THE LESSON LIBRARY / CHEMISTRY
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Oxidation-Number Increase and Decrease
After assigning oxidation numbers, the decisive comparison is directional. Moving to a more positive signed number is oxidation, whether the move is −3 to zero or +2 to +3.…
Explore lesson ChemistryLessonOxidation-State Limits and Covalent Bonds
Oxidation numbers are powerful because they turn a complicated bonding change into a clear numerical comparison. Their power also creates a trap: a formal +4 oxidation state does…
Explore lesson ChemistryLessonOxidation-State Patterns Across the First Row
The first d-block row does not have one preferred oxidation number for every element. Early members can reach high states using several valence electrons; the middle shows broad…
Explore lesson ChemistryLessonOxidation: Gaining Oxygen
Oxygen makes up about one-fifth of the air, and it is a very reactive element. Many substances slowly or quickly join with it. A freshly cut apple goes brown, a shiny bicycle…
Explore lesson ChemistryLessonOxidative Addition and Reductive Elimination Design
Many transition-metal catalytic cycles activate a bond through oxidative addition and release a coupled product through reductive elimination. A catalyst designed only to break…
Explore lesson ChemistryLessonOxidative Addition and Reductive Elimination in a Cycle
Many of the most important catalytic reactions, including every palladium cross-coupling, rely on a metal oscillating between two oxidation states. Oxidative addition breaks a…
Explore lesson ChemistryLessonOxidative Addition Mechanisms
The formal equation LₙM + A–B → LₙM(A)(B) hides several routes. A metal can engage a nonpolar bond through a concerted transition state, a polar alkyl halide may react through…
Explore lesson ChemistryLessonOxidative Addition: Electron and Oxidation Accounting
Oxidative addition is the step that lets a metal break strong bonds such as H–H, C–halogen and even C–H. It begins most cross-coupling cycles, activates hydrogen in hydrogenation…
Explore lesson ChemistryLessonOxidative Phosphorylation Yield and Uncoupling
Older textbooks often assigned exactly 36 or 38 ATP to each glucose molecule, but the modern estimate is lower and depends on coupling assumptions. Glycolysis and the citric acid…
Explore lesson ChemistryLessonOxide Behaviour Across Period Three
Across period three, oxides show a broad shift from basic behaviour on the metallic left toward acidic behaviour on the non-metallic right. Aluminium oxide sits near the middle…
Explore lesson ChemistryLessonOxides Across a Period
Across a period, elements change from metallic to nonmetallic character, and their oxides often change from basic through amphoteric to acidic behavior. Period 3 offers a useful…
Explore lesson ChemistryLessonOxides and Water: Forming Acids and Bases
Some oxides combine with water to form hydroxides or acids. The familiar examples CaO + H₂O → Ca(OH)₂ and SO₃ + H₂O → H₂SO₄ each have one product, so both are combination…
Explore lesson ChemistryLessonOxides of Nitrogen
Nitrogen forms several oxides rather than one fixed “nitrogen oxide.” N₂O, NO, NO₂, N₂O₃ and N₂O₅ illustrate a broad oxidation-state range. Their structures and reactions differ…
Explore lesson ChemistryLessonOxidising Agents
An oxidising agent makes another species undergo oxidation. It does so by accepting electrons or, in a broader formal description, by undergoing reduction itself. The name often…
Explore lesson ChemistryLessonOxidising and Reducing Agents
The names of redox agents sound backwards at first. An oxidising agent causes another species to be oxidised, so the agent itself is reduced. A reducing agent causes another…
Explore lesson ChemistryLessonOxidising and Reducing Agents in Metal Reactions
