THE LESSON LIBRARY / CHEMISTRY
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Sigma Bonds, Pi Bonds and Rotation
Organic line formulas draw one, two or three strokes between atoms, but those strokes have different orbital interpretations. Every ordinary single bond is sigma; a double bond…
Explore lesson ChemistryLessonSigmatropic Rearrangements: Cope and Claisen
Some pericyclic reactions move a sigma bond rather than adding a second molecule or closing a polyene ring. A [3,3] sigmatropic rearrangement shifts a sigma bond across two…
Explore lesson ChemistryLessonSigmatropic Rearrangements: Notation and Principles
A sigmatropic rearrangement moves a σ bond while nearby π bonds shift. Unlike a cycloaddition, it need not join two separate molecules; the starting and product structures often…
Explore lesson ChemistryLessonSignificant Figures and Measurement Uncertainty
A calculator will display more digits than an experiment can justify. Rounding a result prevents the display from implying unrealistic detail, but significant-figure rules alone…
Explore lesson ChemistryLessonSignificant Figures and Reported Uncertainty
A calculator can display twelve digits even when the underlying mass was read only to a milligram and the concentration standard is uncertain. Reporting all twelve suggests…
Explore lesson ChemistryLessonSignificant Figures in Chemical Data
A calculator may display ten digits even when a balance or burette supports only three or four. Significant figures are a way to avoid claiming more numerical precision than…
Explore lesson ChemistryLessonSignificant Figures in Mole Calculations
A calculator may show ten digits for a mole answer, but the input mass may be measured only to three significant figures. Reporting every displayed digit implies a precision the…
Explore lesson ChemistryLessonSignificant Figures in Multi-Step Problems
Multi-step calculations can lose accuracy if an intermediate result is rounded too early. They can also appear more certain than the measurements support if every calculator…
Explore lesson ChemistryLessonSignificant Figures in Multiplication and Division
You measure the mass of a metal block as 52.36 g on a good balance, but its volume as 19 cm³ using a measuring cylinder. The calculator says the density is 2.755 789 474 g/cm³.…
Explore lesson ChemistryLessonSignificant Figures: The Idea
A newspaper reports that a stadium holds "about 60 000 people". A ticket office says it sold 59 874 tickets. Both numbers are about the same size, but they tell you very…
Explore lesson ChemistryLessonSigns of Chemical Change: An Overview
We cannot see atoms rearranging, so how do we know when a chemical reaction has taken place? We look for clues — things we can see, hear, smell or measure that suggest a new…
Explore lesson ChemistryLessonSilicates and Silicones
Much of Earth's crust is built from silicates, while silicone sealants and elastomers are manufactured polymers. Their names resemble each other because both feature Si–O…
Explore lesson ChemistryLessonSilicates and Structural Units
Many minerals can be understood as arrangements of SiO₄ tetrahedra. The tetrahedra may remain isolated, share one corner in pairs, form chains or sheets, or link into…
Explore lesson ChemistryLessonSilicon and Alloying Anodes
Silicon can store far more lithium per gram than graphite, making it attractive for compact batteries. Unlike graphite's largely interlayer insertion, silicon's lithiation…
Explore lesson ChemistryLessonSilicon Dioxide Networks
Silicon dioxide occurs in quartz and is a major component of many sands and rocks. The empirical formula SiO₂ resembles CO₂, but quartz is not made of isolated O=Si=O molecules.…
Explore lesson ChemistryLessonSilicon Solar-Cell Materials
Crystalline silicon is an effective solar absorber only when its optical, electrical and interfacial properties are engineered together. A wafer with a suitable band gap can…
Explore lesson ChemistryLessonSilicones and Si–O Backbones
Silicones are not elemental silicon and are not the same as silica. They are organosilicon materials built around repeating Si–O–Si links, often with organic groups attached to…
Explore lesson ChemistryLessonSimple Cubic Packing
The simple cubic structure is the most direct way to see how a repeating cube can represent a crystal. It has lattice points only at cube corners. Fractional counting gives one…
Explore lesson ChemistryLessonSimple Distillation
Evaporation recovers the dissolved solid from a solution but loses the solvent into the air. What if you want to keep the solvent — for example, to get pure water from salty…
Explore lesson ChemistryLessonSimple Molecular Substances
Oxygen, water and carbon dioxide contain strong covalent bonds, yet their bulk properties differ greatly from diamond's. The crucial distinction is structural: they consist of…
Explore lesson ChemistryLessonSimple Reacting Mass Calculations
A balanced equation gives mole ratios, but laboratory problems often provide a reactant mass and request a product mass. The reliable route has three links: reactant grams to…
Explore lesson ChemistryLessonSimple Symbol Equations
A word equation tells you which substances react, but not how many particles are involved. A symbol equation replaces names with formulae, so you can see every atom. Because…
