 Increasing temperature: Increases the solubility of most solids Decreases the solubility of gases 16.1 Changing pressure Changes in pressure have little effect on the solubility of solids and liquids, but pressure strongly influences the solubility of gases. Gas solubility increases as the vapor pressure of the gas above the solution increases. Open bottles of soda go flat 16.1 A supersaturated solution is unstable and contains more solute than it can theoretically hold at a given temperature. The crystallization of a supersaturated solution can be initiated if a very small crystal, called a seed crystal, of the solute is added. 16.1 A supersaturated solution is clear before a seed crystal is added. 16.1 Crystals begin to form in the solution immediately after the addition of a seed crystal. 16.1 Excess solute crystallizes rapidly. 16.1 Water must be tested continually to ensure that the concentrations of contaminants do not exceed established limits. These contaminants include metals, pesticides, bacteria, and even the by- products of water treatment. 16.2 Molarity How do you determine the concentration of a solution? The concentration of a solution is a measure of the amount of solute that is dissolved in a given quantity of solvent. A dilute solution is one that contains a small amount of solute. A concentrated solution contains a large amount of solute. Molarity (M) is the unit of concentration for solutions. To calculate the molarity of a solution, divide the moles of solute by the volume (in liters) of the solution. Reference Table T 16.2 To make a 0.5-molar (0.5M) solution, first add 0.5 mol of solute to a 1-L volumetric flask half filled with distilled water. 16.2 Swirl the flask carefully to dissolve the solute. 16.2 Fill the flask with water exactly to the 1-L mark. 16.2 Making Dilutions What effect does dilution have on the total moles of solute in a solution? Nothing. Diluting a solution reduces the number of moles of solute per unit volume, but the total number of moles of solute in solution does not change. Making a Dilute Solution 16.2 Volume-Measuring Devices 16.2 Since the total number of moles of solute remains unchanged upon dilution, you can use this equation: M 1 V 1 = M 2 V 2 Where M 1 and V 1 are the molarity and volume of the initial solution, and M 2 and V 2 are the molarity and volume of the diluted solution. Write this in ref. table T 16.2 How do you prepare 100 ml of 0.40M MgSO 4 from a stock solution of 2.0M MgSO 4 ? 16.2 ` Another way to express concentration is parts per million (ppm). This is useful when very little solute is dissolved in a very large volume of water; for example the amount of dissolved oxygen in a lake. Ref. table T ppm = (mass solute/mass solution) X 1,000,000 A 1.0 L sample of water from a stream contains 0.008g of dissolved oxygen. What is the concentration in parts per million? The wood frog is a remarkable creature because it can survive being frozen. Scientists believe that a substance in the cells of this frog acts as a natural antifreeze, which prevents the cells from freezing. A solute can change the freezing point of a solution 16.3 A property that depends only upon the number of solute particles, and not upon their identity, is called a colligative property. 16.3 What are three colligative properties of solutions? vapor-pressure lowering boiling-point elevation freezing-point depression 16.3 Dissolving a solute in a solvent will Lower the vapor pressure Raise the boiling point and Lower the freezing point The more you dissolve the greater the change. 16.3 In a pure solvent, equilibrium is established between the liquid and the vapor. 16.3 In a solution, solute particles reduce the number of free solvent particles able to escape the liquid. Equilibrium is established at a lower vapor pressure. 16.3 Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate. 16.3 Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions. This solution will have a lower vapor pressure than the previous one. 16.3 Three moles of calcium chloride dissolved in water produce 9 mol of particles because each formula unit of CaCl 2 dissociates into three ions. This solution will have the lowest vapor pressure. 16.3 Which solution has the lowest vapor pressure at STP? 1 M KCl 1 M CH 3 OH 1 M MgBr 2 The freezing-point depression of aqueous solutions makes walks and driveways safer when people sprinkle salt on icy surfaces to make ice melt. The melted ice forms a solution with a lower freezing point than that of pure water. 16.3 Dissolving a solute in water will lower its freezing point. The difference in temperature between the freezing point of a solution and the freezing point of the pure solvent is the freezing-point depression. The more concentrated the solution the lower the freezing point. 16.3 Which solution would have the lowest freezing point? 1 mole of MgBr 2 in 1 kg water 1 mole of CH 3 OH in 1 kg water 1 mole NaNO 3 in 1 kg water Dissolving a solute in water will raise its boiling point. The difference in temperature between the boiling point of a solution and the boiling point of the pure solvent is the boiling-point elevation. The more concentrated the solution the higher the boiling point. 16.3 Which solution would have the highest boiling point? 1 mole of NaCl in 1 kg water 1 mole of NaCl in 2 kg water 1 mole of NaCl in 3 kg water