Solutions

heterogeneous mixture

a mixture that does not have a uniform composition and in which the individual substances remain distinct.

Suspensions

heterogeneous mixtures containing particles that settle out if left undisturbed. Examples: sand and silt in river water, paint, etc.

Colloids

heterogeneous mixtures of intermediate sized particles (between 1 nm and 1000 nm) and do not settle out.

Milk

example of a colloid.  Tiny liquid butterfat globules stay suspended in a water dispersion medium. 

Tyndall effect

when dispersed colloid particles scatter light (does not occur with solutions).

Solutions

homogeneous mixtures that contain two or more substances called the solute and solvent.

solvent

the most abundant material in the mixture.  Water is our key solvent

Solutes

the less abundant material that is mixed (“dissolved”) in the solvent

Solvation

the process of surrounding solute particles with solvent particles to form a solution.

hydration

Solvation in water

Solvation of Ionic Compounds

when the attraction between dipoles of a water molecule and the charged ions of a crystal are greater than the attraction among ions of a crystal

Like dissolves like

a general rule in chemistry that describes how solutes and solvents interact. It means that substances with similar intermolecular forces tend to dissolve in each other, and those with vastly different forces are likely to be immiscible (not mixable). Specifically, polar solutes generally dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents. 

endothermic

During solvation, the solute must separate into particles and move apart.  The solvent must also move apart to make room for solute particles.  Both of these actions require energy

exothermic

when the solute and solvent mix and attract each other, bonds form and energy is released

heat of solution

overall energy change that occurs during solution formation

Electrolytes

Compounds that dissociate into separate ions in water and are good conductors

“Strong” electrolytes

100% dissociation Ex: Ionic compounds and strong acids: HCl, HNO₃, HBr

“Weak” electrolytes

< 100% dissociation

Nonelectrolytes

• Do not dissociate into ions in water.  

• Covalent molecules

concentration

a measure of how much solute is dissolved in a specific amount of solvent or solution.

Percent by mass

the mass of solute divided by the total mass of the solution, multiplied by 100.

percent of volume

the volume of solute divided by the total volume of the solution, multiplied by 100.

molarity

moles of solute divided by liters of solution

molality

moles of solute divided by kg of solvent

mole fraction

moles of solute divided by moles of solute plus moles of solvent

To “dilute a solution”

• add more solvent or 

• reduce the amount of solute.

Dilution equation

M₁V₁ = M₂V₂

Stock Solution 

a concentrated solution used as a starting point for making dilutions of lower concentrations

Density

mass / volume

soluble

A substance that dissolves in a solvent

insoluble

A substance that does not dissolve in a solvent

Solubility

the maximum amount a solute will dissolve in a solvent.  It depends on the type of solute and solvent and temperature

Saturated solutions

the maximum amount of dissolved solute for a given amount of solvent at a specific temperature and pressure (i.e. at solubility limit).  

Unsaturated solutions

solutions that contain less dissolved solute for a given temperature and pressure than a fully saturated solution.

maximum solubility

an equilibrium is reached between the ongoing processes of solvation and precipitation.

To form a supersaturated solution

a saturated solution is formed at high temperature and then slowly cooled.  Adding more solid will spark rapid precipitation (‘seeding’). Supersaturated solutions are unstable (they don’t tend to exist).

Agitation

Stirring or shaking moves dissolved particles away from the contact surfaces more quickly and allows new collisions to occur.

For a given amount, which type of solution contains the LEAST amount of solute?  

D. unsaturated 

Solubility of Gases

Gases are less soluble in liquid solvents at high temperatures.  Higher K.E. allows more gas molecules to escape from solution. Solubility of gases increases as its external pressure is increased.  Carbonated sodas!

Colligative properties

physical properties of solutions that are affected by the number of particles but not by the identity of dissolved solute particles.

• Types of colligative properties:

  • Vapor pressure lowering

  • Boiling point elevation

  • Freezing point depression

  • Osmotic pressure

van’t Hoff factor (i)

(moles of particles) / (moles of solute)

Vapor pressure lowering

occurs when a non-volatile solute is added to a solvent, causing the vapor pressure of the solution to be lower than the vapor pressure of the pure solvent. This phenomenon is a colligative property, meaning it depends on the concentration of the solute, not its identity. 

boiling point elevation

ΔT_b = K_b i m 

freezing point depression

ΔT_f = K_f i m