Explore lesson ChemistryLessonSimplifying and Checking Formulae
Crossing valencies is a quick way to write formulae, but it sometimes gives an answer that is not in its final form. Cross the valencies of calcium (2) and oxygen (2) and you get…
Explore lesson ChemistryLessonSingle-Atom and Cluster Catalysts
At the smallest end of nanocatalysis, a metal may be dispersed as individual atoms rather than conventional particles. This can use expensive metal efficiently and create unusual…
Explore lesson ChemistryLessonSingle-Atom Catalysts
Putting catalytic metal atoms on a support one by one can maximise exposure of expensive metal and create distinct coordination environments. But “single atom” is a structural…
Explore lesson ChemistryLessonSingle-Molecule Magnets
A molecule can act as a tiny magnet over a finite time without being a piece of bulk ferromagnetic metal. In a single-molecule magnet (SMM), coupled electronic spins and magnetic…
Explore lesson ChemistryLessonSingle, Double and Triple Bonds Revisited
A carbon–carbon single bond uses one bond order at each carbon, a double bond uses two and a triple bond uses three. Since ordinary neutral carbon commonly totals four bond…
Explore lesson ChemistryLessonSingle, Double and Triple Carbon Bonds
Two carbon atoms can share one, two or three pairs of electrons. Ethane, ethene and ethyne each contain two carbon atoms, yet they behave very differently: ethane is fairly…
Explore lesson ChemistryLessonSinglet and Triplet States
Two excited molecules can have similar energies but different total electron spin. States are often labeled singlet or triplet according to multiplicity, which affects which…
Explore lesson ChemistryLessonSinglet Oxygen in Organic Chemistry
Ordinary molecular oxygen is unusual: its lowest electronic state is a triplet. Light-absorbing sensitizers can transfer excitation energy to O₂ and create reactive singlet…
Explore lesson ChemistryLabSize-dependent properties
Properties change at the nanoscale: quantum dots glow different colours, and small particles are mostly surface.
Explore lesson ChemistryLessonSize-Dependent Properties of Nanomaterials
A small particle may melt at a lower temperature than a bulk crystal of the same substance, dissolve more readily, or show different catalytic behaviour. Those statements…
Explore lesson ChemistryLessonSize-Exclusion Chromatography
Size-exclusion chromatography, also called gel-permeation chromatography for many organic-solvent systems, separates dissolved polymer chains mainly by the space their coils…
Explore lesson ChemistryLessonSkeletal Formulae for Carbon Chains
Skeletal formulae make large organic structures easier to read by leaving most carbon letters and carbon-attached hydrogens unwritten. Each unlabeled line end and corner usually…
Explore lesson ChemistryLessonSlater Determinants
Antisymmetrising two electron orbitals by hand is manageable, but a molecule may have dozens of electrons. A Slater determinant packages all required signed permutations into one…
Explore lesson ChemistryLessonSmall Prefixes: Milli, Micro and Nano
Much of chemistry happens on a scale far too small to see. Atoms and molecules are measured in nanometres; bacteria and cells in micrometres; the samples used in medical tests…
Explore lesson ChemistryLessonSmall-x Approximations
Weakly reacting systems can make ICE equations algebraically awkward even though the equilibrium change is small. Replacing C − x by C may simplify the equation, but the shortcut…
Explore lesson ChemistryLessonSN1 Kinetics
If a tertiary alkyl halide reacts by SN1, adding more external nucleophile generally does not make the slow bond cleavage occur faster. The substrate alone participates in the…
Explore lesson ChemistryLessonSN1 Mechanism
In an SN1 reaction, the leaving group departs before the nucleophile forms its bond. This leaves a carbocation intermediate that can be attacked in a later step. The sequence…
Explore lesson ChemistryLessonSN1 Rate and Carbocation Stability
SN1 is named for its simplest kinetic signature: rate depends on alkyl-halide concentration but not directly on the nucleophile concentration in the slow ionization step. This…
Explore lesson ChemistryLessonSN1 Reaction Coordinate
Tertiary alkyl halides can undergo substitution even when a bulky carbon prevents the backside approach needed for SN2. In a typical SN1 reaction, the C–leaving-group bond breaks…
Explore lesson ChemistryLessonSN1 Rearrangement Problems
An SN1 product does not always retain the starting carbon skeleton. Once the leaving group departs, the resulting carbocation may rearrange before a nucleophile captures it. A…
Explore lesson ChemistryLessonSN1 Stereochemistry
An SN1 reaction breaks the bond to the leaving group before the incoming nucleophile forms its bond. The carbon left behind is approximately planar, so a nucleophile can approach…
Explore lesson ChemistryLessonSN1 Stereochemistry and Rearrangement
SN1 proceeds through a carbocation rather than one concerted displacement. The positively charged carbon is approximately planar, so a nucleophile can often approach from either…
Explore lesson ChemistryLabSN1 via carbocation
The SN1 mechanism: the leaving group goes first, making a flat carbocation, then the nucleophile adds.