Knowing that zinc is oxidised and copper(II) ions are reduced is one step. Chemistry also names the participants by what they cause: zinc acts as a reducing agent, and Cu²⁺ acts…
Explore lesson ChemistryLessonOxoacids and Pauling's Rules
HClO, HClO₂, HClO₃ and HClO₄ all contain chlorine, oxygen and an acidic hydrogen, yet their aqueous acid strengths span an enormous range. Counting every oxygen is a useful first…
Explore lesson ChemistryLessonOxoacids and Their Strength
The strengths of related oxoacids can be predicted from structure rather than memorized as a disconnected list. For acids with the same central element, more terminal oxygen…
Explore lesson ChemistryLessonOxoacids of Chlorine
Chlorine forms a sequence of oxoacids that differ by oxygen count: HClO, HClO₂, HClO₃ and HClO₄. Their names—hypochlorous, chlorous, chloric and perchloric—encode that sequence.…
Explore lesson ChemistryLessonOxoacids of Phosphorus
Phosphorus oxoacids show why a molecular formula alone can mislead. H₃PO₃ and H₃PO₄ each contain three hydrogen atoms, but they do not have the same number of readily ionizable…
Explore lesson ChemistryLessonOxoacids of Sulfur
Sulfur forms several oxygen-containing acids and related anions. H₂SO₃ and H₂SO₄ are familiar formulas tied to sulfur at +4 and +6 respectively, but aqueous speciation and acid…
Explore lesson ChemistryLessonOxoanion Structures
Nitrate, sulfate, phosphate and perchlorate are common polyatomic anions, yet their formulas alone do not show three-dimensional shape. Electron-domain reasoning puts nitrate in…
Explore lesson ChemistryLessonOxygen and Nitrogen Multiple Bonds
Elemental oxygen and nitrogen usually occur as O₂ and N₂ molecules in ordinary air. Their familiar Lewis diagrams make a useful comparison: O₂ is drawn with a double bond and N₂…
Explore lesson ChemistryLessonOxygen and Sulfur Contrasted
Oxygen and sulfur have the same six-valence-electron group pattern but favor different ordinary structures. Dioxygen has an O=O multiple-bond description, whereas elemental…
Explore lesson ChemistryLessonOxygen Electrocatalysis Descriptors
Oxygen electrocatalysis involves several adsorbed oxygen-containing species and more than one possible product. A catalyst can reduce O₂ through a four-electron route to water…
Explore lesson ChemistryLessonOxygen Reduction and Evolution Kinetics
Oxygen reduction at a fuel-cell cathode and oxygen evolution at a water-electrolyser anode are opposite overall reactions under matched conditions. They are not equally easy to…
Explore lesson ChemistryLessonOxygen, Ozone and the Oxides
O₂ and O₃ contain only oxygen, yet they differ in bonding, reactivity and environmental role. Oxygen also appears in ordinary oxides, peroxides and superoxides with different…
Explore lesson ChemistryLessonOxygen: The Gas for Life and Burning
Oxygen makes up only about one-fifth of the air, yet almost every animal on Earth dies within minutes without it, and no ordinary fire can burn in its absence. Unlike nitrogen…
Explore lesson ChemistryLessonOxymercuration–Demercuration
Acid-catalysed hydration gives a Markovnikov alcohol but may rearrange through a free carbocation. Hydroboration–oxidation avoids that cation but gives the opposite OH…
Explore lesson ChemistryLessonOzone as an Oxidizing Agent
Ozone can oxidize many substances, and that reactivity supports uses in water treatment and chemical synthesis. Its spontaneous or catalyzed conversion toward O₂ also matters. To…
Explore lesson ChemistryLessonOzone: Helpful High Up, Harmful Down Low
Ozone is a strange pollutant: the same gas is our protector in one place and our enemy in another. About 20 to 30 km above our heads, a thin layer of ozone blocks most of the…
Explore lesson ChemistryLabOzonolysis
Ozonolysis cuts a C=C double bond; the products tell you where the double bond was.
