Solutions and Solubility in GOB Chemistry

Solutions

Definition and Properties

A solution is a homogeneous mixture composed of at least one solute uniformly dissolved in a solvent. The physical state of the solution is determined by the state of the solvent. Solutions can involve solutes and solvents in any phase (solid, liquid, or gas).

• Solute: The substance dissolved in the solvent.

• Solvent: The substance in which the solute is dissolved; determines the phase of the solution.

Types of Liquid Mixtures

Liquid mixtures can be classified based on their uniformity and particle size:

• Solution: Homogeneous, transparent, not easily separable (e.g., salt in water).

• Colloid: Heterogeneous, opaque, not easily separable (e.g., milk, mayonnaise).

• Suspension: Heterogeneous, opaque, easily separable (e.g., blood, matcha).

Dissolving and Solubility

Dissolving Process

Dissolving is a physical process where a solute in any phase dissolves in a solvent to form a solution. For ionic compounds, this involves ion-dipole interactions.

• Example: Dissolving NaCl in water: $\mathrm{NaCl(s) + H_2O(l) \rightarrow Na^+(aq) + Cl^-(aq)}$

• Writing Dissociation Equations: $\mathrm{MgCl_2(s) \rightarrow Mg^{2+}(aq) + 2Cl^-(aq)}$

Solubility Rules

Solubility depends on the nature of the solute and solvent. The rule of thumb is "like dissolves like": polar solutes dissolve in polar solvents, nonpolar in nonpolar.

• Polar + Polar: Soluble (e.g., sugar in water)

• Nonpolar + Nonpolar: Soluble (e.g., oil in hexane)

• Polar + Nonpolar: Insoluble (e.g., oil in water)

Hydrogen bonding is a special interaction between H and N, O, or F atoms, increasing solubility in water.

Controlling Solubility

• Increasing temperature typically increases solubility of solids in liquids.

• Solubility can be affected by pressure (especially for gases).

Saturation Levels

• Unsaturated Solution: Can dissolve more solute.

• Saturated Solution: Cannot dissolve more solute; equilibrium reached.

• Supersaturated Solution: Contains more solute than can normally be dissolved; unstable and may form crystals.

Electrolytes and Conductivity

Electrolyte Classification

Electrolytes are substances that produce ions when dissolved in water, allowing the solution to conduct electricity. They are classified as strong, weak, or nonelectrolytes.

• Strong Electrolyte: Dissociates completely into ions; conducts electricity well.

• Weak Electrolyte: Partially dissociates; conducts electricity weakly.

• Nonelectrolyte: Does not dissociate; does not conduct electricity.

Solubility Rules for Ionic Compounds in Water

Solubility rules help predict whether an ionic compound will dissolve in water:

• Soluble if contains: Li+, Na+, K+, Rb+, Cs+, NH4+, NO3-, C2H3O2-

• Cl-, Br-, I-: Soluble except with Ag+, Pb2+, Hg22+

• SO42-: Soluble except with Ba2+, Pb2+, Ca2+, Sr2+, Hg22+

• Compounds not containing these ions: Usually insoluble

Concentration of Solutions

Concentration Expressions

The concentration of a solution is the amount of solute per amount of solution. Common expressions include:

• Mass Percent (m/m): $\mathrm{\frac{\text{mass of solute}}{\text{mass of solution}} \times 100\%}$

• Volume Percent (v/v): $\mathrm{\frac{\text{volume of solute}}{\text{volume of solution}} \times 100\%}$

• Mass/Volume Percent (m/v): $\mathrm{\frac{\text{mass of solute}}{\text{volume of solution}} \times 100\%}$

• Molarity (M): $\mathrm{\frac{\text{moles of solute}}{\text{liters of solution}}}$

Dilution

Dilution reduces the concentration of a solute by adding more solvent. The relationship is:

• $\mathrm{C_i V_i = C_f V_f}$

• Where $C_i$ and $V_i$ are initial concentration and volume, $C_f$ and $V_f$ are final concentration and volume.

Osmosis

Osmosis and Solution Types

Osmosis is the movement of water through a semipermeable membrane from low solute concentration to high solute concentration. It only occurs with true solutions, not colloids or suspensions.

• Isotonic: Equal solute concentration inside and outside the cell.

• Hypotonic: Lower solute concentration outside; cell swells.

• Hypertonic: Higher solute concentration outside; cell shrinks.

Equivalents (Eq)

Definition and Calculation

An equivalent (Eq) is a unit measuring the chemical activity and electrical charge of ions in solution.

• 1 mol Na+ = 1 Eq

• 1 mol Ca2+ = 2 Eq

• 1 mol Fe3+ = 3 Eq

• 1 mol CO32- = 2 Eq

Example: To find equivalents, multiply moles by the ion charge.