Liquids & Intermolecular Forces

BIG PICTURE (START HERE)

Everything around you is made of tiny particles called molecules.

How those molecules: • stick together • move • separate

…decides whether something is a solid, liquid, or gas.

This slideshow is about HOW molecules stick to each other and what that causes.

INTRAMOLECULAR vs INTERMOLECULAR (SUPER IMPORTANT)🔹 Intramolecular bonding = INSIDE a molecule

Intramolecular bonds = the LEGO bricks snapped together • VERY strong • Break these → molecule breaks apart

Examples: • Covalent bonds (shared electrons) • Ionic bonds (charged attraction)

⚠ These bonds do NOT break when ice melts or water boils.

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🔹 Intermolecular bonding = BETWEEN molecules

Think of this like VELCRO between LEGO creations.

• Much weaker than intramolecular bonds • Controls state of matter • Break these → solid → liquid → gas

📌 Liquids & solids exist because of intermolecular forces.

CHANGES OF STATE (SOLID → LIQUID → GAS)

When ice melts: • Water molecules stay as H₂O • ONLY spacing & movement change

When you add energy (heat): • Molecules move faster • Spread out more • Intermolecular forces weaken

❌ Covalent bonds do NOT break ✅ Intermolecular forces weaken

3⃣ DIPOLE–DIPOLE FORCES

What is a dipole?

Some molecules have: • One side slightly positive (+) • One side slightly negative (–)

This happens when electrons are pulled unevenly.

➡ That molecule is polar.

Dipole–dipole force

Positive end of one molecule attracts negative end of another • Like tiny magnets

📌 Happens ONLY between polar molecules

Strength: • ~1% as strong as covalent bonds • Gets weaker as molecules get farther apart

4⃣ HYDROGEN BONDING (SPECIAL & STRONG)

Hydrogen bonding is a VERY strong dipole–dipole force.

It happens ONLY when: • Hydrogen is bonded to O, N, or F

Why? • Those atoms pull electrons HARD • Hydrogen becomes very positive • Strong attraction forms

📌 Water has hydrogen bonding → THAT’S why water is weird.

Examples of effects: • High boiling point • Ice floats • Strong surface tension

LONDON DISPERSION FORCES (EVERYWHERE)

These exist in: • ALL molecules • Especially nonpolar molecules

How? • Electrons move randomly • For a split second → uneven charge • That temporary dipole attracts another molecule

⚠ Very weak, BUT: • Bigger atoms = more electrons • More electrons = stronger dispersion forces

This explains why: • CH₄ = gas • CCl₄ = liquid

6⃣ POLARIZABILITY

Polarizability = how EASY it is to squish electrons

• More electrons → easier to distort • Bigger atoms → stronger attraction

📌 Used by nonpolar molecules to stick together

DIPOLE–INDUCED DIPOLE

When: • Polar molecule gets close to nonpolar molecule • Polar molecule forces electrons to shift • Temporary dipole forms

Stronger when: • Polar molecule has strong dipole • Nonpolar molecule is large

8⃣ LIQUID PROPERTIES

Surface Tension

• Liquid surface resists stretching • Stronger intermolecular forces → higher surface tension

Water has high surface tension because of hydrogen bonding.

8⃣ LIQUID PROPERTIES

Surface Tension

• Liquid surface resists stretching • Stronger intermolecular forces → higher surface tension

Water has high surface tension because of hydrogen bonding.

Capillary Action

When liquid climbs up a thin tube.

Two forces: • Cohesion = liquid sticks to itself • Adhesion = liquid sticks to container

Water in glass: • Adhesion > cohesion → concave meniscus

Mercury: • Cohesion > adhesion → convex meniscus

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Viscosity

Viscosity = resistance to flow

• Honey > water • Big molecules + strong forces → thick liquids

EVAPORATION & VAPORIZATION

Evaporation: • Molecules escape surface • Happens at ANY temperature

Heat of vaporization (ΔHvap): • Energy needed to turn liquid → gas • Endothermic (absorbs heat)

🔟 VAPOR PRESSURE

Vapor pressure = pressure of gas above liquid

At equilibrium: • Rate of evaporation = rate of condensation

High vapor pressure: • Weak intermolecular forces • Evaporates easily

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Measuring vapor pressure

Formula: P_vapor = P_atmosphere − P_Hg

This comes from a barometer reading.

1⃣1⃣ VAPOR PRESSURE vs TEMPERATURE

As temperature ↑: • Molecules move faster • Vapor pressure ↑

Equation (Clausius–Clapeyron):

ln(P) = −(ΔHvap / R)(1/T) + C

📌 This is just y = mx + b

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1⃣2⃣ SUBLIMATION

Solid → gas directly

Examples: • Dry ice • Iodine

1⃣3⃣ HEATING CURVES

When heating: • Flat lines = phase change • Sloped lines = temperature change

Heat of fusion: • Energy needed to melt solid

1⃣4⃣ MELTING & BOILING POINTS

Melting point: • Solid & liquid vapor pressures equal

Boiling point: • Vapor pressure = atmospheric pressure

1⃣5⃣ PHASE DIAGRAMS

Shows: • Solid / liquid / gas regions • Pressure vs temperature

Key points: • Triple point → all 3 phases exist • Critical point → gas can’t become liquid

Water is WEIRD: • Ice is less dense than liquid • Solid–liquid line slopes backward

CO₂: • No liquid at 1 atm