Part A · the big picture — what chemistry is actually about
One idea explains most of chemistry
Carbon is the backbone
Carbon can bond to 4 other atoms and to itself — making chains, rings, and branches of any length. This flexibility is why life is carbon-based.
Functional groups give personality
Attach different atom groups to that carbon backbone and the molecule's behaviour changes completely. An –OH group makes it an alcohol. A C=O makes it a ketone. The group determines smell, reactivity, solubility, and toxicity.
Polarity determines mixing
"Like dissolves like." Polar molecules (water, alcohol) mix with each other. Nonpolar molecules (oil, wax, petrol) mix with each other. This one rule explains why oil and water don't mix, and why soap works.
The six atoms that build nearly all organic molecules
Over 99% of biochemistry is built from just six elements: C, H, O, N, P, S. The rest (iron in haemoglobin, magnesium in chlorophyll, etc.) are trace actors.
Carbon and hydrogen together form the skeleton; oxygen and nitrogen add reactivity; phosphorus stores energy; sulfur locks protein shapes.
Part B · polarity — the foundation of everything
What makes a molecule polar or nonpolar?
Polar molecules
Electrons are unevenly distributed — one end is slightly negative (δ−), one slightly positive (δ+). This creates a "dipole" — a tiny magnet.
Examples: Water (H₂O), ethanol, ammonia, sugar Dissolves in: Other polar solvents (water) Property: High boiling points, form hydrogen bonds
Nonpolar molecules
Electrons are evenly distributed — no partial charges. The molecule has no dipole. Atoms pull electrons equally.
Examples: Oils, fats, waxes, petrol, CO₂, O₂ Dissolves in: Other nonpolar solvents Property: Low boiling points, don't bond with water
Why soap works: Soap molecules are amphiphilic — they have a polar (water-loving, hydrophilic) head and a nonpolar (fat-loving, hydrophobic) tail. The tail grabs onto grease; the head stays in water. The soap molecule literally bridges two incompatible worlds, surrounding grease in a micelle that water can then wash away.
Electronegativity — who wins the electron tug-of-war?
Polarity comes from electronegativity: how strongly an atom pulls shared electrons toward itself. When two bonded atoms differ greatly in electronegativity, electrons cluster near the more electronegative one — creating a dipole.
The C–H bond (difference: 0.3) is nearly nonpolar — this is why hydrocarbons don't interact with water. The O–H bond (difference: 1.2) is strongly polar — this is why water forms hydrogen bonds and has an anomalously high boiling point of 100°C despite its tiny molecular weight. Fluorine (EN = 4.0) is the most electronegative element, making C–F bonds extremely strong and chemically inert (used in non-stick coatings like PTFE).
The three intermolecular forces — ranked by strength
① Hydrogen bonds
N–H, O–H, or F–H near a lone pair on N/O/F. Strength: ~20 kJ/mol. Makes water liquid at room temperature. Holds DNA strands together. Determines protein folding.
② Dipole–dipole
Between any two polar molecules. Strength: ~5–15 kJ/mol. Why acetone (a ketone) has a higher boiling point than a similar-sized alkane.
③ London dispersion
Exists between ALL molecules — polar or not. Caused by fleeting electron imbalances. Weak per molecule (~1–5 kJ/mol) but adds up: gecko feet stick to glass via millions of these forces.
Why water is weird: Water (MW = 18) should be a gas at room temperature — H₂S (MW = 34, same group) boils at −60°C. But each water molecule can form up to 4 hydrogen bonds simultaneously, creating a cohesive liquid network. This is why water boils at 100°C, why ice floats (hydrogen-bonded crystal is less dense than liquid), and why water has the highest surface tension of any non-metallic liquid.
Part C · the main functional groups — your chemistry vocabulary
Quick reference — functional group at a glance
Alkane (no functional group)C–C–C (sp³)the simplest carbon chain
The "boring" molecules — chemically unreactive (hence "inert") but excellent fuels because burning them releases a lot of energy. The length of the chain determines the state: methane (1C) = gas, octane (8C) = liquid, paraffin (20+ C) = solid wax.
