Chem 12 - Solubility

What is a solution? (2) What are the 2 components of a solution? Examples (3)

A solution is a homogeneous mixture

• Appears as a pure substance (no visible parts)

Consists of:

• Solute → substance being dissolved (smaller amount)

• Solvent → substance doing the dissolving (larger amount)

Examples:

• Saltwater → solute: NaCl, solvent: H₂O

• Soda → solute: CO₂, solvent: H₂O

• Sugar water → solute: sugar, solvent: water

What is dissolution? What happens when NaCl dissolves? (4)

Dissolution = the process of dissolving a ionic solid in a polar solvent

Example:

NaCl_(s) → Na⁺_(aq) + Cl⁻_(aq)

What happens (Dissociation):

• Water molecules surround ions (hydration)

• Ionic compound breaks apart

• Ions become aqueous (aq)

What is precipitation? (2)

Precipitation = formation of an ionic solid from solution (insoluble compound forms)

Example:

Pb2⁺_(aq) + Cl⁻_(aq) → PbCl_2(s)

• Solid formed = precipitate (ppt)

• Key sign: solution becomes cloudy

What is a saturated solution? What happens at equilibrium? (3)

A saturated solution contains the max amount of dissolved solute at a given temperature/under given conditions

At saturation:

• Rate of dissolving = rate of precipitation

• System is in dynamic equilibrium

Example:

NaCl_(s) ⇌ Na⁺_(aq) + Cl⁻_(aq)

Even though it looks stable:

• Dissolving and crystallizing happen continuously

What is solubility? How is it expressed? (3)

Solubility = amount of solute that dissolves in a given amount of solvent at a given temperature to form a saturated solution

Units:

• mol/L

• g per 100 mL

Temperature must be specified (solubility changes with temperature)

How does temperature affect solid solutes? (4) How does temperature affect gas solutes? (6)

↑ Temperature → ↑ Solubility

Example:

sugar_(s) + energy → sugar_(aq)

• Process is endothermic (absorbs heat)

• Adding heat shifts reaction right

↑ Temperature → ↓ Solubility

Example:

CO2_(g) ⇌ CO_2(aq) + energy

• Process is exothermic

• Adding heat shifts reaction left

Result:

• Warm soda → flat

• Cold soda → fizzy

What is the difference between miscible and immiscible liquids? (3) (3)

Miscible:

• Mix completely

• Unlimited solubility

• Example: ethanol + water

Immiscible:

• Do not mix

• Form layers

• Example: oil + water

What is an electrolyte? What does it include? Why is water a poor conductor? (3)

An electrolyte conducts electricity in water because it dissociates into (mobile) ions.

Includes:

• Acids

• Bases

• Salts

Water:

2H₂O ⇌ H₃O⁺ + OH⁻

• Very few ions → poor conductor

Strong electrolytes (2) (2)

Strong Electrolytes

• 100% dissociation/ionization (completely dissociates into ions)

• Excellent conductors

• Examples:

  • Strong Acid: HCl → H⁺ + Cl^-

  • Strong Base NaOH → Na⁺ + OH^-

Weak Electrolytes (4) (2)

Weak Electrolytes

• Substance that gives few ions in solution

• Most particles remain in molecular form

• Partial dissociation (~0.4%)

• Weak conductors (better than water)

• Examples:

  • Weak Acid: CH₃COOH _(aq) ⇌ CH₃COO⁻_(aq) (99.6%) + H⁺_(aq) (0.4%)

  • Weak Base: NH_{3 (l)} + H₂O _(l) ⇌ NH₄⁺_(aq) (99.6%) + OH^-_(aq) (0.4%)

Non-Electrolytes (2) (3)

Non-Electrolytes

• Does not give any ions when in solution

• Do not conduct

• Examples:

  • Molecular substances that do not form ions: Ethanol

    • C₂H₅OH _(l) → C₂H₅OH _(aq)

  • Non-metallic elements: Br₂ _(l) → Br₂ _(aq)

    • Br_{2 (l)} → Br_{2 (aq)}

  • Covalent compounds: Sucrose

    • C₁₂H₂₂O_{11 (s)} → C₁₂H₂₂O_{11 (aq)}

What affects conductivity? (2) Provide 2 examples

1. Number of ions

• More ions → better conductivity

2. Concentration

• Higher concentration → more ions

Examples:

• 0.1M HCl (low conc) → 1M HCl → 6M HCl (high conc)

• 1M NaCl (2 ions) → 1M MgCl₂ (3 ions) → 1M FeCl₃ (4 ions)

• poor conductor → good conductor

What is water hardness? (3)

Water hardness describes the concentration of dissolved minerals (mainly Ca²⁺ &/or Mg²⁺)

Ranges:

• Soft: 0–17.1 ppm

• Hard: 120–180 ppm

Hard water? (6)

• Common in ground water from dissolved limestone (CaCO₃)

Effects:

• Bitter taste

• Kettle scale buildup (CaCO₃, MgCO₃)

  • Clog hot water pipes & heating element in kettles (appliances = less efficient)

• Soap scum (precipitates with stearate in soap)

  • Soap becomes less effective (need more soap to clean)

  • Creates spots on dishes

  • Dull clothes, rough hair

Not a health hazard

What are temporary vs permanent hardness? (2) (2)

Temporary Hardness

• Caused by Ca(HCO₃)₂, Mg(HCO₃)₂

• Can be removed by boiling

Permanent Hardness

• CaCl₂, MgCl₂, CaSO₄, MgSO₄

• Cannot be removed by boiling

How can hard water be softened? (3) (4) (3)

1. Water softener

Device with ion exchange column that binds/removes Ca²⁺, Mg²⁺

2. Washing Soda (Na₂CO₃)

CO₃²⁻ forms precipitates & reduces amount of laundry detergent needed:

• Ca₂⁺ + CO₃²⁻ → CaCO_3(s)

• Mg₂⁺ + CO₃²⁻ ​→ MgCO_3​(s)

3. Boiling: produces kettle scale, improves taste of drinking water

Only effective on temporary hard water

• Ca²⁺ + 2HCO₃⁻ ​→ CaCO₃​ + CO₂​ + H₂​O

• Mg²⁺ + 2HCO₃⁻​ → MgCO₃ ​+ CO₂ ​+ H₂​O

Interpreting Ksp (2) (2)

• Large Ksp

  • Equilibrium is far to the right

  • High solubility

• Small Ksp

  • Equilibrium is far to the left

  • Low solubility