Explore lesson ChemistryLessonOzonolysis of Alkenes
Ozone can cut an alkene at its carbon–carbon double bond. After an appropriate work-up, each former alkene carbon becomes a carbonyl carbon, yielding aldehydes and/or ketones…
Explore lesson ChemistryLessonp-Block Configurations and Valence
The p-block's group numbers encode a simple outer-electron pattern. For most neutral atoms in groups 13–18, subtract ten from the group number to obtain the count of outer s and…
Explore lesson ChemistryCollectionp-Block Elements
Study p-Block Elements with 50 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonp-Block Elements: Unit Review
The p-block spans groups 13–18, from boron to noble gases. Its breadth makes memorizing a single rule for every element unreliable. A stronger method begins with outer-electron…
Explore lesson ChemistryLessonPacking Efficiency
Packing efficiency compares how much of a unit-cell volume ideal equal spheres occupy. The standard simple cubic, BCC and FCC values are about 52.4%, 68.0% and 74.0%. These…
Explore lesson ChemistryLessonPacking Parameter and Micelle Shape
Why do some surfactants form small spherical micelles while others make rods or bilayers? A useful geometric guide compares how much volume a hydrophobic tail occupies with the…
Explore lesson ChemistryLessonPairing Energy and Its Origins
Electrons prefer low-energy orbitals, but putting two electrons into one orbital is not free. They repel one another, and an arrangement with parallel spins in different orbitals…
Explore lesson ChemistryLessonPaper and Thin-Layer Chromatography
Paper chromatography and thin-layer chromatography, or TLC, make separation visible as spots on a flat medium. They are useful for comparing mixtures, monitoring reactions and…
Explore lesson ChemistryLessonPaper Chromatography
Paper chromatography is the simplest form of chromatography, and it can be carried out with little more than a strip of paper, a beaker and a solvent. Yet every detail of the…
Explore lesson ChemistryLessonParallel and Consecutive Reactions
Many reactions do not lead to one product by one path. A reactant can choose parallel pathways, or a product of an early step can react again in a consecutive sequence. These…
Explore lesson ChemistryLessonParallel and Sequential Pathways
Two products can arise by parallel routes from one reactant or by a sequence in which the first product reacts again. Both arrangements can yield the same final products but…
Explore lesson ChemistryLessonParallel Reaction Networks
A reactant often has more than one way to react. An alkyl halide may undergo substitution or elimination; a radical may add to two different sites; a fuel may burn fully or…
Explore lesson ChemistryLessonParamagnetism and Diamagnetism in Complexes
A metal complex’s d-electron diagram makes a testable prediction. If one or more electrons remain unpaired, their spin magnetic moments usually produce paramagnetism. If all…
Explore lesson ChemistryLessonParamagnetism and Unpaired Electrons
Electron configurations can predict more than chemical group patterns. Unpaired electrons carry magnetic moments that contribute to paramagnetic behavior. Counting them requires…
Explore lesson ChemistryLessonParameter Estimation
A mechanism predicts concentration-time curves once its rate constants and starting conditions are specified. Parameter estimation works backward: it chooses values that make…
Explore lesson ChemistryLessonParameterizing and Validating Force Fields
A force field is an equation plus numbers. Bond equilibrium distances, torsion amplitudes, partial charges and Lennard-Jones radii may be fitted to different data. A successful…
Explore lesson ChemistryLessonParent–Daughter Kinetics and the Bateman Equations
Many radionuclides decay into daughters that are themselves radioactive. Uranium-238 starts a chain of fourteen decays ending at lead-206; molybdenum-99 decays to technetium-99m…
Explore lesson ChemistryLessonPartial Molar Quantities
Adding one mole of liquid to a mixture need not increase its volume by the pure liquid's molar volume. Molecules reorganise, and the change depends on what is already present.…
Explore lesson ChemistryLessonPartially Miscible Liquids
Some liquids mix completely at one temperature but separate into two liquid layers at another. The layers are not pure starting substances; each may contain both components in…
Explore lesson ChemistryLessonParticle Counts in Charged Ions
A sodium atom becomes Na⁺ by losing an electron, while chlorine becomes Cl⁻ by gaining one. Neither process changes the element's proton number. Ion questions often combine…
Explore lesson ChemistryLessonParticle Energy and Temperature
When you warm your hands on a mug of hot chocolate, what is actually happening inside the drink? The particle model gives a clear answer: the particles in hot things move faster…
Explore lesson ChemistryLabParticle in a box
An electron in a box: only certain standing waves, and so certain energies, are allowed.
Explore lesson ChemistryLessonParticle in a Box: Problem-Solving Workshop
The particle in a box is the first quantum model you can solve completely, and it rewards practice. Almost every problem reduces to three tools: the energy formula, the…
Explore lesson ChemistryLessonParticle in a Three-Dimensional Box
Molecules in a gas move in three dimensions, bouncing around a container. Treating each molecule as a particle in a three-dimensional box gives the quantised translational energy…
Explore lesson ChemistryLessonParticle in a Two-Dimensional Box
Real molecules are not one-dimensional. Electrons in aromatic rings, in graphene sheets and on metal surfaces move in two dimensions. The next step beyond the one-dimensional box…
Explore lesson ChemistryLessonParticle Mass and the Rate of Diffusion
Helium balloons go flat within a day, while air-filled balloons stay firm for weeks. A leak of hydrogen spreads through a room faster than a leak of a heavy gas such as carbon…
Explore lesson ChemistryLabParticle model of states
100 g of ice is heated from −20 °C. You watch the particles and the heating curve at the same time.
