Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
1,200 fundamentals
“In water (H₂O), hydrogen is +1 and oxygen –2, together giving a neutral molecule.”
Free elements have oxidation number 0; oxygen is usually –2 (except peroxides and OF₂); hydrogen is +1 (except metal hydrides). The sum of oxidation numbers equals the ion’s charge. Forgetting the exceptions (e.g., O in OF₂) is a frequent error.
Memory trick: O –2, H +1, free 0; sum = charge
“If a lab synthesis should give 80 g of aspirin but you collect 60 g, the percentage yield is 75 % – the efficiency of the experiment.”
% yield = (actual yield ÷ theoretical yield) ×100. Reflects losses and side reactions. Trap: using mass for one side and moles for the other without proper conversion.
Memory trick: Yield % = (real ÷ theoretical)×100
“Dissolving 1 g of salt in 1 m³ (1000 L) of water gives a concentration of 1 ppm.”
ppm means one part solute per 10⁶ parts of solution by mass. For water, 1 ppm ≈ 1 mg of solute per litre because 1 L ≈ 1 kg. The common mistake is treating ppm as a volume fraction or using it for non-aqueous media without converting mass to volume.
Memory trick: ppm = mg / L (water)
“In a sealed pressure cooker, the total mass of ingredients before cooking equals the mass of steam and food after.”
In any closed chemical reaction the total mass of reactants equals the total mass of products because atoms are neither created nor destroyed. This is the law of conservation of mass. In open systems mass appears to change if gases escape, so always consider a closed vessel.
Memory trick: Closed system → mass in = mass out
“Table salt always contains sodium and chlorine in the same 23:35 mass ratio, no matter where it’s mined.”
A pure compound always contains its constituent elements in a fixed mass ratio, independent of the source or amount. This is the law of definite proportions. If the observed ratio varies, the sample is a mixture, not a single compound.
Memory trick: Pure compound = fixed % by mass
“On a dartboard, hitting the bullseye once is accurate; hitting the same spot many times shows precision.”
Accuracy measures how close a result is to the true value; precision measures the repeatability of results. High accuracy with low precision gives scattered values around the true value, while high precision with low accuracy clusters away from it. Exams often swap the terms.
Memory trick: Bullseye once = accurate; cluster = precise
“When you make lemonade, 2 lemons plus 1 cup sugar always yields 3 cups drink – the ratio is fixed by the balanced recipe.”
Stoichiometry uses a balanced chemical equation to relate the mole amounts of reactants and products via their coefficients. After balancing, the ratio of coefficients gives the quantitative relationship. A common trap is to use the equation before it is balanced.
Memory trick: Balance → Ratio → Amount
“Plant roots pull water from dry soil through membranes toward the salty cell interior.”
Osmosis: solvent moves from lower to higher solute concentration across a semipermeable membrane, creating osmotic pressure π = i·M·R·T. The flow stops only when an external pressure equal to π is applied.
Memory trick: Water → higher solute side (π = iMRT)
“Raisins left in water swell as water rushes in, and the pressure that stops further inflow is the osmotic pressure.”
Osmotic pressure is the hydrostatic pressure needed to halt net solvent flow through a semipermeable membrane. For an ideal dilute solution π = i C R T, with i the van’t Hoff factor (i=1 for non-electrolytes). Forgetting i for salts or using the formula for concentrated solutions gives wrong results.
π = CRT
Memory trick: π = i·C·R·T – i counts particles
“When you stir NaCl into water, it instantly splits into Na⁺ and Cl⁻, doubling the number of particles.”
The van’t Hoff factor i equals the actual number of particles produced per formula unit in solution. For strong electrolytes i ≈ number of ions (NaCl →2). For weak electrolytes i < theoretical because of incomplete dissociation; for nonelectrolytes i = 1.
Memory trick: i = particles ÷ formula units
“When you add a pinch of salt to water, the solution conducts better per mole of salt as it gets more diluted, like spreading traffic over wider lanes.”
Molar conductivity Λm = κ·1000/c (κ = specific conductivity, c = mol L⁻¹). As concentration decreases, ion mobility rises, so Λm increases, even though κ itself falls. The trap: confusing rise of Λm with rise of κ on dilution.
Memory trick: Λm up, κ down on dilution
“When you zero a kitchen scale before weighing, you’re setting the same reference point the SHE uses for all electrode potentials.”
The Standard Hydrogen Electrode is the half-cell H₂(g, 1 atm)│Pt│H⁺(aq, 1 M). By definition its potential is 0.00 V at all temperatures, so every other electrode potential is measured relative to it. Remember: the 0 V is assigned, not experimentally obtained.
Memory trick: SHE = H2|H+ at 1 atm/1 M → 0 V
“Think of two pool balls: they must hit each other hard enough and with the right spin to pocket a ball, just like reactants need sufficient energy and proper orientation.”
Collision theory states a reaction occurs only when reactant particles collide with energy ≥ Ea and with the correct orientation (steric factor). Both factors determine the frequency of effective collisions, which sets the rate constant. Trap: ignoring orientation can make a seemingly fast reaction appear slow on paper.
Memory trick: Energy ≥ Ea + right angle = effective collision.
“When you dissolve a drop of dye in a huge tank of water, the dye’s disappearance looks like a first-order decay because water’s concentration stays essentially constant.”
