Chemistry B11
Chapter 6
Solutions and Colloids
Solutions: solutions have some properties: 1. The distribution of particles in a solution is
uniform. Every part of the solution has exactly the same composition and properties as every
other part (a single phase-homogenous). 2. Solutions are almost always transparent (solid
solutions are exceptions). 3. A solution cannot be separated into its components by filtration.
4. The components of a solution do not separate on standing. 5. Solutions can be separated
into pure components (distillation, chromatography).
Note: we have different types of solutions: gas in gas (air), liquid in liquid (alcohol in water),
solid in liquid (sugar in water), solid in solid (alloys), and gas in liquid (gas in cokes).
Note: All mixtures of gases are solutions. Because gas molecules are far apart from each
other and much empty space separates them. Whenever we mix solids, we almost always get a
heterogeneous mixture.
Note: a solution consists of two parts:
1. Solvent: the component present in the greater amount in a solution (when one liquid is
dissolved in another).
2. Solute: the component(s) present in the smaller amount in a solution (when one liquid is
dissolved in another).
Miscible: some liquids are completely soluble in other liquids to form a solution (no matter
what quantities of each are mixed) (for example: methanol and water).
Immiscible: some liquids cannot mix together and they produce the different phases (for
example: oil and water).
Saturated solution: when a solution contains all the solute it can hold at a given temperature,
we call the solution saturated.
Unsaturated solution: any solution containing a lesser amount of solute than a saturated
solution at a given temperature is unsaturated (so we can dissolve more solute in the solvent).
Supersaturated solution: when a solution contains more solute in the solvent than it can
normally hold at a given temperature under equilibrium conditions. A supersaturated solution
is not stable; when disturbed in any way, such as by stirring or shaking, the excess solute
precipitates.
Solubility: the maximum amount of a solute that will dissolve in a given amount of a
particular solvent (at a given temperature).
Note: The more similar two compounds are (similar in term of polarity), the more likely that
one will be soluble in the other. Like dissolves like. Polar compounds dissolve in polar
Dr. Behrang Madani
Chemistry B11
Bakersfield College
solvents, and nonpolar compounds dissolve in nonpolar solvents. For example, water
dissolves NaCl (two polar compounds) and CCl4 dissolves C6H14 (two nonpolar compounds).
Note: For most solids and liquids that dissolve in liquids, solubility increases with increasing
temperature (except for gases, solubility in liquids almost always decreases with increasing
temperature).
Henry’s law: the solubility of a gas in a liquid is directly proportional to the pressure (the
higher the pressure, the greater the solubility of a gas in a liquid). Pressure has little effect on
the solubility of liquids or solids.
Concentration: amount of a solute dissolved in a given quantity of solvent. Many methods of
expressing concentration exist. We learn the three most important here:
1. Percent concentration:
Weight / Volume (W / V) % =
Weight solute
× 100
Volume of solution (mL)
Weight / Weight (W / W) % =
Volume / Volume (V / V) % =
Weight solute
× 100
Weight of solution
Volume solute (mL)
× 100
Volume of solution (mL)
2. Molarity: the number of moles of solute dissolved in 1 liter (L) of solution. The units of
molarity are moles per liter.
Molarity (M) =
moles solute (n)
volume of solution (L)
or
Molarity (M) × V = number of moles (n)
Note: we can prepare a solution of a given molarity (with a known volume).
3. Pert per million (ppm) and part per billion (ppb): for very dilute solutions.
ppm =
g solute
× 10 6
g solvent
ppb =
g solute
× 10 9
g solvent
Dilution: we frequently prepare solutions by diluting concentrated solutions (stock solutions)
rather than by weighing out pure solute.
When we add only solvent during dilution, the number of moles of solute remains unchanged:
Dr. Behrang Madani
Chemistry B11
Bakersfield College
M1V1 = moles before dilution
M2V2 = moles after dilution
Therefore,
M1V1 = M2V2
%1V1 = %2V2
(using percent concentrations)
Note: All nitrates (NO3-) and acetate (CH3COO-) are soluble in water.
-
Note: Most chlorides (Cl ) and sulfates (SO42-) are soluble in water (except: AgCl, BaSO4,
PbCl2, Hg2Cl2, and PbSO4).
Note: Most carbonates (CO32-), phosphates (PO43-) and hydroxides (OH-) are insoluble in
water (except: NaOH, LiOH, KOH, and NH3).
Hydrate and hydration: when a solid ionic compound is added to water, water molecules
surround the ions at the surface of the crystal. Water is a polar molecule. The negative ions
(anions) attract the positive poles of water molecules, and the positive ions (cations) attract
the negative poles of water molecules. Each ion attracts multiple water molecules and the ion
remove from the crystal. We say ions are hydrated and this phenomenon is called hydration (a
more general term, covering all solvents, is solvated).
Electrolyte: substances that conduct an electric current when dissolved in water or when in
the molten state are called electrolytes. These substances can be ionized and produce ions.
