Attractive Forces, Solubility, and Solutions: Study Notes for GOB Chemistry

Chapter 7: Solubility

Attractive Forces and Solubility

Solubility is the ability of a substance (solute) to dissolve in another substance (solvent), forming a homogeneous mixture. The solubility of compounds in water or oil depends on the types of attractive forces present between molecules.

• Polar Compounds: Compounds like sucrose (table sugar) have multiple hydroxyl (–OH) groups, making them polar. This polarity allows them to interact with water molecules through dipole–dipole and hydrogen-bonding interactions.

• Example: Sucrose dissolves in water because both are polar and can form hydrogen bonds.

• Ionic Compounds: When ionic compounds dissolve in water, water molecules surround the ions, forming ion–dipole attractions. Although each ion–dipole interaction is weaker than an ionic bond, the cumulative effect of many water molecules hydrating an ion can overcome the ionic bond strength. This process is called hydration.

• Example: Sodium chloride (NaCl) dissolves in water as Na+ and Cl– ions are hydrated by water molecules.

Dipole Interactions and Drug Solubility

Drug molecules must be soluble in water to be absorbed in the digestive tract. Many drugs are designed to have polar groups (such as amines or carboxylic acids) that can be charged at physiological pH, increasing their solubility.

• Example: Pseudoephedrine is converted to pseudoephedrine hydrochloride by adding HCl, which protonates the amine group, making it more soluble in water.

Predicting Solubility

Solubility in water or oil can be predicted based on molecular polarity:

• Water Solubility: Polar molecules and ionic compounds are generally more soluble in water.

• Oil Solubility: Nonpolar molecules are more soluble in oil.

• Example Questions:

  • Which is more soluble in water: CH3CH2OH, CH3OCH3, or CH3CH3? Answer: CH3CH2OH (ethanol) is most soluble due to its –OH group.

  • Which is more soluble in oil: C5H16, C5H9OH, or NaCl? Answer: C5H16 (pentane) is most soluble due to its nonpolar nature.

Amphipathic Compounds

Some molecules, such as fatty acids, have both polar and nonpolar regions. These are called amphipathic compounds.

• Structure: Fatty acids have a long nonpolar hydrocarbon tail and a polar carboxylic acid head (R–COOH).

• Behavior: The large nonpolar tail dominates, making fatty acids behave like nonpolar molecules.

• Soaps: Soaps are amphipathic molecules derived from fatty acids. They have ionic (carboxylate) heads and long nonpolar tails, allowing them to interact with both water and oil.

Chapter 8: Solutions

Definition and Properties of Solutions

A solution is a homogeneous mixture of two or more substances. The solute is the substance present in a smaller amount, and the solvent is present in a larger amount.

• Key Properties:

  • Components are evenly distributed and do not react chemically.

  • Solutions are transparent and do not separate upon standing.

  • Concentration can be changed by adding more solute or solvent.

• Types of Solutions: Solutions can be made of gases, liquids, or solids. For example, air is a solution of gases, and brass is a solution of solids.

• Aqueous Solutions: Solutions where water is the solvent.

Unique Behavior of Water

Water molecules can form up to four hydrogen bonds, giving water unique properties:

• High specific heat (energy absorption)

• High boiling point and low vapor pressure

• Ice is less dense than liquid water due to hydrogen bonding

Colloids and Suspensions

Not all mixtures are solutions. Colloids and suspensions are heterogeneous mixtures:

• Colloids: Particles are 1–1000 nm in diameter and remain suspended (e.g., milk).

• Suspensions: Particles are larger than 1000 nm and settle upon standing (e.g., muddy water, blood).

Formation of Solutions: Solvation and Hydration

Solvation is the process by which solvent molecules surround and interact with solute molecules. In aqueous solutions, this process is called hydration.

• Polar and Ionic Compounds: Dissolve in water due to strong attractive forces.

• Nonpolar Compounds: Do not dissolve in water.

• Amphipathic Molecules: May interact with water through their polar part but do not form true solutions.

Saturated and Unsaturated Solutions

A solution is unsaturated if it can dissolve more solute. It is saturated if it contains the maximum amount of dissolved solute. Adding more solute to a saturated solution results in undissolved solute and establishes a physical equilibrium:

• $ext{solute (solid)} ightleftharpoons ext{solute (dissolved)}$

Solubility and Temperature

• Solids: Solubility in water generally increases with temperature.

• Gases: Solubility in water decreases as temperature increases.

Solubility and Pressure: Henry's Law

Henry's Law states that the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the liquid:

• $S = kP$ where $S$ is solubility, $k$ is a constant, and $P$ is pressure.

• Application: Gas exchange in the lungs and tissues is governed by differences in partial pressures of oxygen and carbon dioxide.

Additional info:

• These notes cover the core concepts of intermolecular forces, solubility, and solution chemistry, including biological and pharmaceutical applications.

• Key terms: solute, solvent, hydration, amphipathic, colloid, suspension, saturated solution, Henry's Law.