If one reactant is in large excess, its concentration change is negligible, so the rate law k[A]^m[B]ⁿ reduces to k' [A]^m where k' = k[B]ⁿ (constant). The reaction then follows apparent first-order kinetics. Trap: verify that the excess reactant truly remains constant throughout the experiment.
Memory trick: Excess ≈ constant → absorb into k'.
“In a relay race, the slowest runner sets the team’s finish time; similarly, the slowest elementary step controls the overall reaction rate.”
The step with the highest activation energy (or smallest rate constant) is the rate-determining step; the overall rate law is derived from this step alone, with intermediates expressed via steady-state or pre-equilibrium approximations. Trap: writing the overall rate from a fast step gives an incorrect expression.
Memory trick: RDS = slowest link → overall rate.
“Carbon dioxide can’t be turned into liquid by squeezing a soda can at room temperature because its critical temperature is 31 °C.”
Critical temperature is the highest temperature at which a gas can be liquefied by pressure alone. Above this temperature, no amount of pressure will condense the gas. Each substance has its own Tc, a key property for liquefaction.
Memory trick: Critical T = ‘Can’t’ condense above it
“A school ledger balances total marks; each subject’s credit adjusts the final score just as formal charge balances electrons on an atom.”
Formal charge = valence electrons – (non-bonding electrons + ½ bonding electrons). The best Lewis structure has the smallest absolute FC on each atom and puts negative charge on the more electronegative atom. In NO₃⁻ only one O carries –1 in any resonance form; the overall –1 charge is delocalised over the three oxygens, not –1 on each.
Memory trick: FC = V – (L + B/2)
“Lithium iodide melts at a lower temperature than sodium chloride because Li⁺ small size pulls electron density from I⁻, giving partial covalent character.”
Fajans' rules state that a small, highly charged cation and a large, polarizable anion increase covalent character of an ionic bond. This explains LiI’s lower melting point compared with NaCl. High charge on either ion (e.g., Al³⁺) also pushes covalency.
Memory trick: Small⁺ + Large⁻ + high charge → covalent
“Liquid oxygen sticks to a magnet because two electrons occupy the degenerate π* antibonding orbitals singly, giving O₂ two unpaired spins.”
MOT places O₂’s 12 valence electrons in σ2s, σ*2s, σ2p, π2p, π*2p; the last two electrons occupy separate π* orbitals, resulting in two unpaired electrons and paramagnetism. Mistake to avoid: using the wrong orbital order, which would predict diamagnetism.
Memory trick: O₂: π* each gets 1 e⁻ → paramag
“Like a group of friends sharing a heavy bag, a σ-bond donates electron density to a neighboring empty p-orbital, easing the carbocation.”
Hyperconjugation is delocalisation of σ-C–H or σ-C–C electrons into an adjacent empty or partially filled orbital, stabilising carbocations, alkenes and radicals. The overlap lowers the system’s energy. Do not mix it up with resonance; only σ bonds participate.
Memory trick: Hyper = over, conj = join → σ-C–H joins empty p
“Like a left-handed glove fits only the left hand, a molecule that cannot be overlaid on its mirror image is chiral.”
A molecule is chiral when it has no improper symmetry element (plane, centre, Sn). A tetrahedral carbon with four different groups often gives chirality, but axial, planar or helical arrangements can also be chiral. Conversely, a molecule with stereogenic centres can be achiral (meso) if an internal symmetry plane exists.
Memory trick: Four different groups → chiral; internal mirror → meso.
“Keto-enol tautomerism is like a dancer swapping a hat (hydrogen) and a cape (double bond) between carbon and oxygen.”
Tautomerism is the rapid interconversion of isomers that differ by the position of a proton and a double bond, the classic example being keto-enol equilibrium. The shift is facilitated by acid or base catalysis and is significant when the enol form is stabilised by conjugation or intramolecular H-bond. Do not confuse it with constitutional isomerism; only certain functional groups show tautomerism.
Memory trick: TAUT = Transfer A Ubiquitous Hydrogen.
“Ammonia fertilizer is made in a Haber reactor where N₂ and H₂ are forced together like a high-pressure soda can.”
The Haber reaction N₂ + 3H₂ ⇌ 2NH₃ is exothermic. According to Le Chatelier, high pressure (≈150–200 atm) shifts equilibrium toward NH₃, while moderate temperature (400–500 °C) balances rate and yield. An iron catalyst with promoters lowers activation energy. Trap: raising temperature improves speed but cuts yield; optimal compromise is essential.
N₂+3H₂⇌2NH₃
Memory trick: Haber: high P, moderate T, Fe catalyst.
“The towering contact converters at a fertilizer plant turn SO₂ into SO₃ over V₂O₅, the heart of sulfuric acid production.”
In the Contact process SO₂ is oxidised to SO₃ using O₂ over a V₂O₅ catalyst at 450–550 °C. The catalyst provides a lower-energy pathway, increasing rate without being consumed. The reaction is exothermic, so too high a temperature shifts equilibrium left; plants use a compromise temperature and recycle gases. A frequent mistake is writing Pt as the catalyst or ignoring the temperature-equilibrium trade-off.
Memory trick: V-2-O-5 = V for Victory over SO₂ → SO₃.