The positively charged ions (cations) migrate to the negative electrode (cathode) and the
negatively charged ions (anions) migrate to the positive electrode (anode). The movement of
ions constitutes an electric current.
Note: compounds that dissociate (ionize) completely are called strong electrolytes (most of
the ionic compounds and some acids). Compounds that dissociate partially are called weak
electrolytes (such as CH3COOH). Compounds that do not dissociate (do not conduct
electricity) are called nonectrolytes (such as distilled water).
Dr. Behrang Madani
Chemistry B11
Bakersfield College
Solubility of covalent compounds in water: some acids are soluble in water. Covalent
compounds will dissolve in water if they can form hydrogen bonds with water. In general,
they should have no more than three C atoms for each O or N atom. For example, acetic acid,
CH3COOH, is soluble in water, but benzoic acid, C6H5COOH, is not. The exception to this
generalization is the rare case where a covalent compound reacts with water-for instance,
HCl.
Colloids: in a colloid, the diameter of the solute particles ranges from about 1 to 1000 nm
(this diameter is under 1 nm in a rue solution). The colloids are not uniform and transparent
(they appear cloudy and milky). Colloidal systems are stable and their components do not
separate on standing (for example: milk, butter, smoke, and fog).
Tyndall effect: if light passes through a colloidal system, we can see the pathway of the light
without seeing the colloidal particles themselves (they are too small to see). This method is
used to distinguish a colloid from a solution (because we cannot see the pathway of the light
in a solution).
Brownian motion: colloidal particles are in constant motion in a solvent (randomly). For
example, the motion of the dust particles dispersed in air. This motion creates favorable
conditions for collisions between particles. Why do colloidal particles remain in solution
despite all the collisions?
1. Most colloidal particles carry a large solvation layer. They do not actually touch each
other; instead, only their solvent layers collide.
2. All colloids in a particular solution acquire the same kind of charge. Therefore, the
like charges repel each other.
Emulsion: a mixture of immiscible substances (liquid-liquid). Emulsion is a type of the
colloidal systems (usually as droplets of larger than colloidal size). The emulsion systems are
usually stable. Milk and mayonnaise are two examples of the emulsion systems.
Suspension: when the diameter of the solute particles is greater than 1000 nm, we have a
suspension system. Suspension is not a type of the colloidal systems. The suspension systems
are not stable and separate into phases (for example: sand in water).
Note: by adding a solute, the boiling point of a liquid increases and the freezing point of a
liquid decreases (compare to a pure solvent). We can find any changes of the temperature by
using the following formulas:
∆tb = iKbM
∆tf = iKfM
∆tb: change of the boiling point (ºC), ∆tf: change of the freezing point (ºC), M: molarity
(mole/L), Kf and Kb: constant (depend on nature of a solute), i: number of particles (if we
have NaCl, two particles will be formed. Because it dissociates to Na+ and Cl-. However, if
we have C2H6O2, only one particle will be produced. Because this covalent compound does
not dissociate).
Dr. Behrang Madani
Chemistry B11
Bakersfield College
Osmotic membrane (semipermeable membrane): is a selective membrane that contains
very tiny pores, large enough to allow solvent molecules to pass through them but not the
larger solvated molecules.
Osmotic pressure: imagine that we have a concentrated sugar solution on one side of a
semipermeable memberane and a dilute sugar solution on other side. Water molecules are
smaller than sugar molecules and they move back and forth across the membrane. Water
flows from the dilute solution into the concentrated solution (because molecules will always
diffuse from an area of higher concentration to an area of lower concentration). However,
sugar molecules cannot pass through the membrane (because the size of these molecules is
bigger). The amount of external pressure that must be applied to the more concentrated
solution to stop the passage of solvent molecules across a semipermeable membrane is called
osmotic pressure and this phenomenon is called osmosis.
Note: The solution of higher concentration always has a higher osmatic pressure than the one
of lower concentration, which means that the flow of solvent molecules always occurs from
the more dilute solution to the more concentrated solution.
Osmolarity:
Osmolarity = M × i
M: molarity
i: number of particles
Isotonic solutions: two solutions with the same osmolarity. Plasma should be isotonic with
red blood cells.
Hypertonic solutions: a hypertonic solution has a greater osmolarity (and greater osmotic
pressure) than the red blood cells. If the blood cells are placed in a hypertonic solution, water
flows from the cells into the plasma. This process is called crenation, shrivels the cells.
Hypotonic solutions: a hypotonic solution has a lower osmolarity (and lower osmotic
pressure) than the red blood cells. If the blood cells are placed in a hypotonic solution, water
flows from the plasma into the cells. This process is called hemolysis, swells the cells (the red
blood cells eventually burst).
Dr. Behrang Madani
Chemistry B11
Bakersfield College
Dr. Behrang Madani
Chemistry B11
Bakersfield College