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Solutions

Solutions. Chapter 18.1 Objectives: Identify the factors that determine the rate at which a solute dissolves; Calculate the solubility of a gas in a liquid under various pressure conditions. Chapter 18.2

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Solutions

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  1. Solutions Chapter 18.1 Objectives: Identify the factors that determine the rate at which a solute dissolves; Calculate the solubility of a gas in a liquid under various pressure conditions. Chapter 18.2 Objectives: solve problems involving the molarity of a solution; describe how to prepare a dilute solution from a concentrated solution; explain what is meant by percent by volume and percent mass per volume.

  2. Solution Formation Why is rock salt added to the ice surrounding the ice cream mixture when ice cream is made? Remember: When a solute dissolves, the dissolving process occurs at the surface of the crystal.

  3. Formation cont. • Recall that “like dissolves like”. A polar solvent dissolves polar solutes. • Other factors in the solvation process include: • Stirring (agitation) • Temperature • Particle size • Agitation affects only how fast a solute dissolves, not how much. • Temperature affects rate of dissolving AND how much can be dissolved. • Particle size affects the rate: the smaller the particle the faster it dissolves.

  4. Solubility Water is in constant motion, colliding with particles and taking them into solution. As new ions enter solution, other ions move out and settle to the bottom. A saturated solution is a solution that contains all the dissolved solute it can for a given amount of solvent.

  5. Solubility The solubility of a substance is the amount of solute that dissolves in a given quantity of solvent at a given temperature. An unsaturated solution contains less solute than a saturated solution. If two liquids dissolve into one another they are miscible: water and alcohol. If two liquids do not dissolve into one another they are immiscible: oil and water.

  6. Factors Affecting Solubility Temperature. It is easier to dissolve sugar in hot tea than iced tea. You can dissolve MORE sugar in hot tea than iced tea. However, for CO2 in coke, as temperature increases solubility decreases; the CO2 escapes more readily. Pressure. It is easier to keep CO2 in your Coke when it is unopened than when you open the bottle and leave the cap off.

  7. Henry’s Law In a closed bottle of coke, the dissolved CO2 is kept in solution by the vapor pressure at the top of the bottle. When the bottle is opened, the pressure is released and the dissolved CO2 begins to bubble out of the coke. Henry’s law states that the solubility of a gas is directly proportional to the pressure of the gas above the liquid. Henry’s Law: S1/P1 = S2/P2 Scuba divers know Henry’s law very well. If they rise to the surface of the sea too quickly, the dissolved N2 in their blood will accumulate in their joints and give them “the bends”.

  8. Supersaturation When a solution is heated and excess solute is added to the system, the solute dissolves. As the solution is slowly cooled, the excess solute may stay dissolved below a temperature where it would normally crystallize out. The only way to tell if you have a supersaturated solution is to add a seed crystal (one extra crystal). If the solution is supersaturated, all the excess solute will deposit on the seed crystal and come out of solution.

  9. Concentrations of Solutions The concentration of a solution is a measure of the amount of solute that is dissolved in a given quantity of solvent. In chemistry concentration is known as: Molarity. Molarity is the number of moles of solute per given volume of solution.

  10. Concentrations cont. A dilute solution is one that contains only a low concentration of solute. A concentrated solution contains a large amount of solute. Molarity = moles of solute/liters of solution The units for molarity are : moles/liter or M

  11. Making Dilutions • Who drinks orange juice? From the carton or from frozen concentrate? • If you use the frozen concentrated orange juice, it must be diluted with water before consumed. Usually it takes three cans of water to one can of concentrate. • This is making a dilution. • The formula for dilutions is: • M1 x V1 = M2 x V2 • Molarity of the concentrated times its’ volume equals the molarity of the dilute times its’ volume

  12. Percent Solutions • Often solutions are made of two miscible liquids. • Hydrogen peroxide is a percent solution. • The best way to indicate the amount of solute is by percent solution by volume. • If 20 mL of alcohol is dissolved in 100 mL of water, the solution is said to be 20% alcohol by volume. • Percent mass per volume is the mass of solute dissolved per solution volume: • Mass solute(g)/ volume solution(mL)

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