Section 6.7—Properties of Solutions How do all those dissolved things affect the properties of the drink?

What’s Vapor Pressure? Vapor Pressure – Pressure created above a sample by particles evaporating from the sample and becoming gas particles.

Vapor Pressure & Temperature Temperature is proportional to the average kinetic energy of the molecules. Average means some will have more and some will have less! To evaporate, molecules must break intermolecular forces—this requires a minimum amount of energy More molecules will have the minimum needed to evaporate from the liquid As temperature increases, the average energy of the molecules increase As temperature increases, the vapor pressure increases.

Vapor Pressure of Solutions Only solvent particles on the very top layer of the sample can evaporate Looking down on the top of beaker: If a solvent particle on the top layer has enough energy, it can break the IMF’s and evaporate Solvent particles Beaker with solvent only Once evaporated, they cause vapor pressure

Vapor Pressure of Solutions Only solvent particles on the very top layer of the sample can evaporate Looking down on the top of a solution in a beaker: Solute particles Beaker with solvent only Solvent particles The solvent and solute form intermolecular forces (connections) with each other. The solvent must now break those connections in order to evaporate. The connections are holding the solvent particles back.

Vapor Pressure of Solutions Only solvent particles on the very top layer of the sample can evaporate Looking down on the top of a solution in a beaker: Beaker with solvent only The fewer particles that evaporate, the lower the vapor pressure. The vapor pressure of a solution is always less than the pure solvent.

When does something boil? Atmospheric pressure pushes down on the top of the liquid Molecules are gaining the energy to break intermolecular forces and become a gas Heat source (usually underneath) heats the molecules closest to it the fastest

When does something boil? When enough water molecules turn to gas and create as much pressure as the atmosphere is pushing down with, a bubble can form (counter-act the atmospheric pressure)

Boiling and Atmospheric Pressure Boiling occurs when vapor pressure of liquid = atmospheric pressure The vapor pressure of the liquid doesn’t need to be as high to boil with lower atmospheric pressure Higher altitude means lower atmospheric pressure The bubbles can form at a lower temperature The boiling point of a liquid is always lower at higher altitude

Boiling Points of Solutions Boiling occurs when vapor pressure of liquid = atmospheric pressure The temperature needs to be raised until the vapor pressure of the solution = atmospheric pressure The solution does not have high enough vapor pressure to boil at the solvent’s boiling point Solutions have lower vapor pressure than the pure solvent. The boiling point of a solution is always higher than the pure solvent

When do things freeze? When you’re above the freezing point, solid will melt to liquid When you’re below the freezing point, liquid will freeze to solid Freezing point is when there is equilibrium between solid & liquid—the amount of solid and liquid stay the same This occurs when the rate of evaporation from the solid is the same as the rate of evaporation from the liquid Every time a molecule evaporates from the solid, one also evaporates from the liquid. Every time a molecule re-forms into the solid, one also reforms into the liquid. Neither one can “get ahead”—it’s at equilibrium

Freezing Points of Solutions In order for a liquid to freeze, the solid’s vapor pressure and the liquid’s vapor pressure must be equal This is the point where the speed of molecules joining to form a solid equals the speed molecules leave the solid to be liquid The temperature is lowered until the solid’s vapor pressure = the liquid solution’s vapor pressure The vapor pressure of the liquid (solution) is lower than the solid’s (solvent) The solid is the pure solvent. The liquid is the solution. The freezing point of a solution is always lower than the pure solvent

What effects do electrolytes cause? When solutes are electrolytes, the impact is greater Electrolytes break up into more than one particle when added to water. Therefore, there are even more particles when considering colligative properties. For every 1 mole of ___ added __ moles of particles are in solution Sugar (non-electrolyte) 1 (C6H12O6 stays together) NaCl (electrolyte) 2 (Na+ + Cl-) CaCl2 3 (Ca2+ + 2 Cl-)

What did you learn about sports drinks?

Sports Drink Properties Solution Concentrations Electrolytes pH Is a Properties Differ from pure liquids in Solution How much solute is in it? With Concentrations Electrolytes Some affect Can be determined by that need to all dissolve when mixed together pH Solubility Titrations