Explore lesson ChemistryLabSN2 backside attack
The SN2 mechanism: the nucleophile attacks from the back as the leaving group leaves, in one step.
Explore lesson ChemistryLessonSN2 in Depth: Backside Attack and Inversion
SN2 is more than the arrow “Nu⁻ replaces X.” It is a one-step bond exchange with a characteristic rate law and stereochemical outcome. A nucleophile attacks opposite the leaving…
Explore lesson ChemistryLessonSN2 Kinetics
The "2" in SN2 refers to the molecularity of the rate-limiting elementary step, not to the number of products or curved arrows. Both the nucleophile and the alkyl substrate…
Explore lesson ChemistryLessonSN2 Mechanism
An SN2 reaction replaces a leaving group in one concerted event. The nucleophile bonds to carbon while the carbon–leaving-group bond breaks. There is no isolable carbocation…
Explore lesson ChemistryLessonSN2 Rate and Substrate Effects
The SN2 mechanism predicts a testable kinetic pattern. The substrate and nucleophile meet in the same elementary step, so changing either concentration changes the rate. At the…
Explore lesson ChemistryLessonSN2 Reaction Coordinate
When hydroxide replaces bromide in bromoethane, the new C–O bond is not completed before the C–Br bond starts to break. The two changes occur together in a single elementary…
Explore lesson ChemistryLessonSN2 Stereochemistry — Haloalkanes and Haloarenes
SN2 is especially informative when the carbon bearing the leaving group is stereogenic. Backside attack makes its three unchanged groups flip their spatial arrangement as the…
Explore lesson ChemistryLessonSN2 Stereochemistry — Organic Mechanisms and Named Reactions
An SN2 reaction has an unmistakable three-dimensional signature. A nucleophile approaches a saturated carbon from the side opposite its leaving group, and the three other…
Explore lesson ChemistryLessonSoap in Hard Water
Soap works less efficiently in hard water because dissolved Ca²⁺ and Mg²⁺ can pair with fatty-acid carboxylate ions. The resulting salts have low solubility and separate as scum.…
Explore lesson ChemistryLessonSoap Molecules and Their Two Ends
A simple soap molecule has two contrasting regions. Its long hydrocarbon tail mixes poorly with water but interacts with oily material, while its charged carboxylate head…
Explore lesson ChemistryLessonSodium Carbonate and Washing Soda
Sodium carbonate, Na₂CO₃, is a soluble salt with a basic carbonate ion. A common crystalline form known as washing soda is the decahydrate Na₂CO₃·10H₂O. The distinction matters…
Explore lesson ChemistryLessonSodium Chloride in Chemistry and Daily Life
Sodium chloride is familiar as table salt, but its chemistry reaches beyond seasoning. Its NaCl formula records a one-to-one ratio of sodium and chloride ions. In water those…
Explore lesson ChemistryLessonSodium Compounds and Applications
Sodium appears in compounds with very different uses and reactions while normally remaining at oxidation state +1. NaCl, NaOH, Na₂CO₃ and NaHCO₃ differ mainly in their anions…
Explore lesson ChemistryLessonSodium Hydrogen Carbonate
Sodium hydrogen carbonate, NaHCO₃, connects salt formulas, acid–base chemistry and gas formation. Commonly called baking soda, it can release carbon dioxide when it reacts with…
Explore lesson ChemistryLessonSodium to Argon Configurations
Neon completes the second shell with ten electrons. Sodium begins period three by placing its eleventh electron in 3s. As atomic number rises to argon, 3s fills and then 3p…
Explore lesson ChemistryLessonSodium-Ion Battery Materials
Sodium-ion batteries use the same broad rocking-chair idea as lithium-ion cells: ions move between two host electrodes during charge and discharge. They do not use the same…
Explore lesson ChemistryLessonSoftening Water by Boiling
The simplest way to soften some hard water needs no chemicals at all: just boil it. For centuries people have noticed that boiled water lathers better with soap than water…
Explore lesson ChemistryLessonSoftening Water with Washing Soda
Boiling can only deal with temporary hardness, but there is a cheap chemical that removes both kinds: washing soda, or sodium carbonate. For generations it was added to laundry…
Explore lesson ChemistryLessonSoil Chemistry and Contaminant Mobility
Soil is chemically active, not a passive sieve. Clay minerals, iron oxides and organic matter bind and release ions and organic molecules as water moves through pores. Their…
Explore lesson ChemistryLessonSoil pH and Agriculture
Farmers and gardeners talk about soil being "sour" or "sweet". What they really mean is acidic or alkaline. The pH of soil controls which nutrients roots can take up, which…
Explore lesson ChemistryLessonSolar Radiation and Atmospheric Photochemistry
Almost all atmospheric chemistry is driven by sunlight. The Sun delivers energy that splits molecules into reactive fragments — atoms and radicals that then set off chains of…
Explore lesson ChemistryLessonSolar-Fuel Selectivity
A solar-fuel catalyst may have several chemically available electron acceptors. Protons or water can become H₂, oxygen can consume electrons, and CO₂ can become CO, formate…
Explore lesson ChemistryLessonSolid Diffusion and Rate Limits in Li-ion Electrodes
