Solutions - Key Concepts

Solutions

• Solutions are homogeneous mixtures of two or more pure substances.
• The solvent is the substance present in the greatest abundance.
• All other substances are solutes.
• Aqueous solutions have water as the solvent.

Aqueous Solutions

• Ionic compounds dissolve via dissociation, water surrounds separated ions.
• Molecular compounds typically interact with water but do not dissociate.
• Some molecular substances react with water when they dissolve.
• All substances dissolve by solvation.

Electrolytes and Nonelectrolytes

• Electrolytes dissociate into ions when dissolved in water.
• Nonelectrolytes dissolve in water but do not dissociate into ions.
• Strong electrolytes dissociate completely.
• Weak electrolytes dissociate partially.

Strong vs. Weak Electrolytes—Equilibrium

• Strong electrolytes dissociate completely, represented by a forward reaction:
  $HCl (aq) → H^+ (aq) + Cl^- (aq)$
• Weak electrolytes dissociate partially, establishing equilibrium (forward and backward reactions):
  $CH<em>3COOH (aq) ⇌ H^+(aq) + CH</em>3COO^− (aq)$

Intermolecular Forces

• Intermolecular forces of attraction exist between solute and solvent molecules.

Opposing Processes

• Solution-making and crystallization are opposing processes:
  $solute + solvent ⇌ solution$
• Saturated solution: rate of dissolving equals the rate of crystallization.
• Unsaturated solution: has not reached the amount that will result in crystallization.

Solubility

• Solubility: the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature.
• Saturated solutions contain the maximum amount of solute dissolved.
• Unsaturated solutions contain less than the maximum amount of solute.

Solubility Units

• Solubility is a fixed relationship between solute and solvent at a particular temperature (e.g., solubility of $NaCl$ in $H_2O$ @ $20^oC = 360 g/L$).

Supersaturated Solutions

• Supersaturated solutions hold more solute than normally possible at a given temperature and are unstable.

Solute–Solvent Interactions

• "Like dissolves like."
• Stronger solute-solvent interactions lead to greater solubility.

Liquid/Liquid Solubility

• Miscible liquids mix in all proportions.
• Immiscible liquids do not mix.

Solution Formation

• If the polarities of the solute and solvent are similar, dissolving is likely.

Solubility and Biological Importance

• Fat-soluble vitamins (nonpolar) are stored in fatty tissue.
• Water-soluble vitamins need to be included in the daily diet.

Solubility of Ionic Compounds

• Not all ionic compounds dissolve in water.
• Solubility rules determine which combinations of ions will dissolve.

Pressure Effects

• Pressure has little effect on the solubility of solids and liquids.
• Gas solubility is affected by pressure.

Temperature Effects

• For most solids, solubility increases with temperature, but this varies.
• For all gases, solubility decreases as temperature increases.

Solution Concentration

• Qualitative descriptions: saturated, unsaturated, supersaturated.
• Quantitative measures: mass percentage, parts per million (ppm), molarity.

Percent Composition

• $Mass \% = (mass \ of \ component \ in \ solution / total \ mass \ of \ solution ) × 100$

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

• $ppm = (mass \ of \ component \ in \ solution / total \ mass \ of \ solution) × 10^6$

Molarity

• $Molarity (M) = moles \ of \ solute / volume \ of \ solution \ in \ liters$
• Molarity is a conversion factor between moles and liters.

Mixing a Solution

• To make a solution of known molarity, weigh out a known mass of solute and add solvent to a volumetric flask to the required volume.

Stoichiometry Applied to Solutions

• Use molarity and volume to find moles, and balanced equations for mole ratios.

Titration

• Titration is an analytical technique to calculate the concentration of a solute in a solution.

Dilution

• Dilution: adding solvent to lower the concentration.
• Moles of solute do not change: $M<em>1V</em>1 = M<em>2V</em>2$

Colligative Properties

• Colligative properties depend on the quantity of solute particles, not their identity.
• Include vapor-pressure lowering, boiling-point elevation, freezing-point depression, and osmotic pressure.

Vapor Pressure

• Nonvolatile solutes lower the vapor pressure of a solution.

Osmosis and Osmotic Pressure

• Osmosis: passage of water through a semipermeable membrane from less to more concentrated side.
• Osmotic pressure: pressure to stop osmosis; $\Pi = MRT$

Types of Solutions

• Hypertonic: higher solute concentration outside the cell, water moves out, cell shrinks.
• Isotonic: equal solute concentration inside and outside the cell, no net water movement.
• Hypotonic: lower solute concentration outside the cell, water moves in, cell swells.

Some Uses of Colligative Properties

• Reverse osmosis: Applying pressure greater than osmotic pressure to force water through a membrane, used in water purification.