Explore lesson ChemistryLessonParticle on a Ring
So far our quantum particles have moved along straight lines between hard walls. Many real systems are round instead: the π electrons of benzene circulate around a ring, a…
Explore lesson ChemistryLessonParticle on a Sphere and Spherical Harmonics
A particle on a ring rotates in one plane. Let the particle roam over the whole surface of a sphere and rotation becomes three-dimensional. This is the model for a rotating…
Explore lesson ChemistryLessonParticles and Moles in a Reaction
Reaction questions sometimes start or end with a number of molecules rather than a mass. A balanced equation can connect those counts directly, but moles make large numbers…
Explore lesson ChemistryLessonParticles and Temperature
We use thermometers every day, but what is a thermometer really measuring? The particle theory gives a clear answer: temperature tells us how fast, on average, the particles of a…
Explore lesson ChemistryLessonParticles Are Always Moving
Even a rock sitting perfectly still on a table is full of motion: its particles are vibrating billions of times per second. Particle motion never stops, although its type and…
Explore lesson ChemistryLessonParticles in a Gas
The air around you is full of particles — nitrogen, oxygen, argon, carbon dioxide and water vapour — yet you can walk straight through it, and it weighs very little. Gases behave…
Explore lesson ChemistryLessonParticles in a Liquid
Water can be poured from a jug, sloshed around a bath and dripped from a tap, yet a litre of it stays a litre wherever it goes. Liquids sit between solids and gases: they flow…
Explore lesson ChemistryLessonParticles in a Solid
A steel spoon, a grain of salt and an ice cube all have something in common: they keep their shape and cannot be squashed. The reason lies inside them, at the level of particles.…
Explore lesson ChemistryLessonParticles in Physical and Chemical Changes
From the outside, physical and chemical changes can look alike: both may involve heating, colour changes or gases. The real difference is hidden at the level of particles. In a…
Explore lesson ChemistryLessonPartition Functions: Worked Problems
Partition function formulae are compact, but using them well requires care with units, degeneracies and zero-point conventions. This page works through three linked problems that…
Explore lesson ChemistryLessonPartition, Adsorption and Distribution Coefficients
Chromatographic retention is often summarised as a component's preference for stationary versus mobile phase. That preference can arise from dissolution into an immobilised…
Explore lesson ChemistryLessonPartitioning of Pollutants Between Phases
After a pollutant enters a river, some may remain dissolved, some may move into air and some may attach to sediment or organic matter. A molecule's destination is influenced by…
Explore lesson ChemistryLessonParts per Billion by Mass
Parts per billion, or ppb, reports very small ratios compactly. In the mass convention, one ppb is one microgram of a named substance per kilogram of total sample. The word…
Explore lesson ChemistryLessonParts per Million by Mass
Trace amounts are awkward as ordinary percentages. Parts per million, or ppm, often gives a compact number. For a mass-based ppm statement, one ppm means one mass unit of the…
Explore lesson ChemistryLessonPauli Exclusion in Orbital Boxes
An orbital box diagram turns a subshell configuration into individual orbital occupancies. A box represents one orbital; arrows represent electrons. Pauli's exclusion principle…
Explore lesson ChemistryLessonPauli Exclusion Principle
The Pauli exclusion principle governs how identical electrons occupy quantum states. In chemistry's four-number description, no two electrons in one atom can have the same…
Explore lesson ChemistryLessonPeer Review and Reproducibility
Peer review can improve a chemistry paper by exposing missing controls, unclear methods or claims that exceed data. Reviewers usually do not rerun the reaction, inspect every raw…
Explore lesson ChemistryLessonPeierls Distortion and Low-Dimensional Solids
A uniform one-dimensional chain with one electron per site seems metallic in elementary tight binding: one spin-degenerate band is half filled. Yet a chain can lower its total…
Explore lesson ChemistryLessonPeptide Bonds and Sequence
Proteins become sequence-defined polymers because amino acids can join through a recurring amide linkage. The order of residues matters, and the chain has direction: its N…
Explore lesson ChemistryLessonPercent Ionic Character as a Continuum
Classifying a bond as ionic or covalent is a useful first approximation, but the electron distribution need not obey a sharp border. H–Cl shares electrons unequally, while…
Explore lesson ChemistryLessonPercent Ionization of Weak Acids
A weak acid's Ka remains fixed at a given temperature, but the fraction of a prepared sample that ionizes can change with starting concentration. Percent ionization measures that…