Fast charging requires lithium to cross electrolyte-filled pores, pass an interphase, react at particle surfaces and redistribute inside active solids. A slow step anywhere can…
Explore lesson ChemistryLessonSolid Electrolyte Families
A solid electrolyte can carry lithium ions between electrodes without a flowing liquid. That possibility motivates batteries with different cell architectures and potentially…
Explore lesson ChemistryLessonSolid Electrolytes and Ionic Conduction
A piece of zirconia can separate two gases while letting oxide ions pass through its lattice. This seems contradictory if a crystal is pictured as a rigid collection of locked…
Explore lesson ChemistryLessonSolid Oxide and High-Temperature Fuel Cells
Solid oxide fuel cells, or SOFCs, use a ceramic electrolyte and operate at elevated temperature. Unlike a typical PEM cell, many SOFCs conduct oxide ions from the oxygen side…
Explore lesson ChemistryLessonSolid Solutions
A binary alloy need not form separate pure-A and pure-B solids. If its components substitute for one another in one crystal structure over the entire composition range, the solid…
Explore lesson ChemistryCollectionSolid-State and Materials Chemistry
Study Solid-State and Materials Chemistry with 50 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonSolid-State and Materials Chemistry: Unit Review
The unit began with periodic atomic arrangements and ended with functional materials. Along the way, ideal lattice models explained bands, while departures from the…
Explore lesson ChemistryLessonSolid-State NMR and Magic-Angle Spinning
In solution, rapid molecular tumbling averages many orientation-dependent magnetic interactions, so sharp NMR lines can appear. A solid usually lacks that motion, and its powder…
Explore lesson ChemistryLessonSolid–Liquid Diagrams: Simple Eutectics
When two substances mix as liquids but have limited solid solubility, their melting diagram may contain a eutectic. At the eutectic composition, liquid freezes at one lowest…
Explore lesson ChemistryLessonSolid–Solid Battery Interfaces
A liquid electrolyte wets a porous electrode and can reach many surface sites. Two solids must be brought into direct contact, and their contact can change as the electrode…
Explore lesson ChemistryLessonSolubilisation, Detergency and Microemulsions
Oil and water normally separate, yet a surfactant solution can carry some oil-like molecules through water. Above its micellisation range, a surfactant offers nonpolar interiors…
Explore lesson ChemistryLessonSolubility
Stir sugar into tea and it disappears; stir sand into water and it settles to the bottom. Whether a substance dissolves — and how much of it can dissolve — is its solubility.…
Explore lesson ChemistryLessonSolubility and Boiling Trends of Amines
Amine physical properties reflect competing parts of the molecule. Nitrogen can interact with water, while a growing hydrocarbon region resists mixing with it. N–H bonds…
Explore lesson ChemistryLessonSolubility and Precipitation Formulae
The solubility product is an equilibrium expression for ions released by a slightly soluble solid. It does not say that every salt with the same Ksp has the same molar…
Explore lesson ChemistryLessonSolubility and Temperature
You can stir far more sugar into hot tea than into iced tea before crystals start collecting at the bottom. The amount of a substance that can dissolve is not fixed: it depends…
Explore lesson ChemistryLessonSolubility at a Stated Temperature
A solubility number is meaningful only with its conditions and basis. “36 g per 100 g water at 25 °C” is a different statement from “36 g per 100 g solution,” and a change in…
Explore lesson ChemistryLessonSolubility Data
A solubility value is not an intrinsic number attached to a chemical name alone. It depends on solvent, temperature, pressure for gases, and often pH, ionic strength or complex…
Explore lesson ChemistryLessonSolubility Equilibrium and Ksp
A sparingly soluble salt still dissolves to a measurable extent. When undissolved solid coexists with dissolved ions at equilibrium, the ion activities satisfy a solubility…
Explore lesson ChemistryLessonSolubility of Organic Molecules
Many small oxygen- or nitrogen-containing organic molecules dissolve in water, while many hydrocarbons do not. The useful idea is competition between water-compatible polar…
Explore lesson ChemistryLessonSolubility Product and Ion Product
When two clear ionic solutions are mixed, a precipitate may form even though neither starting bottle contained a solid. The prediction rests on the ion product immediately after…
Explore lesson ChemistryLessonSolubility Product and Precipitation
A solubility rule suggests which solid might form, but Ksp gives a quantitative equilibrium threshold. Precipitation depends on the product of the free ion activities at the…
Explore lesson ChemistryLessonSolubility Product as an Equilibrium Constant
Silver chloride is called "insoluble", yet a tiny amount does dissolve. Add solid silver chloride to water and, after a while, the solution contains about 1.3 × 10⁻⁵ mol dm⁻³ of…
Explore lesson ChemistryLessonSolubility Product Tables
Ksp tables describe equilibria between a specified solid and dissolved ions. A low Ksp often suggests low solubility, but Ksp alone is not a direct concentration because…
