Solutions and Colloids: Structure, Properties, and Calculations

Solutions and Colloids

Types of Solutions

Solutions are homogeneous mixtures composed of a solvent and one or more solutes. The solvent is the component present in the greater amount, while the solute is dissolved in the solvent. Solutions can exist in various physical states depending on the nature of the solute and solvent.

• Gas in Liquid: Example: Carbonated water

• Liquid in Liquid: Example: Wine

• Solid in Liquid: Example: Salt water

• Gas in Gas: Example: Air

• Solid in Solid: Example: 14-carat gold

Solute	Solvent	Example
Gas	Liquid	Carbonated water
Liquid	Liquid	Wine
Solid	Liquid	Salt water
Gas	Gas	Air
Solid	Solid	14-carat gold

Characteristics of Solutions

• The distribution of particles is uniform.

• Components do not separate on standing.

• Cannot be separated by filtration.

• Solutions can have various compositions for a given solute-solvent pair.

• Usually transparent (except solid solutions).

• Separation into pure components is a physical change.

Solubility

Definition and Types

Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a specific temperature. Solubility is a physical constant and varies for each solute-solvent pair.

• Saturated Solution: Contains the maximum amount of solute at equilibrium.

• Unsaturated Solution: Contains less than the maximum solute.

• Supersaturated Solution: Contains more than the equilibrium amount; excess solute will precipitate if disturbed.

Example: If 0.32 g of aspartic acid is dissolved in 115.0 mL of water and a white powder forms after two days, the solution became saturated as water evaporated, causing excess aspartic acid to precipitate.

Example: Dissolving 0.251 g aspartic acid in 50.0 mL water at 50°C and cooling to 25°C without disturbance results in a supersaturated solution, as the solubility at 25°C is 0.250 g/50.0 mL.

Factors Affecting Solubility

• Nature of Solvent and Solute: "Like dissolves like"—polar solutes dissolve in polar solvents, nonpolar in nonpolar.

• Hydrogen Bonding: Molecules with O, N, or F atoms can form hydrogen bonds, increasing solubility in water.

Temperature and Pressure Effects

• Temperature: Solubility of solids in liquids generally increases with temperature; for gases, it decreases.

• Pressure: Has little effect on solids/liquids, but increases gas solubility in liquids (Henry's Law).

Concentration of Solutions

Percent Composition

• Weight/Volume Percent (w/v): $\frac{\text{grams solute}}{\text{mL solution}} \times 100$

• Weight/Weight Percent (w/w): $\frac{\text{grams solute}}{\text{grams solution}} \times 100$

• Volume/Volume Percent (v/v): $\frac{\text{mL solute}}{\text{mL solution}} \times 100$

Example: 5.0% w/v acetic acid in 240 mL vinegar contains 12 g acetic acid.

Example: 6.0 g NaCl in 300 mL solution is 2.0% w/v.

Molarity (M)

Molarity is the number of moles of solute per liter of solution.

$\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}$

Example: 18.0 g Li2O (molar mass = 29.9 g/mol) in 500 mL water gives 1.20 M solution.

Dilution of Solutions

When diluting a solution, the number of moles of solute remains constant:

$M_1 V_1 = M_2 V_2$

Example: To prepare 200 mL of 3.5 M acetic acid from 6.0 M stock, use 120 mL of stock and dilute to 200 mL.

Parts per Million (ppm) and Parts per Billion (ppb)

Used for very dilute solutions:

$\text{ppm} = \frac{\text{grams solute}}{\text{grams solution}} \times 10^6$

$\text{ppb} = \frac{\text{grams solute}}{\text{grams solution}} \times 10^9$

Water as a Solvent

Dissolving Ionic Compounds

Water, a polar molecule, dissolves ionic solids by surrounding ions with water molecules (hydration), preventing them from recombining.

Dissolving Covalent Compounds

Some covalent compounds react with water (e.g., HCl, SO3), while others (e.g., methanol) are simply solvated without reaction.

Electrolytes

Types of Electrolytes

• Strong Electrolytes: Completely dissociate into ions (e.g., NaCl, HCl).

• Weak Electrolytes: Partially dissociate (e.g., CH3COOH).

• Nonelectrolytes: Do not dissociate (e.g., CH3OH).

Colloids and Suspensions

Colloids

Colloids are mixtures with particle sizes between 1 nm and 1000 nm. They do not settle out and exhibit the Tyndall effect (scattering of light).

Type	Example
Gas in liquid	Whipped cream
Liquid in liquid	Milk, mayonnaise
Solid in liquid	Jelly
Solid in solid	Dried paint

Suspensions have larger particles (>1000 nm) and will settle out.

Properties of Mixtures

Property	Solutions	Colloids	Suspensions
Particle size (nm)	0.1–1.0	1–1000	>1000
Filterable	No	No	Yes
Homogeneous	Yes	Borderline	No
Settles on standing	No	No	Yes
Behavior to light	Transparent	Tyndall effect	Translucent/opaque

Colligative Properties

Freezing Point Depression

The freezing point of a solvent decreases when a solute is dissolved in it. The decrease depends only on the number of solute particles, not their identity.

$\Delta T_f = K_f \cdot n$

• Kf for water: 1.86 °C/mol particles

Boiling Point Elevation

The boiling point of a solvent increases when a solute is dissolved in it.

$\Delta T_b = K_b \cdot n$

• Kb for water: 0.512 °C/mol particles

Osmosis and Osmotic Pressure

Osmosis is the movement of solvent through a semipermeable membrane from lower to higher solute concentration. Osmotic pressure is the pressure required to stop osmosis.

• Osmolarity: $\text{Osmolarity} = M \times i$ (where i is the number of particles per formula unit)

Biological Importance of Osmosis

• Isotonic Solution: Same osmolarity as blood plasma; no net water movement.

• Hypotonic Solution: Lower osmolarity; cells swell and may burst (hemolysis).

• Hypertonic Solution: Higher osmolarity; cells shrink (crenation).

Dialysis

Dialysis uses a membrane that allows small solute and solvent molecules to pass but retains larger molecules. Hemodialysis is a medical application for patients with kidney failure.

Additional info: This summary covers the essential concepts of solutions and colloids, including their types, properties, calculations, and biological relevance, as required for an introductory college chemistry course.