Jöns Jacob Berzelius developed an alphabetic system of chemical symbols for expressing composition. Combining element symbols with numerical proportions made a chemical formula compact enough to write and print while retaining information about a compound's constituents. His original notation placed the numbers differently from the subscripts familiar in modern formulas.^[ref]

Wilhelm Ostwald made catalysis, chemical equilibrium, and reaction rates central subjects of physical chemistry. His account of catalysis emphasized changes in the speed of a chemical reaction; this connects a catalyst's practical effect with the time needed to approach chemical equilibrium.^[ref]

Official lecture page, with a portrait: www.nobelprize.org/prizes/chemistry/1909/ostwald/lecture/

S. P. L. Sørensen introduced the pH scale in 1909 while investigating the importance of acidity for biochemical processes. Expressing acidity logarithmically made it practical to compare conditions spanning many powers of ten and to describe the conditions affecting an enzyme or other biochemical chemical reaction.^[ref]

An organic acid is an organic compound capable of donating a proton in an acid-base reaction. Carboxylic acids and sulfonic acids are important families, but acidic organic compounds need not all contain the same functional group.

Acid strength, quantified by an acid dissociation constant, depends on the stability and solvation of the conjugate base. Comparing different organic acids therefore requires a specified solvent and a defined equilibrium, rather than simply counting acidic hydrogen atoms.

A chemical formula uses symbols of chemical elements and numerical subscripts to record composition. In the molecular formula ${\mathrm{H}}_2\mathrm{O}$ of water, the subscript $2$ counts two hydrogen chemical atoms, and the omitted subscript after $\mathrm{O}$ means one oxygen chemical atom. A molecular formula describes one molecule; a chemical formula need not describe a discrete molecule.

Read a preceding coefficient separately from the subscripts. One ${\mathrm{H}}_2\mathrm{O}$ contains two hydrogen and one oxygen chemical atom; $2{\mathrm{H}}_2\mathrm{O}$ contains four hydrogen and two oxygen chemical atoms distributed between two water molecules. In ${\mathrm{H}}_2{\mathrm{O}}_2$, the second subscript instead puts two oxygen chemical atoms into each hydrogen peroxide molecule. Thus $2{\mathrm{H}}_2\mathrm{O}\ne {\mathrm{H}}_2{\mathrm{O}}_2$: the coefficient scales the whole chemical formula, while a subscript specifies its composition. See Flowers et al., Chemistry 2e, §2.4: chemical formulas.

For crystalline $\mathrm{NaCl}$, the chemical formula specifies a $1:1$ ratio of sodium ions ${\mathrm{Na}}^{+}$ to chlorine ions ${\mathrm{Cl}}^{-}$. Its formula unit is the accounting entity; the ordinary crystal is an extended arrangement of ions, rather than a collection of discrete $\mathrm{NaCl}$ molecules. Specifying the entity matters when assigning an amount of substance. See formula unit and 2.6 Ionic and Molecular Compounds (§2.6: ionic and molecular compounds).

A molecular formula gives an atom inventory, not the full arrangement of chemical bonds or the molecular geometry. For example, ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$ counts six carbon, twelve hydrogen, and six oxygen chemical atoms in a glucose molecule, but does not specify the pattern shown by its Fischer projection. Different structures can share a molecular formula. This compact inventory is useful for chemical equation accounting precisely because it leaves the structural detail to other representations.

The choice of letters has an early explanation in Jöns Jacob Berzelius, Essay on the Cause of Chemical Proportions (1813–1814), §III, transcribed by Carmen Giunta: letters made chemical signs easier to write and print, and formulas compressed information about composition. His numerical notation differed from modern subscripts; the lasting idea was to make proportions readable in ordinary text.

Phosphate can mean the anion $\mathrm{P}{\mathrm{O}}_4^{3-}$ or, more broadly, its protonated forms and salts. In aqueous solution the distribution among these forms depends on pH.

Near neutral conditions, hydrogen phosphate and dihydrogen phosphate form an important buffering pair. A phosphate ester instead has covalent phosphorus-oxygen-carbon connectivity, so free phosphate and a phosphorylated biomolecule are related but distinct chemical species.

Carbohydrates are a family of chemical substances encompassing monosaccharides, combinations of monosaccharides, and their derivatives. Glucose is a small carbohydrate; cellulose is a long-chain polysaccharide. Deoxyribose illustrates why composition alone does not define the family: replacing a hydroxy group in ribose by hydrogen still leaves a carbohydrate. See IUPAC Gold Book: carbohydrates.