Explore lesson ChemistryLessonSolubility Rules for Ionic Compounds
Solubility rules help decide whether exchanging ions will create a solid. They are patterns with exceptions, not an instruction to call every salt simply “soluble” or “insoluble”…
Explore lesson ChemistryLessonSolubility Rules for Salts
Stir sodium chloride into water and it vanishes; stir in calcium carbonate and the water just turns cloudy before the powder settles out. Both are salts, yet they behave very…
Explore lesson ChemistryLessonSolubility Rules Revisited
Solubility rules summarize common patterns: many sodium, potassium and ammonium salts are soluble; many nitrates are soluble; some silver halides and many metal carbonates are…
Explore lesson ChemistryLessonSolubility Trends in Group 2 Salts
Two sets of group 2 compounds show opposite broad solubility trends. Hydroxides tend to become more soluble from magnesium toward barium, while sulfates tend to become less…
Explore lesson ChemistryLessonSolubility Versus Concentration
A concentration states what is in the solution now. Solubility states an equilibrium capacity under specified conditions. Comparing the two can reveal whether a sample is…
Explore lesson ChemistryLessonSoluble and Insoluble Salts
Knowing a salt's formula is only half of a preparation problem. We also need to know whether the product stays dissolved in water or forms a solid. Solubility patterns give a…
Explore lesson ChemistryLessonSoluble and Insoluble Substances
Salt vanishes when stirred into water, but sand just swirls around and settles again. Nail varnish will not wash off with water, yet it wipes away easily with nail-varnish…
Explore lesson ChemistryLessonSolute and Solvent Roles
School examples often place a solid into a liquid and call the solid the solute. That pattern is useful, but it is not the definition. Gases can dissolve in liquids, liquids can…
Explore lesson ChemistryLessonSolution and Colligative Formulae
Adding a nonvolatile solute to a solvent can lower vapour pressure, raise boiling temperature, lower freezing temperature and create osmotic pressure. Simple formulae relate…
Explore lesson ChemistryLessonSolution Kinetics and Ionic Strength
Solution reactions occur in a medium that can stabilize ions, reorganize around reactants and influence how molecules meet. Two experiments with identical nominal reactant…
Explore lesson ChemistryLessonSolution Volume to Product Amount
The concentration–volume relationship tells how much reactant a measured solution contains. A balanced equation then predicts product amount, which may be reported as moles…
Explore lesson ChemistryCollectionSolutions and Colligative Properties
Study Solutions and Colligative Properties with 30 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonSolutions and Colligative Properties: Integrated Review
Solutions are best understood as particles interacting in a mixture whose composition can be reported in several ways. The same solute sample has a mass percentage, mole…
Explore lesson ChemistryCollectionSolutions and Concentration
Study Solutions and Concentration with 50 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonSolutions and Concentration: Unit Review
Solution chemistry connects three views of one sample: what particles are present, how much can remain dissolved, and how much is present per chosen amount of mixture. A strong…
Explore lesson ChemistryLessonSolutions and Equilibria Map
Solutions bring species into contact, but their behavior depends on more than the written amounts. Solvation influences chemical form and activity; concentration measurements…
Explore lesson ChemistryLessonSolutions as Homogeneous Mixtures
When a small amount of sugar dissolves completely in water, every well-mixed sample has the same sweet composition. The resulting liquid is a solution. It contains more than one…
Explore lesson ChemistryLessonSolutions at the Molecular Level
A solution is uniform on the scale of an ordinary sample, but its particles remain solute and solvent species. Dissolving NaCl in water separates ions from a crystal and…
Explore lesson ChemistryLessonSolutions: Solute, Solvent and Solution
Stir a spoonful of sugar into a cup of tea and it seems to vanish. The tea still tastes sweet, so the sugar has not disappeared — it has dissolved. Chemists use three precise…
Explore lesson ChemistryLessonSolvation Free Energies and Thermodynamic Cycles
A gas-phase quantum calculation is often easier to define than a solution reaction. Thermodynamic cycles connect the two by adding the free-energy cost or benefit of transferring…
Explore lesson ChemistryLessonSolvent Effects in Mechanisms
The same alkyl halide and nucleophile can react at very different rates in different liquids. The solvent is not an inert stage: its molecules surround ions and transition…
Explore lesson ChemistryLessonSolvent Effects on Substitution
The solvent is not just a container for reactants. It stabilizes charged species and can cover a nucleophile with a shell of interacting molecules. These effects alter the energy…
Explore lesson ChemistryLessonSolvent Friction and Barrier Crossing
A gas-phase reaction profile displays the energy of a molecule or small collision complex. In a liquid, countless solvent molecules surround the reactants and rearrange during…