Explore lesson ChemistryLessonPercentage by Mass of an Element — Atoms and Molecules: First Look
Water is made of hydrogen and oxygen, but how much of the mass of a glass of water is actually hydrogen? Only about one-ninth. The rest is oxygen. The percentage by mass of an…
Explore lesson ChemistryLessonPercentage by Mass of an Element — The Mole Concept: Introduction
Chemical formulas show atom ratios, but many questions ask for mass percentages. Oxygen is two of the three atoms in a CO₂ molecule, yet its mass percentage is not simply two…
Explore lesson ChemistryLessonPercentage by Mass of an Element in a Compound
How much iron can be extracted from a tonne of iron ore? Which fertiliser supplies the most nitrogen per bag? Both questions ask what fraction of a compound's mass comes from one…
Explore lesson ChemistryLessonPercentage Composition
A formula specifies atom counts, but it can also reveal what fraction of a compound's mass comes from each element. These percentages help connect measured elemental analysis to…
Explore lesson ChemistryLessonPercentage Composition from a Formula
A chemical formula fixes not only atom ratios but also the share of a pure compound's mass contributed by each element. Percentage composition uses that formula and atomic…
Explore lesson ChemistryLessonPercentage Error and Percentage Uncertainty
Saying a result is "a bit out" is not very scientific. How far out? Is that a lot or a little? Two percentages answer these questions. Percentage error compares your result with…
Explore lesson ChemistryLessonPercentage of Water in a Hydrated Salt
A hydrate's formula tells how much of its mass belongs to water of crystallisation. Heating can sometimes remove that water, allowing an experimental mass loss to be compared…
Explore lesson ChemistryLessonPercentage Yield Defined
Theoretical and actual yields can be compared with a percentage. It answers how much of the calculated target-product maximum was obtained. The formula is short, but it is…
Explore lesson ChemistryLabPericyclic reactions
Ring closures that happen in one concerted step. Orbital symmetry decides the direction of rotation.
Explore lesson ChemistryLessonPericyclic Reactions in Nature and Synthesis
Pericyclic reactions can build several bonds and stereochemical relationships in one operation, making them attractive both to synthetic chemists and to living systems. A ring…
Explore lesson ChemistryLessonPericyclic Reactions: Problem Solving
Pericyclic problems can look like memory tests because many named reactions appear in the unit. A more reliable method starts with the bonds and atoms, then selects the…
Explore lesson ChemistryLessonPeriod Three as a Trend Case Study
Period three is a compact laboratory for periodic reasoning. Sodium through argon fill outer 3s and 3p states while sharing a [Ne] core. Along the row, atomic size broadly…
Explore lesson ChemistryLessonPeriodic Boundaries and Long-Range Forces
A finite molecular-dynamics box has walls unless boundary conditions say otherwise. Periodically repeating the box allows a small cell to mimic a patch of bulk liquid or solid…
Explore lesson ChemistryLessonPeriodic Classification and Materials
Periodic position can suggest a metal for wiring, silicon for an electronic device or argon for a low-reactivity atmosphere. It cannot by itself tell which wire survives a…
Explore lesson ChemistryCollectionPeriodic Classification and Trends
Study Periodic Classification and Trends with 60 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonPeriodic Classification and Trends: Unit Review
Periodic classification orders elements by proton number and reveals recurring outer-electron patterns. It helps predict families, ion formulas, radius directions and broad…
Explore lesson ChemistryLessonPeriodic Data and Model Limits
A table of ionisation energies is data; a sentence about shielding is a model used to explain the pattern. Both matter, but neither can replace the other. A measured value may…
Explore lesson ChemistryLessonPeriodic Electronic-Structure Calculations
A crystal contains an immense number of atoms, yet a periodic calculation can model it through a small repeating cell. The price of that efficiency is a set of assumptions…
Explore lesson ChemistryLessonPeriodic Law as a Predictive Pattern
The periodic table is more than a catalogue of element names. It arranges elements by increasing atomic number so related patterns reappear. Sodium and potassium, for example…
Explore lesson ChemistryLessonPeriodic Table Terms
Periodic-table words let a chemist compress many observations into a small map. But the labels are not all measurements of the same thing. Group and period are positions; valence…
Explore lesson ChemistryLabPeriodic trend explorer
Colour the periodic table by a property and see the trends across periods and down groups.