Alcohol–OHhydroxyl group
Key feature
–OH group attached to carbon. The oxygen pulls electrons, making the group polar.
Polarity
Polar. Short-chain alcohols fully mix with water. Longer chains become less soluble.
The –OH group forms hydrogen bonds with water — which is why ethanol and water mix completely. Ethanol is polar enough to dissolve in water but also has a nonpolar carbon chain — so it can dissolve some oils too. This dual nature makes it a versatile solvent (perfumes, cleaning agents). The difference between drinkable and poisonous alcohol is a single carbon: ethanol (C₂) is metabolised to acetaldehyde then to acetic acid (harmless); methanol (C₁) is metabolised to formaldehyde — toxic and fatal even in small doses.
Aldehyde–CHO (R–C(=O)–H)carbonyl at chain end
Key feature
C=O double bond at the END of a carbon chain, with an H attached.
Polarity
Polar. The C=O is strongly polarised. Mix with water at short chain lengths.
Aldehydes are often responsible for strong, distinctive smells. Many flavour compounds (vanilla, cinnamon, almond) are aldehydes. The smell of fresh-cut grass is partly due to short-chain aldehydes. Formaldehyde — the simplest aldehyde — is toxic and used in preservation (embalming fluid). Alcohol oxidises to aldehyde: when you drink ethanol, your liver converts it to acetaldehyde — the molecule largely responsible for the hangover.
KetoneR–C(=O)–R'carbonyl in the middle
Key feature
C=O double bond in the MIDDLE of a chain, flanked by two carbon groups.
Polarity
Polar. Excellent solvents — dissolve both polar and many nonpolar compounds.
Ketones are superb solvents because their polarity is intermediate — they dissolve many things water won't. Acetone (nail polish remover) is the simplest ketone, miscible with water but also dissolves plastics, paints, and oils. The body produces ketones (like acetoacetate) when burning fat instead of glucose — this is the basis of the "keto diet." Breath smelling of nail polish remover can indicate diabetic ketoacidosis — a medical emergency.
Carboxylic acid–COOHthe classic organic acid
Key feature
Contains both C=O and –OH. The –OH proton can be released → makes it acidic.
Polarity
Strongly polar. Dissolves in water and can form strong hydrogen bonds — hence high boiling points and distinctive sharp smells.
The sharp smell of vinegar is pure acetic acid (ethanoic acid). The soreness you feel after intense exercise comes partly from lactic acid accumulating in muscle. Long-chain carboxylic acids are called fatty acids — the building blocks of fats and oils. When you react a carboxylic acid with an alcohol, you get an ester (see below) — which is how many flavours and fragrances are made.
Ester–COO– (R–CO–O–R')acid + alcohol → ester + water
Key feature
Formed by reacting a carboxylic acid with an alcohol, releasing water. The –OH and –COOH combine.
Polarity
Moderately polar but less than acids/alcohols. Many are volatile liquids with fruity or floral smells.
Esters are responsible for most fruit and flower smells. "Artificial banana" flavour is almost pure isoamyl acetate — a single ester molecule. All fats and vegetable oils are triglycerides: three fatty acid chains attached to a glycerol backbone via ester bonds. When you digest fat, enzymes break those ester bonds. Polyester fabric is also an ester polymer — long chains of ester-linked monomers.
Amine–NH₂ (or –NHR, –NR₂)nitrogen-containing — basic, often smelly
Key feature
Contains nitrogen. The lone pair on N can accept a proton → makes amines basic (opposite of acids).
Polarity
Polar. Short-chain amines are water-soluble. Most have strong, unpleasant fishy or decaying smells.
