1. Chapter 14: Mixtures and Solutions

2. 14.1 Types of Mixtures

3. Heterogeneous Mixtures
A mixture is a combination of two or more pure substances in which each pure substance retains its individual chemical properties.
A suspension is a mixture containing particles that settle out if left undisturbed
Some suspensions separate into a solid-like mixture on the bottom and water on the top. When the solid-like mixture is stirred or agitated, it flows like a liquid.
Called thixotropic mixtures

4. A colloid is a heterogeneous mixture of intermediate sized particles that do not settle out
Particles between atomic-scale size of solution particles and the size of suspension particles (1 nm-1000 nm diameter)
Colloids are categorized according to the phases of their dispersed particles and dispersing mediums
Table 1 lists the types of colloids

5. The erratic movement of colloid particles is called Brownian motion
First observed by botanist Robert Brown
Results from collisions of particles of the dispersion medium with the dispersed particles
The collisions prevent the colloid particles from settling out of the mixture

6. Concentrated colloids are often cloudy or opaque while dilute colloids sometimes appear as clear solutions.
Dilute colloids appear to be homogeneous because their disperse particles are so small.
Dispersed colloid particles are large enough to scatter light, a phenomenon known as the Tyndall effect.
A beam of light is visible in a colloid
Suspensions can also exhibit the Tyndall effect, but solutions cannot

7. Homogeneous Mixtures
Solutions are homogeneous mixtures that contain two or more substances called the solute and solvent.
The solute is the substance that dissolves
The solvent is the dissolving medium
A solution might exist as a gas, liquid, or solid depending on the state of its solvent
Most are liquids
A substance that dissolves in a solvent is said to be soluble in that solvent.
Substances that are soluble in each other in any proportion are said to be miscible
A substance that does not dissolve in a solvent is said to be insoluble in that solvent.
Two liquids that can be mixed together but separate shortly after are said to be immiscible.

8. 14.2 Solution Concentration

9. The concentration of a solution is a measure of how much solute is dissolved in a specific amount of solvent or solution.
Concentration can be described qualitatively using the words concentrated or dilute
Commonly used quantitative descriptions are percent by either mass or volume, molarity, and molality.
Express concentration as a ratio of measured amounts of solute and solvent or solution

10. Percent by mass
Percent by mass is the ratio of the solute’s mass to the solution’s mass expressed as a percent.
Percent by mass = (mass of solute/ mass of solution) x 100

11. Percent by volume
Usually describes solutions in which both solute and solvent are liquids
Percent by volume is the ratio of the volume of the solute to the volume of the solution, expressed as a percent
Percent by volume = (volume of solute/volume of solution) x 100

12. Molarity
Molarity (M) is the number of moles of solute dissolved per liter of solution
Also known as molar concentration
The unit M is read as molar.
Molarity (M) = (moles of solute/liters of solution)

13. Preparing Molar Solutions
To prepare 1L of 1.75M aqueous solution of sucrose…
The solution contains 1.75 moles of sucrose dissolved in 1 L of solution
Since the molar mass of sucrose is 342g, 1.75 moles of sucrose is equal to 599 g of sucrose
Sucrose will add volume to the solution so you must use slightly less than one liter of water to make one liter of solution

14. Diluting solutions
You can prepare a less-concentrated solution by diluting the stock solution with additional solvent
When you add solvent you increase the number of solvent particles among which the solute particles move – decreasing the solution’s concentration
Moles of solute do not change during dilution
M1V1 = M2V2
M1V1 represent molarity and volume of stock solution
M2V2 represent molarity and volume of dilute solution

15. Molality
Molality (m) is the ratio of the number of moles of solute dissolved in one kilogram of solvent
The unit m is read as molal
Molality (m) = moles of solute = moles of solute
1 kg of solvent g of solvent

16. Mole fraction
A mole fraction is the ratio of the number of moles of solute or solvent in solution to the total number of moles of solute and solvent
The symbol X is commonly used for mole fraction, with a subscript to indicate the solvent or solute
XA = solvent
XB = solute
Mole fraction equations:
XA = nA/ (nA + nB)
XB = nB/ (nB + nA)
nA and nB represent the number of moles of each substances

17. 14.3 Factors Affecting Solvation

18. The Solvation Process
The process of surrounding solute particles with solvent particles to form a solution is called solvation.
Solvation in water is called hydration
“Like dissolves like” is the general rule used to determine whether solvation will occur in a specific solvent.

19. Aqueous solutions of ionic compounds
Water molecules are polar molecules
Charged ends attract ions from the surface of a crystal when a collision occurs
Water molecules surround the ions, and the solvated ions move into the solution, exposing more ions on the surface of the crystal until the entire crystal has dissolved
Not all ionic substances are solvated by water molecules
Attractive forces between the ions in the crystal are so strong that they cannot be overcome by the attractive forces of the water molecules

20. Aqueous solutions of molecular compounds
The attractive forces among the solute are overcome by the attractive forces between the polar water molecules and the polar solute molecules.
Nonpolar solutes are more readily dissolved in nonpolar solvents

21. Heat of solution
Energy is required to overcome the attractive forces within the solute and within the solvent – endothermic process
Solute particles separate
Solvent particles separate
When solute and solvent particles mix, the particles attract each other and energy is released – exothermic process
The overall energy change that occurs during the solution formation process is called the heat of solution.