Explore lesson ChemistryLessonPeriodic Trends as Data
Periodic trends are often taught as arrows on a chart, but the arrows summarize measured or evaluated data with definitions and exceptions. Ionization energy, electron affinity…
Explore lesson ChemistryLessonPeriodic Trends Versus Individual Exceptions
The periodic table is often taught with arrows for size, ionisation energy and electronegativity. The arrows are useful first guides, but chemistry becomes more accurate when the…
Explore lesson ChemistryLessonPeriodicity of Valency and Oxidation States
The periodic repetition of outer-electron patterns is reflected in recurring compound formulas. Group-1 metals often form +1 ions, group-2 metals often +2, and halogens often −1…
Explore lesson ChemistryLessonPeriods and Occupied Electron Shells
Neutral sodium is [Ne]3s¹ and neutral potassium is [Ar]4s¹. Their outer electrons occupy different principal shells, which corresponds to their different rows of the periodic…
Explore lesson ChemistryLessonPeriods: The Horizontal Rows
Read a line of text from left to right and the story moves forward one word at a time. The periodic table's horizontal rows, called periods, work in a similar way. Moving one box…
Explore lesson ChemistryLessonPermanent Dipole Interactions
Polar molecules have uneven charge distributions with partially positive and negative regions. When two such molecules approach in a favourable orientation, opposite ends…
Explore lesson ChemistryLessonPermanent Hardness
If you boil some hard water and then test it with soap, you may find that it still refuses to lather properly. Boiling has removed part of the hardness but not all of it. The…
Explore lesson ChemistryLessonPermanganate Titration Calculations
Permanganate titration problems combine two distinct calculations: chemical stoichiometry and sample-volume bookkeeping. In acid, MnO₄⁻ to Mn²⁺ accepts five electrons. That sets…
Explore lesson ChemistryLabPeroxide effect
The peroxide effect: with peroxide, HBr adds to an alkene the “wrong” way (anti-Markovnikov).
Explore lesson ChemistryLessonPeroxide Effect in HBr Addition
HBr addition to an unsymmetrical alkene can give a different major regioisomer when radical initiators such as peroxides are present under suitable conditions. The change is…
Explore lesson ChemistryLessonPersistent Organic Pollutants
Some organic pollutants can be detected far from where they were manufactured or used. Their environmental story cannot be explained by toxicity alone. They must last long enough…
Explore lesson ChemistryLessonPersonal Protective Equipment
Even when experiments are carefully planned, small splashes, sprays of hot liquid and flying fragments of glass can happen. Personal protective equipment, usually shortened to…
Explore lesson ChemistryLessonPerturbation Theory versus the Variation Method
Chemistry relies on two great strategies for approximating the Schrödinger equation. The variation method guesses a trial function and minimises its energy. Perturbation theory…
Explore lesson ChemistryLessonpH and Electrode Potential
Many electrode reactions include hydrogen ions or hydroxide ions. Their potentials therefore vary with pH. The hydrogen electrode offers a clear example: at fixed hydrogen…
Explore lesson ChemistryLessonpH and Hydrogen Ion Concentration
Lemon juice has a pH of about 2 and black coffee a pH of about 5. It is tempting to think the lemon juice is a little more than twice as acidic. In fact it contains roughly a…
Explore lesson ChemistryLessonpH and pOH
Hydronium and hydroxide concentrations vary over many powers of ten, so logarithmic scales make them easier to compare. A one-unit pH change represents a tenfold change in…
Explore lesson ChemistryLessonpH and Salts: Integrated Review
This unit connects proton transfer, pH, salt formulas, reaction amounts and practical observations. A strong solution to an integrated problem does not rely on one memorised…
Explore lesson ChemistryLessonpH Control and Sulfide Precipitation
Metal sulfides often have low solubilities, but the amount of free S²⁻ in water depends strongly on pH. Hydrogen sulfide behaves as a weak diprotic acid: H₂S, HS⁻ and S²⁻ are…
Explore lesson ChemistryLessonpH in the Human Body
Your body contains some of the most acidic and most tightly controlled liquids you will ever meet. The fluid in your stomach is strongly acidic, around pH 1 to 2, yet a few…
Explore lesson ChemistryLessonpH of a Strong Hydroxide Base
Calculating pH from a dissolved strong hydroxide base takes one extra step compared with a strong monoprotic acid. First count how many OH⁻ ions each dissolved formula unit…