Real examples
Putrescine and cadaverine (smell of decay and rotting meat), trimethylamine (fishy smell), adrenaline, dopamine, serotonin, amino acids
Amines are biochemically critical — all amino acids contain an amine group, and neurotransmitters (dopamine, serotonin, adrenaline) are amines. The fishy smell of old fish is trimethylamine — an amine produced as bacteria break down proteins. Lemon juice on fish works because the citric acid (an acid) reacts with the amine (a base), neutralising it and reducing the smell. This is real chemistry on your plate.
EtherR–O–R'oxygen bridge between two carbons
Key feature
An oxygen atom bonded to two carbon groups. No O–H bond, so can't form hydrogen bonds as a donor.
Polarity
Moderately polar but far less than alcohols. Mostly insoluble in water. Dissolves many organic compounds.
Diethyl ether was the first widely used surgical anaesthetic (1846). Its pleasant smell and rapid onset made it transformative for surgery — though highly flammable. THF is one of the most common solvents in organic chemistry labs — it dissolves almost everything. Note: "ether" in everyday speech often refers to diethyl ether specifically, but chemically it names any R–O–R structure.
Amide–CO–NH–the peptide bond — life's connector
Key feature
A C=O next to a nitrogen. The peptide bonds linking amino acids into proteins are amide bonds.
Polarity
Polar. Strong hydrogen bonding — hence proteins fold into complex 3D shapes via amide H-bonds.
Every protein in your body — muscle, enzymes, hair, antibodies — is built from amino acids joined by amide (peptide) bonds. Nylon is a synthetic polyamide: long chains of amide-linked monomers that mimic the structural properties of proteins. Paracetamol contains an amide group, which is why it behaves differently from aspirin (which is an ester). The amide bond is exceptionally stable — which is why proteins can persist for thousands of years in fossils.
Part D · interactive — what's in this molecule?
Pick a familiar substance — see its chemistry
Part E · pH — the acid-base scale
pH 0–14: from battery acid to drain cleaner
0
Battery acid, stomach acid (pH ~1.5)
2
Lemon juice (~2.2), vinegar (~2.5)
4
Tomato juice (~4), coffee (~5)
7
Pure water — neutral
7.4
Human blood — must stay here
9
Baking soda (~8.3), seawater (~8.1)
11
Ammonia (~11), bleach (~12)
14
Drain cleaner, NaOH
pH is logarithmic. Each step is 10× more acidic or alkaline. pH 3 is 10× more acidic than pH 4, and 100× more acidic than pH 5. Stomach acid (pH ~1.5) is roughly 30,000× more acidic than blood (pH 7.4). Your body maintains blood pH within ±0.1 of 7.4 — outside pH 6.8 or 7.8, death occurs.
Interactive: how much more acidic is X than Y?
pH of substance A
pH of substance B
Part F · key reactions — chemistry in action
Click a reaction type to see it in action
Part G · smell and taste — chemistry you can sense
Every smell is a molecule landing on a receptor
Smell (olfaction) works by molecules binding to ~400 different receptor proteins in your nose. Each receptor fires for a range of molecular shapes. Your brain reads the pattern as a smell. Pick a familiar smell below to see the chemistry behind it.
Select a smell above.
The five tastes — and their chemical triggers
Sweet
Sugars (glucose, fructose, sucrose) bind to T1R2/T1R3 receptor dimers. Fructose is ~1.7× sweeter than glucose despite being the same formula (C₆H₁₂O₆) — different shape, different receptor fit.
Sour
H⁺ ions (from acids like citric, acetic, lactic) activate proton-sensing channels. The more H⁺, the lower the pH, the sourer the taste. Vinegar (pH 2.5) vs lemon (pH 2.2) — directly measurable acidity.
Salty
Na⁺ ions from sodium chloride pass directly through ENaC ion channels on taste cells. Other salts (KCl, MgCl₂) taste differently because their cations activate different channels or in different proportions.
Bitter
~25 different TAS2R receptors detect a huge variety of compounds — mostly alkaloids and glucosinolates that are toxic in plants. Caffeine, quinine, and Brussels sprouts all trigger bitterness. Evolved as a toxin-detection system.