Explore lesson ChemistryLessonSolvent Selection Guides
Choosing a solvent by habit can lock in avoidable health or environmental burdens. Solvent selection guides collect property and hazard information so chemists can compare…
Explore lesson ChemistryLessonSolvent-Free and Mechanochemical Reactions
Removing bulk solvent can cut a major source of process mass. Some reagents react directly when mixed or heated neat; others benefit from grinding, shear or ball milling.…
Explore lesson ChemistryLessonSolving a Simple Kc Problem
Writing an ICE table is the setup; solving its equilibrium equation yields actual composition. A simple one-to-one reaction can often be solved by rearranging a linear ratio…
Explore lesson ChemistryLessonSolving Mechanism Problems Systematically
Mechanism questions become manageable when the substrate and reagents are read as evidence rather than as a prompt to recall a name. First classify the transformation, then…
Explore lesson ChemistryLessonSolving Mixed Particle-Count Problems
A mixed atomic-structure question may give a nuclide symbol, an ion charge and a configuration, then ask for missing quantities. The safest method is to keep three counts…
Explore lesson ChemistryLessonSolving Multi-Step Stereochemistry Problems
Long stereochemistry questions become manageable when each spatial feature is analysed separately. A structure can contain several stereocentres, an alkene and a ring whose chair…
Explore lesson ChemistryLessonSolving Periodic Table Problems
Periodic-table problems often combine several clues: a shell arrangement, a period, a group, an ion charge or a broad property. The fastest reliable approach is to translate each…
Explore lesson ChemistryCollectionSome Basic Concepts of Chemistry
Study Some Basic Concepts of Chemistry with 30 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonSome Basic Concepts of Chemistry: Unit Review
Chemical calculation begins with identifying what is measured, what particles are represented, and what must be found. The unit's topics form one connected story: fixed chemical…
Explore lesson ChemistryLessonSources and Preparation of Alkanes
Most everyday alkane supplies come from natural gas and crude oil mixtures rather than from synthesizing each alkane separately. Distillation separates existing molecules by…
Explore lesson ChemistryLessonsp Hybridisation and Linear Centers
An sp label describes an atom with two principal local sigma directions in an elementary hybrid model. The directions are approximately opposite, and two p orbitals remain…
Explore lesson ChemistryLessonsp² Hybridisation and Trigonal Centers
An sp² centre has three principal local sigma directions arranged roughly in one plane and one p orbital perpendicular to it in the elementary hybrid model. Ethene's carbons are…
Explore lesson ChemistryLessonsp³ Hybridisation and Tetrahedral Centers
Methane provides the standard sp³ example: carbon has four C–H sigma bonds pointing toward tetrahedral directions. The label describes a useful local orbital arrangement. Other…
Explore lesson ChemistryLessonSpaces Between Particles
If you mix 50 mL of water with 50 mL of ethanol, you might expect exactly 100 mL of mixture. Measure it carefully and you find only about 96–97 mL. Where did the missing volume…
Explore lesson ChemistryLessonSpecific Conductivity
The same solution can show different measured resistance in different cells. Long electrode spacing makes charge transport harder, while larger electrode area offers a wider…
Explore lesson ChemistryLessonSpecific Heat and Temperature-Change Problems
Heating or cooling a substance changes its thermal energy without necessarily causing a chemical reaction or phase change. Over a range where specific heat capacity is…
Explore lesson ChemistryLessonSpecific, Molar and Total Heat Capacity
Heat-capacity data are reported in several forms. A calorimeter may have a total C in J K⁻¹, water a specific c in J g⁻¹ K⁻¹, and a gas a molar C_m in J mol⁻¹ K⁻¹. The formulas…
Explore lesson ChemistryLessonSpecifying the Particles: Atoms, Molecules, Ions and Formula Units
“One mole of oxygen” sounds precise, but it may mean oxygen atoms or oxygen molecules. The distinction changes the number of atoms and the mass. The mole is a counting unit, so…
Explore lesson ChemistryLessonSpectator Ions and Net Ionic Equations
Spectator ions remain in solution while other ions form a precipitate or water. A net ionic equation removes those unchanged species from the written account, leaving the…
Explore lesson ChemistryLessonSpectator Ions in Redox Equations
Aqueous metal-displacement equations often include ions that keep dissolved salts electrically balanced but do not change during the redox step. Such spectator ions appear in a…
Explore lesson ChemistryLessonSpectra of Tetrahedral Complexes
Tetrahedral cobalt(II) species often appear deep blue, whereas aqueous octahedral Co²⁺ is commonly pale pink. The difference is not a change in cobalt's d count: Co²⁺ remains d⁷.…
Explore lesson ChemistryLessonSpectrochemical Series
Ligands do not all split a metal's d orbitals by the same amount. The spectrochemical series arranges them by typical field strength, from smaller to larger splitting under…