Many familiar monosaccharides have a molecular formula of the form ${\mathrm{C}}_n{\mathrm{H}}_{2n}{\mathrm{O}}_n$, equivalently ${\mathrm{C}}_n({\mathrm{H}}_2\mathrm{O}{)}_n$. This explains the historical name, “hydrate of carbon”, but does not mean the molecule consists of carbon mixed with intact water molecules. Nor is it a universal formula: deoxyribose has ${\mathrm{C}}_5{\mathrm{H}}_{10}{\mathrm{O}}_4$. The family is defined through chemical structure and relationships, not that ratio. See IUPAC's definition and origin of the name carbohydrate.

The useful surprise is the variety of roles obtainable from related building blocks. Glucose participates in metabolism, ribose forms part of RNA, and linked glucose units make structural cellulose in a plant cell wall. Understanding carbohydrates therefore connects cellular energy, molecular information, and material structure. See Alberts et al., Molecular Biology of the Cell: the chemical components of a cell.

An aldehyde has a group $\mathrm{RC}(=\mathrm{O})\mathrm{H}$, written $\mathrm{CHO}$ at the top of the illustrated open-chain Fischer projections; $R$ denotes the attached remainder of the molecule.

Lipids are a chemically diverse family of chemical substances, conventionally associated with biological origin and solubility in nonpolar solvents. They include triglycerides, phospholipids, and cholesterol. The name does not specify one molecular formula, one repeating monomer, or one kind of polymer. Some lipids have both hydrophilic and hydrophobic regions rather than being uniformly nonpolar. See IUPAC Gold Book: lipids.

Structure explains the different jobs. A triglyceride has three fatty acid-derived acyl groups attached to glycerol and serves as an energy store. A typical glycerophospholipid instead combines two such chains with a phosphate-containing head, making it an amphiphile suited to a lipid bilayer. Cholesterol has a different structure and also contributes to cell membrane properties. Thus a lipid is a chemical building material, while a lipid bilayer is an organization of many molecules. See Clark, Douglas and Choi, Biology 2e, 3.3: lipid structures and functions.

A zwitterion has opposite formal electric charges in different regions of a molecule, with zero net electric charge. Phosphatidylcholine provides a membrane-related example. This is stronger than merely having chemical polarity through fractional partial atomic charges. See IUPAC Gold Book: zwitterions.

Sphingosine is an eighteen-carbon amino alcohol with a long unsaturated hydrocarbon chain. Its amino group can form an amide with a fatty acid, producing a ceramide; the long chains help anchor these lipids in a cell membrane.

Sphingosine therefore supplies a structural backbone for many sphingolipids, whereas glycerol supplies the backbone of glycerophospholipids. The two lipid families can have similar membrane roles while differing in their underlying chemical bonds.^[ref]

For the carboxylic acid-derived examples here, an acyl group is the group remaining after formal removal of $\mathrm{OH}$ from $\mathrm{R}-\mathrm{C}(=\mathrm{O})-\mathrm{OH}$. It includes the oxygen at the end of the fatty acid-derived chain, so it is more than just a hydrocarbon region. See IUPAC class names: carboxylic acids and derivatives.

Hydrogen peroxide is ${\mathrm{H}}_2{\mathrm{O}}_2$, containing an oxygen-oxygen single bond. Each oxygen has formal oxidation state $-1$, unlike the $-2$ state in ordinary water.

It can act as either an oxidant or reductant depending on its reaction partner. Its decomposition gives water and oxygen; catalysts alter the rate without changing that overall stoichiometry.

Here an amide means a carbonyl group whose carbon is bonded directly to nitrogen; each of $R$, $R^{\prime }$ and $R^{\prime \prime }$ may be hydrogen or a group attached through carbon. A peptide bond contains this linkage. Chemical resonance distinguishes its nitrogen from that of an amine. ^[ref] and ^[ref]

A simple neutral amine has nitrogen bonded to one, two, or three groups attached through carbon, with hydrogen filling the remaining positions in the neutral three-bond form. These are the primary, secondary, and tertiary forms respectively. An amide has different connectivity to a carbonyl group. ^[ref]

Copper(II) sulfate is $\mathrm{CuS}{\mathrm{O}}_4$, with copper formally in oxidation state +2. Hydrated crystals include water of crystallization, so the hydrate must be specified when converting a weighed mass into a chemical amount.

In the catalysed peroxide comparison, dissolved copper sulfate is treated with sodium hydroxide to form a precipitate. That experiment tests the resulting solid-forming treatment; it does not establish identical catalytic behaviour for every dissolved copper species.

A chemical species on which a reagent or catalyst acts in a chemical reaction. An enzyme acts on a biochemical substrate, a particular instance of this broader reaction role.

A product is a species formed by a chemical reaction. In a balanced reaction equation, products are conventionally written on the right, although a reversible reaction can produce the original reactants in its reverse direction.

A reaction mixture can contain a desired product, by-products, intermediates, and unreacted starting material. Chemical yield measures the amount of a specified product relative to a stated reference, whereas selectivity concerns preference among competing transformations or products. Identifying a product therefore requires both composition and its role in the reaction being described.