Umami
Glutamate (an amino acid) and nucleotides (IMP, GMP) bind the T1R1/T1R3 receptor. Identified in 1908 by Kikunae Ikeda studying kombu seaweed. Naturally concentrated in Parmesan, anchovies, mushrooms, tomatoes, and soy sauce.
Part H · boiling points — how structure determines state of matter
Same molecular weight, wildly different boiling points
Molecular weight alone doesn't determine boiling point — intermolecular forces do. Two molecules of similar mass but different functional groups can have boiling points 100°C apart.
Butanone (ketone, MW 72) and pentane (alkane, MW 72) have identical molecular weights yet boil 44°C apart — because the C=O group creates dipole-dipole attractions that pentane lacks. Water (MW 18) should boil around −80°C based on size alone; its actual boiling point of 100°C is entirely due to the network of hydrogen bonds each molecule forms.
Part I · chemistry in the kitchen — reactions you've already done
Six kitchen reactions — and what's actually happening
🥩 Maillard reaction
When protein (amine groups on amino acids) reacts with sugar (carbonyl groups) at temperatures above ~140°C, hundreds of new flavour and colour molecules form. This is browning: bread crust, seared meat, coffee roasting. It is NOT caramelisation (sugar-only) — it requires both amino acids and sugars.
🍰 Baking soda + acid
NaHCO₃ (baking soda) is a base. When it contacts an acid (buttermilk, vinegar, lemon juice), it reacts: NaHCO₃ + H⁺ → Na⁺ + H₂O + CO₂. The CO₂ gas creates bubbles that make cakes rise. Baking powder is baking soda premixed with a dry acid (cream of tartar) — just add water to activate.
🥚 Denaturing proteins (cooking eggs)
Egg white is ~10% protein (mainly ovalbumin). Heat breaks the hydrogen bonds and disulfide bridges that keep the protein folded in its 3D shape. The protein unfolds ("denatures"), exposing hydrophobic regions that stick to each other — forming the opaque solid you see. This is irreversible: you cannot un-cook an egg.
🍬 Caramelisation
Heating sucrose above ~160°C breaks the glycosidic bond into glucose + fructose, then drives dehydration and polymerisation reactions, forming hundreds of compounds including furans (nutty), diacetyl (buttery), and brown polymers (caramelans). This is purely sugar chemistry — no proteins involved. The smell of caramel is a complex mixture, not a single molecule.
🫒 Emulsification (making mayonnaise)
Oil and water don't mix — but egg yolk contains lecithin (phosphatidylcholine), an amphiphilic molecule with a polar head and nonpolar tail. It coats tiny oil droplets and keeps them suspended in water. The result: a stable emulsion. The same principle applies to milk, cream, and vinaigrette (which lacks a proper emulsifier, which is why it separates).
🍋 Acid ceviche (cold cooking)
Citric acid in lime juice (pH ~2.2) denatures fish proteins without heat. The H⁺ ions disrupt the hydrogen bonds maintaining the protein's native structure, unfolding and cross-linking the proteins — the fish turns opaque and firms up exactly as heat would. The result is chemically "cooked," though not safe from all pathogens.
Part J · test yourself
1. Why does lemon juice stop cut fruit from browning?
Two reasons, both chemical. First, lemon juice is highly acidic (pH ~2.2) — it lowers the pH on the fruit's surface, and the enzyme responsible for browning (polyphenol oxidase) is inactivated at low pH. Second, lemon juice contains ascorbic acid (vitamin C), which is a reducing agent — it reacts preferentially with oxygen before the fruit's compounds can. Both effects deny the browning enzyme what it needs (neutral pH and oxygen). The same principle explains why blanching (brief boiling) stops browning: heat denatures the enzyme permanently.
