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.

To evaporate, molecules must break intermolecular forces—this requires a minimum amount of energy

As temperature increases, the average energy of the molecules increase

More molecules will have the minimum needed to evaporate from the liquid

As temperature increases, the vapor pressure increases.

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!

Vapor Pressure of SolutionsOnly solvent particles on the very top layer of the sample can evaporate

Looking down on the top of beaker:

Solvent particles

Beaker with solvent only

If a solvent particle on the top layer has enough energy, it can break the IMF’s and evaporate

Once evaporated, they cause vapor pressure

Vapor Pressure of SolutionsOnly 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 Solvent particles

Solute 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 SolutionsOnly 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?

Heat source (usually underneath) heats the molecules closest to it the fastest

Molecules are gaining the energy to break intermolecular forces and become a gas

Atmospheric pressure pushes down on the top of the liquid

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

Higher altitude means lower atmospheric pressure

The vapor pressure of the liquid doesn’t need to be as high to boil with 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

Solutions have lower vapor pressure than the pure solvent.

The solution does not have high enough vapor pressure to boil at the solvent’s boiling point

The temperature needs to be raised until the vapor pressure of the solution = atmospheric pressure

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

In order for a liquid to freeze, the solid’s vapor pressure and the liquid’s vapor pressure must be equal

The solid is the pure solvent. The liquid is the solution.

The vapor pressure of the liquid (solution) is lower than the solid’s (solvent)

The temperature is lowered until the solid’s vapor pressure = the liquid solution’s vapor pressure

The freezing point of a solution is always lower than the pure solvent

Freezing Points of Solutions

This is the point where the speed of molecules joining to form a solid equals the speed molecules leave the solid to be liquid

When solutes are electrolytes, the impact is greater

For every 1 mole of ___ added

__ moles of particles are in solution

Sugar (non-electrolyte) 1 (C6H12O6 stays together)

NaCl (electrolyte)

CaCl2

2 (Na+ + Cl-)

3 (Ca2+ + 2 Cl-)

Electrolytes break up into more than one particle when added to water.

Therefore, there are even more particles when considering colligative properties.

What effects do electrolytes cause?

What did you learn about sports drinks?

Sports Drink

SolutionSolution

Is a

ElectrolytesElectrolytes

SolubilitySolubility

With

that need to all dissolve when mixed together

ConcentrationsConcentrations

How much solute is in it?

pHpH

Some affect

TitrationsTitrations

Can be determined by

Differ from pure liquids in PropertiesProperties