Packing efficiency

“Oranges stacked in a grocer's pyramid mimic close packing.”

How much of a crystal's space is filled: fcc/hcp ~74%, bcc ~68%, simple cubic ~52%.

Memory trick: Close packing ≈ 74%.

Crystal defects

“A pinch of impurity turns pure silicon into a useful semiconductor.”

Real crystals have point defects; doping semiconductors deliberately adds them.

Memory trick: Imperfections can be useful.

Raoult's law

“Salt on icy roads works by lowering water's freezing point via this idea.”

The vapour pressure of a solvent falls in proportion to the mole fraction of solute dissolved.

Memory trick: More solute → lower vapour pressure.

Colligative properties

“Antifreeze protects a car radiator by depressing the freezing point.”

Properties depending on the number of solute particles, not their identity (freezing-point depression, boiling-point elevation).

Memory trick: Count particles, not type.

Henry's law

“A fizzy bottle hisses and bubbles the moment you release the pressure.”

The solubility of a gas is proportional to its partial pressure above the liquid.

Memory trick: More pressure → more dissolved gas.

Molality

“For precise low-temperature work, chemists prefer molality.”

Moles of solute per kilogram of solvent — unlike molarity, it doesn't change with temperature.

m = mol/kg

Memory trick: Per kg of solvent.

Adsorption vs absorption

“A sponge absorbs water; charcoal adsorbs impurities on its surface.”

Adsorption is sticking to a surface; absorption is soaking through the bulk.

Memory trick: Surface vs throughout.

Colloids and the Tyndall effect

“Sunbeams through mist or a projector's dusty light are Tyndall in action.”

Colloids scatter light, making a visible beam (the Tyndall effect).

Memory trick: Colloids scatter light.

Roasting and calcination

“The first step of turning rock into metal is driving off the unwanted parts.”

Roasting heats an ore in air (for sulphides); calcination heats it without air (for carbonates).

Memory trick: Air = roasting; no air = calcination.

Bond order

“A triple bond (order 3) is shorter and stronger than a single bond.”

Half the difference between bonding and antibonding electrons; higher bond order means stronger, shorter bonds.

Memory trick: Higher order = shorter, stronger.

Dipole moment

“Water's non-zero dipole is why it dissolves salt so well.”

A measure of molecular polarity; it is a vector that can cancel by symmetry.

Memory trick: Symmetry can zero it out.

Bond enthalpy

“Breaking bonds costs energy; forming them releases it — the balance sets ΔH.”

The energy needed to break one mole of a particular bond in the gas phase.

Memory trick: Break costs, form releases.

Solubility product (Ksp)

“It predicts whether a precipitate will form when solutions are mixed.”

The equilibrium constant for a sparingly soluble salt dissolving.

Memory trick: Exceed Ksp → it precipitates.

Salt hydrolysis

“Baking soda solution is mildly basic because of hydrolysis.”

Salts of weak acids or bases react with water to give acidic or basic solutions.

Memory trick: Weak parent → non-neutral salt.

Nernst equation

“It's why a battery's voltage sags as it drains.”

Adjusts electrode potential for non-standard concentrations.

Memory trick: Concentration shifts the voltage.

Corrosion

“Rust silently costs the world billions each year.”

The slow electrochemical oxidation of metals, like iron rusting.

Memory trick: An unwanted galvanic cell.

Fuel cell

“Hydrogen fuel cells power spacecraft and emit only water.”

Converts a fuel's chemical energy directly and cleanly into electricity.

Memory trick: Clean direct-to-electricity.

Molecularity vs order

“Order is measured; molecularity is mechanistic.”

Molecularity is the number of species in an elementary step (a whole number); order is experimental and can be fractional.

Memory trick: Molecularity is always a whole number.

First-order half-life

“Radioactive decay's steady half-life is first-order kinetics.”

For a first-order reaction, the half-life is constant, independent of concentration.

t½ = 0.693/k

Memory trick: Constant half-life = first order.

SN1 vs SN2

“Bulky groups jam the backside attack, pushing the reaction toward SN1.”

SN1 goes via a carbocation in two steps (favours 3°); SN2 is a one-step backside attack (favours 1°).

Memory trick: 3°→SN1, 1°→SN2.

Haloarenes are less reactive

“Chlorobenzene stubbornly holds its chlorine, unlike a simple alkyl halide.”

The C–X bond in haloarenes resists substitution due to resonance and partial double-bond character.

Memory trick: Resonance strengthens the C–X bond.

Oxidation of alcohols

“A breathalyser detects alcohol by oxidising it and reading the colour change.”

Primary alcohols oxidise to aldehydes then acids; secondary to ketones; tertiary resist oxidation.

Memory trick: 1°→acid, 2°→ketone, 3°→no.

Acidity of phenol

“The ring 'spreads out' the negative charge, making phenol give up its H more easily.”

Phenol is more acidic than alcohols because its conjugate base is resonance-stabilised.

Memory trick: Resonance makes phenol acidic.

Aldol condensation

“A classic carbon–carbon bond-builder in organic synthesis.”

Aldehydes/ketones with α-hydrogens combine to form β-hydroxy carbonyl compounds.

Memory trick: Needs an α-hydrogen.