Explore lesson ChemistryLessonSpectroscopic Reference Data
Spectral tables are guides to structure, not dictionaries in which one peak has one unique meaning. IR, NMR and UV–visible measurements probe different molecular properties and…
Explore lesson ChemistryLessonSpectroscopy Formulae
Spectroscopy formulae transform measured light into quantities that can be compared with concentration or molecular energy levels. The transformation is not automatic…
Explore lesson ChemistryCollectionSpectroscopy I
Study Spectroscopy I with 70 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonSpectroscopy I: Unit Review
This unit has introduced four techniques that together allow chemists to identify almost any small organic molecule. Each technique uses a different interaction between matter…
Explore lesson ChemistryLessonSpectroscopy in Forensics, Medicine and Industry
The techniques in this unit are not only tools for exam questions. Mass spectrometry confirms drug use in athletes and road users, the physics of NMR produces detailed images of…
Explore lesson ChemistryLessonSpectroscopy of Metalloproteins
A metal ion in a protein is usually a tiny part of a very large molecule — one iron atom in 20 000 other atoms, perhaps. Yet the metal is where the chemistry happens.…
Explore lesson ChemistryLessonSpectroscopy Terms
Spectroscopy turns interactions between matter and electromagnetic radiation into chemical evidence. Its terms appear across UV–visible, infrared, Raman, atomic and NMR methods…
Explore lesson ChemistryLessonSpin and Magnetic-Moment Estimates
Counting unpaired electrons predicts more than a yes/no magnetic label. A spin-only formula estimates the effective magnetic moment for a suitable isolated or weakly interacting…
Explore lesson ChemistryLessonSpin Crossover Complexes
Some d⁴–d⁷ octahedral complexes sit close to the balance between orbital splitting and pairing cost. They are neither permanently high spin nor permanently low spin under all…
Explore lesson ChemistryLessonSpin Crossover in Depth
Some coordination compounds lie near the boundary between high-spin and low-spin electronic states. Temperature, pressure or light can change which state is populated, producing…
Explore lesson ChemistryLessonSpin Crossover Points on Tanabe–Sugano Diagrams
Some octahedral d counts can rearrange their electrons when ligand-field splitting becomes large. For d⁴–d⁷, a high-spin state can be lowest in a weak field while a lower-spin…
Explore lesson ChemistryLessonSpin States and Open-Shell Systems
Radicals, transition-metal complexes and stretched bonds often have unpaired electrons. Their energies and reactions can depend on whether spins are paired, parallel or coupled…
Explore lesson ChemistryLessonSpin-Forbidden Bands and the Spectrum of Manganese(II)
Many Mn²⁺ aqua solutions are very pale even though manganese is a transition metal with five d electrons. The explanation is not that no excited states exist. Rather, the…
Explore lesson ChemistryLessonSpin–Orbit Coupling and Deviations from Spin-Only Moments
An orbitally nondegenerate ground state has no large first-order orbital moment, yet its measured magnetic moment can still differ slightly from the spin-only prediction.…
Explore lesson ChemistryLessonSpin–Spin Coupling and the n+1 Rule
Look closely at a high-resolution ¹H NMR spectrum and many signals are not single peaks at all: they are split into clusters of two, three, four or more lines. This splitting…
Explore lesson ChemistryLessonSpinel and Olivine Cathodes
Layered oxides are only one way to host lithium. Spinel LiMn₂O₄ offers a three-dimensional network of lithium migration pathways. Olivine LiFePO₄ has a more anisotropic…
Explore lesson ChemistryLessonSplitting of Free-Ion Terms in Octahedral Fields
An isolated metal ion is approximately spherical, so L labels such as D and F are appropriate. Six ligands on octahedral axes reduce that symmetry to O_h. The free-ion terms do…
Explore lesson ChemistryLessonSquare-Planar Complexes and d⁸ Ions
Four-coordinate complexes need not be tetrahedral. Many d⁸ ions, especially Pd²⁺ and Pt²⁺, form square-planar complexes, and Ni²⁺ can do so with a suitable ligand environment.…
Explore lesson ChemistryLessonSquare-Planar Crystal Fields
Four-coordinate does not uniquely mean tetrahedral. A square-planar complex also has four donor atoms, but they lie approximately on x and y axes in one plane. This arrangement…
Explore lesson ChemistryLessonSquare-Planar Molecular Shape
XeF₄ has four bonded fluorine atoms but six electron domains in the familiar Lewis/VSEPR model because xenon also has two lone pairs. An octahedral electron-domain scaffold with…
Explore lesson ChemistryLessonStability of Oxidation States in Solution
An element in an intermediate oxidation state may be able to oxidize some of its own atoms while reducing others. That is disproportionation. The reverse combination of high and…
Explore lesson ChemistryLessonStability Tests and Lifetime Claims
“Stable for 100 hours” is incomplete unless the measured product rate, selectivity and operating conditions are known. A catalyst can look stable because conversion is capped…
Explore lesson ChemistryLessonStable and Unstable Nuclides
