Unit 4: Liquids and Solids

Table of Contents

Liquid State: Surface Tension

Definition: The property of a liquid that causes its surface to contract to the minimum possible area. It is the force per unit length (N/m) or energy per unit area (J/m2) of the surface.

Cause: Molecules at the surface experience a net inward pull from the bulk molecules, as they lack neighbors above them. This creates tension, like a "skin" on the liquid.

Determination using Stalagmometer

A stalagmometer is a glass tube used to measure surface tension via the drop-count method. It compares the number of drops formed by a fixed volume of a test liquid (1) and a reference liquid (2, usually water).

Formula: (γ1) / (γ2) = (n2 d1) / (n1 d2)
Where: γ = surface tension, d = density, n = number of drops.

Effect of Temperature on Surface Tension (Qualitative)

Surface tension decreases as temperature increases.
Reason: As temperature rises, the kinetic energy of the molecules increases. This increased movement overcomes the intermolecular forces, reducing the net inward pull on the surface molecules.

Liquid State: Viscosity

Definition: The measure of a fluid's internal resistance to flow. (e.g., Honey has high viscosity, Water has low viscosity).

Cause: It arises from the intermolecular forces that make it difficult for one layer of liquid to slide past another.

Determination using Ostwald Viscometer

This method compares the time (t) it takes for a fixed volume of liquid to flow through a narrow capillary tube under the force of gravity.

  • Principle: Based on Poiseuille's equation. For a given viscometer, the time of flow is proportional to the viscosity (η) and inversely proportional to the density (d).
  • By comparing a test liquid (1) to a reference liquid (2, water):
Formula: (η1) / (η2) = (d1 t1) / (d2 t2)
Where: η = viscosity, d = density, t = flow time.

Effect of Temperature on Viscosity (Qualitative)

Viscosity of a liquid decreases as temperature increases.
Reason: As temperature rises, molecules gain kinetic energy and can more easily overcome the intermolecular forces, allowing the layers to slide past each other with less resistance.

Solid State: Classification and Structure

Forms of Solid

Crystalline solids are classified based on their constituent particles and the bonding forces between them.

Solid Type Particles Bonding Forces Properties Examples
Ionic Cations and Anions Electrostatic Attraction Hard, brittle, high MP, conduct only when molten/aqueous. NaCl, K2SO4
Covalent (Network) Atoms Covalent Bonds Very hard, very high MP, non-conductors (except graphite). Diamond, SiO2 (Quartz)
Molecular Molecules Intermolecular Forces (H-bonds, Dipole, London) Soft, low MP, non-conductors. H2O (Ice), I2, Dry Ice (CO2)
Metallic Cations in a "sea" of electrons Metallic Bonding Ductile, malleable, variable hardness, high conductivity. Cu, Fe, Ag

Crystal Systems and Bravais Lattice Types

  • Space Lattice: A regular 3D arrangement of points (lattice points) in space.
  • Unit Cell: The smallest repeating unit that generates the entire lattice.
  • Crystal Systems: Based on unit cell edge lengths (a, b, c) and angles (α, β, γ), there are 7 crystal systems (Cubic, Tetragonal, Orthorhombic, Monoclinic, Triclinic, Hexagonal, Rhombohedral).
  • Bravais Lattices: Including variations (Primitive, Body-Centered, Face-Centered, End-Centered), there are a total of 14 Bravais Lattices.

Different types of Cubic Unit cells

Unit Cell Type Atom Locations Atoms per Unit Cell (Z) Coordination No.
Simple Cubic (SC) 8 corners 8 × (1) / (8) = 1 6
Body-Centered (BCC) 8 corners + 1 in center (8 × (1) / (8)) + 1 = 2 8
Face-Centered (FCC) 8 corners + 6 face centers (8 × (1) / (8)) + (6 × (1) / (2)) = 4 12

Solid State: Defects in Crystals

Crystal defects are imperfections or deviations from the perfect, ordered arrangement in a crystal.

Point defect vs. Line defect

  • Point Defects: Irregularities or vacancies at specific lattice points. (e.g., Schottky, Frenkel).
  • Line Defects: Irregularities or dislocations along an entire row or line of lattice points. (e.g., edge dislocation).

Schottky & Frenkel Defect

These are stoichiometric point defects (they do not change the chemical formula).

Feature Schottky Defect Frenkel Defect
Description A pair of vacancies: one cation and one anion are missing from the lattice. A smaller ion (usually the cation) is dislocated from its site to an interstitial site.
Effect on Density Decreases density (mass is lost, volume is constant). No significant change (mass is conserved).
Condition Found in compounds with similar cation/anion sizes and high coordination numbers. Found in compounds with a large difference in cation/anion sizes.
Examples NaCl, KCl, CsCl AgCl, AgBr, ZnS