2. Why does oil and water not mix, and how does soap change that?
Oil is nonpolar — its C–C and C–H bonds share electrons equally. Water is polar — the oxygen pulls electrons away from hydrogen, creating partial charges. When you mix them, water molecules strongly prefer interacting with each other (via hydrogen bonds) over oil, and oil molecules prefer their own nonpolar interactions. The system minimises energy by separating. Soap molecules have a long nonpolar hydrocarbon tail (soluble in oil) and a polar ionic head (soluble in water). They surround oil droplets in microscopic spheres called micelles — nonpolar tails pointing inward at the oil, polar heads facing outward into the water. The whole assembly is then water-soluble and can be rinsed away.
3. Paracetamol and aspirin are both painkillers. What is the key chemical difference between them?
Aspirin contains an ester group (–COO–) and a carboxylic acid (–COOH). It is acidic and irritates the stomach lining directly, and inhibits COX enzymes throughout the body — including in the stomach. Paracetamol contains an amide group (–CO–NH–) — it acts primarily in the central nervous system and has little effect on the stomach. The amide bond makes paracetamol significantly less acidic than aspirin and much gentler on the digestive tract. However, paracetamol's metabolism produces a toxic intermediate (NAPQI) that the liver neutralises with glutathione — which is why overdose causes liver failure, not stomach bleeding (as aspirin overdose does).
4. Why does fresh fish smell fishy, and why does lemon juice reduce the smell?
The fishy smell is primarily trimethylamine (TMA) — a small amine molecule produced as bacteria break down trimethylamine oxide (TMAO) in fish tissue after death. TMA is a base (it contains nitrogen with a lone pair that accepts protons). Lemon juice is acidic (citric acid, pH ~2.2). When acid meets base: the acid donates a proton to the amine, converting TMA into trimethylammonium salt — a non-volatile ionic compound with essentially no smell, because it can't easily evaporate. The chemistry is: acid + amine → ammonium salt. This is a genuine acid-base neutralisation happening on your plate. Very fresh fish has almost no TMA yet — which is why sashimi-quality fish barely smells at all.
5. What functional groups does caffeine contain, and why does it dissolve in both water and fat?
Caffeine contains amide-like C=O groups and tertiary amine-like nitrogen atoms in a bicyclic ring system (it's a methylxanthine). Its molecular structure gives it intermediate polarity: polar enough to dissolve readily in water (which is why coffee is water-extracted), but also soluble in fats and oils. This mixed character is exactly what allows caffeine to cross the blood-brain barrier — which is a lipid (fat) membrane. A molecule that can't dissolve in fats cannot penetrate cell membranes. Caffeine's mixed polarity is the chemical reason it reaches your brain rapidly after drinking coffee.
6. Why is methanol so much more toxic than ethanol when both are alcohols?
Both are alcohols with an –OH group, but differ by one carbon. The toxicity comes from their metabolites — what your body converts them into. Ethanol (C₂H₅OH) → acetaldehyde → acetic acid (effectively harmless; acetic acid is vinegar). Methanol (CH₃OH) → formaldehyde → formic acid. Formaldehyde is highly reactive and damages proteins and DNA in any tissue it contacts. Formic acid disrupts the mitochondrial electron transport chain, starving cells of ATP — particularly in the optic nerve (causing blindness) and systemically causing fatal acidosis. As little as 10 mL of methanol can cause permanent blindness; 30 mL can be fatal. The antidote is ethanol — it competes for the same enzyme (alcohol dehydrogenase), slowing methanol metabolism until the methanol can be excreted.
7. The Maillard reaction and caramelisation both produce browning. How are they different?
Caramelisation is purely thermal decomposition of sugars — no protein required. When sugar is heated above ~160°C, it begins to break down and re-polymerise into brown compounds with characteristic caramel flavours. It's a reaction of sugars with themselves. The Maillard reaction (discovered by Louis-Camille Maillard in 1912) requires both amino acids (from protein) and reducing sugars, and begins at around 140°C. The amine groups on amino acids react with the carbonyl groups of sugars through a complex cascade of reactions, producing hundreds of new compounds that give browned meat, bread crust, and roasted coffee their characteristic flavours and aromas — many of which can't be produced by caramelisation alone. This is why a sugar glaze browns differently from a meat surface browning.