States of Matter and Their Attractive Forces: Gas Laws, Solubility, and Cell Membranes

States of Matter and Their Attractive Forces

Introduction

This chapter explores the physical states of matter, the forces that hold molecules together, and how these concepts relate to biological systems such as cell membranes. It covers the behavior of gases, changes of state, intermolecular forces, solubility, and the structure of biological lipids.

Gases and Pressure

Pressure and Its Units

• Pressure is defined as the force exerted per unit area.

• Common units of pressure include:

  • Atmosphere (atm): Standard atmospheric pressure at sea level.

  • Pascals (Pa): The SI unit of pressure.

  • Pounds per square inch (psi): Used in engineering and physiology; atmospheric pressure at sea level is about 14.7 psi.

  • Millimeters of mercury (mmHg): Used in measuring blood pressure and atmospheric pressure.

Behavior of Gases: Kinetic Molecular Theory

• Gas particles are far apart, with most of the volume being empty space.

• They move in constant, random motion and possess kinetic energy proportional to absolute temperature.

• There are no significant attractive forces between gas particles.

• An ideal gas perfectly follows these assumptions.

Gas Laws

Boyle’s Law: Pressure and Volume

• At constant temperature, the volume of a fixed amount of gas is inversely proportional to its pressure.

• Equation: $P_1 V_1 = P_2 V_2$

• Example: Doubling the pressure on a gas halves its volume.

Boyle’s Law and Breathing

• Inhalation increases chest cavity volume, decreasing lung pressure and drawing air in.

• Exhalation decreases volume, increasing pressure and expelling air.

Charles’s Law: Temperature and Volume

• At constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature (in Kelvin).

• Equation: $\frac{V_1}{T_1} = \frac{V_2}{T_2}$

• Example: Doubling the absolute temperature doubles the volume.

Gay-Lussac’s Law: Temperature and Pressure

• At constant volume, the pressure of a fixed amount of gas is directly proportional to its absolute temperature.

• Equation: $\frac{P_1}{T_1} = \frac{P_2}{T_2}$

The Combined Gas Law

• Combines Boyle’s, Charles’s, and Gay-Lussac’s laws for a fixed amount of gas.

• Equation: $\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}$

Changes of State

Phase Transitions

• Heating increases particle motion, weakening intermolecular interactions.

• Common changes of state:

  • Freezing/Melting: Between solid and liquid.

  • Evaporation/Condensation: Between liquid and gas.

  • Sublimation/Deposition: Between solid and gas.

Vapor Pressure and Boiling Point

• Vapor pressure: The pressure exerted by vapor molecules above a liquid in a closed container.

• Each substance has a characteristic vapor pressure that increases with temperature.

• Boiling point: The temperature at which vapor pressure equals atmospheric pressure.

Intermolecular Forces (IMF)

Types of Intermolecular Forces

Type	Strength	Occurs Between	Example
London Forces (Dispersion)	Weakest	All molecules (especially nonpolar)	Methane (CH4)
Dipole-Dipole	Moderate	Polar molecules	Acetone
Hydrogen Bonding	Strong	H bonded to O, N, or F	Water (H2O)
Ion-Dipole	Very Strong	Ions and polar molecules	Na+ in water
Ionic Attraction (Salt Bridge)	Strongest	Oppositely charged ions	NaCl

London Forces

• Result from temporary uneven electron distribution (induced dipoles).

• Significant in nonpolar molecules.

Dipole-Dipole Attractions

• Occur between polar molecules with permanent dipoles.

• Stronger than London forces.

Hydrogen Bonding

• Special case of dipole-dipole attraction involving H bonded to O, N, or F.

• Much stronger than other dipole-dipole forces.

• Requires a donor hydrogen and an acceptor electron pair.

• Water can act as both donor and acceptor.

Ion-Dipole Attraction

• Occurs between ions and polar molecules (e.g., Na+ and H2O).

• Important in biological systems.

Ionic Attraction (Salt Bridge)

• Strongest attractive force; occurs between oppositely charged ions.

• Important in protein structure (e.g., between carboxylate and protonated amine groups).

Predicting Physical Properties

• Stronger intermolecular forces lead to higher boiling and melting points.

• Straight-chain alkanes (more surface area) have higher boiling points than branched alkanes with the same number of carbons.

• Boiling and melting points increase with molecular size and strength of IMFs.

Solubility

The Golden Rule: Like Dissolves Like

• Substances with similar polarity and IMFs are soluble in each other.

• Hydrophilic: Water-loving; soluble in water (polar or ionic).

• Hydrophobic: Water-hating; not soluble in water (nonpolar).

Solubility of Different Compounds

• Nonpolar compounds (e.g., oils): Interact via London forces; not soluble in water.

• Polar compounds (e.g., sugar): Interact via dipole-dipole and hydrogen bonding; soluble in water.

• Ionic compounds (e.g., NaCl): Dissolve via ion-dipole interactions; hydration occurs when water molecules surround ions.

Drug Solubility

• Orally administered drugs must be water-soluble for absorption.

• Pharmaceuticals often include amine or carboxylic acid groups to increase water solubility by forming charged species at physiological pH.

Amphipathic Compounds and Emulsifiers

• Amphipathic: Molecules with both polar and nonpolar regions (e.g., fatty acids).

• Fatty acids are mainly nonpolar due to their long hydrocarbon chains, despite having a polar carboxylic acid group.

• Amphipathic molecules like soaps act as emulsifiers, allowing polar and nonpolar substances to mix.

Biological Applications: Lipids and Cell Membranes

Dietary Lipids

• Fats: Derived from animals; solid at room temperature.

• Oils: Derived from plants; liquid at room temperature.

• Both are triglycerides (hydrolyzable lipids).

• Physical structure (degree of saturation, chain length) determines whether a lipid is solid or liquid at room temperature.

Phospholipids and Cell Membranes

• Phospholipids: Main structural components of cell membranes.

• Structure: Glycerol backbone, two fatty acids (nonpolar tails), and a phosphate-containing group (polar head).

• Form bilayers in water: Polar heads face aqueous environments; nonpolar tails face inward, creating a hydrophobic interior.

Steroids and Cholesterol

• Steroids: Lipids with a four-ring structure (steroid nucleus); nonpolar.

• Cholesterol: A steroid that regulates membrane fluidity and serves as a precursor for hormones and bile acids.

• The hydroxyl group of cholesterol aligns with the aqueous environment; the rest of the molecule embeds in the membrane's hydrophobic core.

• Excess cholesterol increases membrane rigidity.