22. Factors that affect rate of solvation
Three common ways to increase the collisions between solute and solvent particles and thus increase the rate at which the solute dissolves
Agitating the mixture
Stirring or shaking
Moves dissolved solute particles away from the contact surfaces, allowing new collisions to occur

23. Increasing the surface area of the solute
Breaking solute into pieces
Allows more collisions to occur
Increasing the temperature of the solvent
Increases the kinetic energy of particles, resulting in more frequent collisions and collisions with greater energy
As temperature increases, the rate of solvation for solids increases
Solvation of gases decreases at higher temperatures

24. Solubility
Solubility refers to the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature and pressure.
Some dissolved solute particles collide with and rejoin the crystal – crystallization
As long as the solvation rate is greater than the crystallization rate, the net effect is continuing solvation
When no more solute appears to dissolve, a state of dynamic equilibrium exists between crystallization and solvation

25. An unsaturated solution is one that contains less dissolved solute for a given temperature and pressure than a saturated solution.
A saturated solution contains the maximum amount of dissolved solute for a given amount of solvent at a specific temperature and pressure.

26. Temperature and supersaturated solutions
Many substances are more soluble at high temperatures than at low temperatures
A supersaturated solution contains more dissolved solute than a saturated solution at the same temperature
Formed at a higher temperature and cooled slowly
If a seed crystal (solute) is added, the excess solute precipitates quickly

27. Solubility of gases
Gases are less soluble at higher temperatures than at lower temperatures
The kinetic energy of gas particles allows them to escape from a solution more readily at higher temperatures

28. Pressure and Henry’s Law
Pressure affects the solubility of gaseous solutes in solutions
Solubility of a gas in any solvent increases as its external pressure increases
Ex: carbonated beverages
Henry’s law states that at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid.
(S1/P1) = (S2/P2)

29. 14.4 Colligative Properties of Solutions

30. Electrolytes and Colligative Properties
Physical properties of solutions that are affected by the number of particles but not the identity of dissolved solute particles are called colligative properties.
Colligative = “depending on the collection”
Include vapor pressure lowering, boiling point elevation, freezing point depression and osmotic pressure

31. Electrolytes in aqueous solution
Ionic compounds are called electrolytes because they dissociate in water to form a solution that conducts electric current.
Some molecular compounds ionize in water and are also electrolytes
Electrolytes that produce many ions in solution are called strong electrolytes; those that produce only a few ions in solution are called weak electrolytes.

32. Nonelectrolytes in aqueous solution
Many molecular compounds dissolve in solvents but do not ionize
These solutions do not conduct electricity
Ex: sucrose

33. Vapor Pressure Lowering
Vapor pressure is the pressure exerted in a closed container by liquid particles that have escaped the liquid’s surface and entered the gaseous state.
Experiments show that adding a nonvolatile solute (one that has little tendency to become a gas) to a solvent lowers the solvent’s vapor pressure.
When the solvent contains a solute, a mix of solute and solvent particles occupies the surface area. With fewer solvent particles at the surface, fewer particles enter the gaseous state, and the vapor pressure is lowered.
Vapor pressure lowering is due to the number of solute particles in solution

34. Boiling Point Elevation
The temperature difference between a solution’s boiling point and a pure solvent’s boiling point is called the boiling point elevation.
Symbolized as ∆Tb
For nonelectrolytes, the value of the boiling point elevation is directly proportional to the solutions molality.
∆Tb = Kb x m
Kb = the molal boiling point elevation constant is the difference in boiling points between a 1m nonvolatile, nonelectrolytes solution and a pure solvent
Units = °C / m
Table 5 lists several common solvents
The value of the boiling point elevation is directly proportional to the solution’s solute molality
The greater the number of solute particles in the solution, the greater the boiling point elevation

35. Freezing Point Depression
In a solution, the solute particles interfere with the attractive forces among the solvent particles, preventing the solvent from entering the solid state at its normal freezing point.
The freezing point of a solution is always lower than that of a pure solvent
A solution’s freezing point depression (∆Tf ) is the difference in temperature between its freezing point and the freezing point of its pure solvent.
For nonelectrolytes, the value of the freezing point depression is directly proportional to the solution’s molality
Equation:
∆Tf = Kf x m
Kf = molal freezing point depression constant
Specific to their solvents
Table 6 lists several common solvents

36. Osmotic Pressure
Osmosis is the diffusion of a solvent through a semipermeable membrane
Movement from an area of higher solvent concentration to an area of lower solvent concentration.
The amount of additional pressure caused by the water molecules that moved into the solution is called the osmotic pressure.
Depends on the number of solute particles in a given volume of solution