Carbon-12 and carbon-14 are both carbon, yet one is stable and the other is radioactive. Their six protons give them the same element name, while their different neutron counts…
Explore lesson ChemistryLessonStable Outer Shells and the Noble Gases
Most elements are restless: sodium reacts with water, chlorine attacks metals, oxygen supports burning. But one column of the periodic table is strikingly calm. The noble gases…
Explore lesson ChemistryLessonStages in Extracting a Metal
Extracting metal from ore is usually a sequence rather than one equation. The ore may first be crushed and concentrated, then its valuable mineral chemically converted, reduced…
Explore lesson ChemistryLessonStages of a Life-Cycle Assessment
Life-cycle thinking becomes a formal study when its question, data and interpretation are organised. A life-cycle assessment (LCA) has four connected phases: goal and scope…
Explore lesson ChemistryLessonStandard Biochemical Free Energy
A tabulated biochemical free-energy change is not the free-energy change that every cell experiences. Tables commonly use a transformed standard convention with proton activity…
Explore lesson ChemistryLessonStandard Electrode Potentials
A table of standard electrode potentials ranks reduction half-reactions relative to a chosen reference. It does not assign a directly measurable absolute voltage to an isolated…
Explore lesson ChemistryLessonStandard Enthalpy of Formation
Standard enthalpies of formation provide a common set of building blocks for reaction enthalpy calculations. A formation equation makes exactly one mole of a specified substance…
Explore lesson ChemistryLessonStandard Entropy Data
Entropy tables provide another state-function tool for predicting reaction behavior. Unlike standard formation enthalpies, standard molar entropies of elemental substances at…
Explore lesson ChemistryLessonStandard Form for Very Large and Very Small Numbers
Mole calculations are full of extreme numbers: 6.022 × 10²³ particles in a mole, 1.99 × 10⁻²³ g for a carbon atom. Writing these out with all their zeros is slow and invites…
Explore lesson ChemistryLessonStandard Hydrogen Electrode
The standard hydrogen electrode provides the conventional zero of the reduction-potential scale. It consists of hydrogen gas, hydrogen-ion activity at the standard value, and an…
Explore lesson ChemistryLessonStandard Molar Entropy
Standard molar entropy tables provide values in J mol⁻¹ K⁻¹ for substances at stated temperatures and reference states. Unlike standard enthalpies of formation, elemental values…
Explore lesson ChemistryLessonStandard Potentials and Redox Behaviour
Transition metals have several oxidation states, so tables of standard potentials help compare specific redox couples. A potential belongs to a balanced half-reaction under…
Explore lesson ChemistryLessonStandard Reduction Potentials
An electrode-potential table is not a list of voltages for isolated half-cells floating without reference. Each entry is a written reduction reaction compared with a reference…
Explore lesson ChemistryLessonStandard States and Standard Enthalpy
Thermochemical values need common reference conditions so reactions can be compared and combined. A standard-state symbol indicates a convention for the physical and pressure…
Explore lesson ChemistryLessonState Functions and Path Functions
Two routes can bring a system from the same initial state to the same final state while transferring different amounts of heat and work. Internal energy and enthalpy changes are…
Explore lesson ChemistryLessonState Symbols in Equations
An equation's formulas identify substances, while state symbols describe their physical form under the stated conditions. The labels (s), (l), (g) and (aq) help explain why a gas…
Explore lesson ChemistryLessonStates of Matter in the Water Cycle
The water in your glass today may once have fallen as snow on a mountain, flowed down a river, floated in a cloud or been part of an ocean wave. Water on Earth is constantly…
Explore lesson ChemistryCollectionStates of Matter: Gases and Liquids
Study States of Matter: Gases and Liquids with 30 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonStates of Matter: Integrated Review
Gas and liquid problems often begin with an everyday observation and end with a calculation. A vessel warms, a gas is collected over water, or a liquid boils at a different…
Explore lesson ChemistryCollectionStates of Matter: Particle Model
Study States of Matter: Particle Model with 50 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonStates of Matter: Unit Review
This unit has built one powerful idea: all matter is made of tiny particles that are always moving and that attract one another. From that single picture you can explain the…
Explore lesson ChemistryLessonStationary Points on a Surface
A potential energy surface has special geometries where its local slope vanishes. These stationary points organise much of a proposed reaction mechanism. A local minimum can…
Explore lesson ChemistryLessonStatistical Basis of Reaction Equilibrium
An equilibrium constant is often calculated from tabulated Gibbs energies. Statistical thermodynamics asks where those Gibbs energies come from. Molecular ground energies favour…
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