Explore lesson ChemistryLessonpH of a Strong Monoprotic Acid
For a strong monoprotic acid in a simple dilute aqueous solution, the path from preparation concentration to pH is direct: one dissolved acid unit produces approximately one…
Explore lesson ChemistryLessonpH of Amphiprotic Salt Solutions
An aqueous salt containing the intermediate ion HA⁻ of a diprotic acid H₂A has two competing reactions: HA⁻ can donate or accept a proton. Under common simplifying conditions…
Explore lesson ChemistryLessonpH Titration Curves
A pH titration curve makes the changing chemical composition visible. Before equivalence, the original analyte and its reaction products determine pH. Near equivalence, a small…
Explore lesson ChemistryLessonpH-Dependent Solubility
Some sparingly soluble solids contain anions that react with hydronium. Adding acid can remove the free anion from solution by protonation. The solid can then dissolve further…
Explore lesson ChemistryCollectionpH, Salts and their Uses
Study pH, Salts and their Uses with 50 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonPhase Coexistence as Equality of Chemical Potential
Liquid water and its vapour can share a container at equilibrium even though their densities, entropies and molecular arrangements differ. The thermodynamic requirement is not…
Explore lesson ChemistryLessonPhase Diagrams of Water and Carbon Dioxide
Water and carbon dioxide make an instructive pair because their phase diagrams have the same basic ingredients but different practical paths. Water's ice–liquid boundary leans…
Explore lesson ChemistryLessonPhase Rule and Phase Diagrams: Worked Problems
Phase-diagram problems often combine three distinct calculations: a phase-rule count, a material-balance lever rule and a boundary-slope equation. Each has different inputs and…
Explore lesson ChemistryLessonPhase Stability and Chemical Potential
At a given temperature and pressure, a pure substance may be capable of appearing as solid, liquid or vapour. Which phase is stable? For a pure phase, chemical potential equals…
Explore lesson ChemistryLessonPhases, Components and Degrees of Freedom
The Gibbs phase rule is compact, but it is useful only when its letters are counted correctly. P denotes the number of coexisting phases, C the number of independent components…
Explore lesson ChemistryLessonPhenol Preparation Routes
Phenol is not usually obtained by treating chlorobenzene exactly like a primary alkyl chloride with mild aqueous hydroxide. The aryl C–Cl bond has different geometry and…
Explore lesson ChemistryLessonPhenol Reactions and Recognition
Phenol combines a weakly acidic O–H group with an activated aromatic ring. Those two features support common recognition tests: base can form phenoxide, bromine water can react…
Explore lesson ChemistryLessonPhosphate Buffer Design
Phosphate is the classic buffer for work near neutral pH. Its second dissociation, H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻, has a thermodynamic pKa₂ of 7.20 at 25 °C, placing it right in the range…
Explore lesson ChemistryLessonPhosphine Ligands: Sterics and Electronics
Phosphines are among the most adjustable ligands in molecular catalysis. Changing the three groups attached to phosphorus alters how much space the ligand occupies and how it…
Explore lesson ChemistryLessonPhosphorescence and Spin–Orbit Coupling
Phosphorescence commonly follows a route in which an excited singlet crosses into a triplet state and then emits while returning to a singlet ground state. Because the emission…
Explore lesson ChemistryLessonPhosphoric Acid as a Triprotic Example
Phosphoric acid is the textbook triprotic acid, and for good reason. Its three pKa values are spread so widely that its behaviour is almost like three separate monoprotic acids…
Explore lesson ChemistryLessonPhosphorus Allotropes
Phosphorus has several allotropes that differ despite containing only phosphorus atoms. White phosphorus consists of P₄ molecules, red phosphorus has more extended bonded…
Explore lesson ChemistryLessonPhosphorus Chemistry
Phosphorus is a Group 15 nonmetal with several elemental forms. White phosphorus consists of strained P₄ tetrahedra and is highly reactive; red and black forms have more extended…
Explore lesson ChemistryLessonPhosphorus Chlorides
Phosphorus forms both PCl₃ and PCl₅. The formulas demonstrate +3 and +5 formal oxidation states, while their shapes show how different numbers of bonding regions alter molecular…
Explore lesson ChemistryLessonPhotocatalyst Band Alignment
A light-driven catalyst needs more than a colour that absorbs sunlight. It must absorb suitable photons, separate the resulting electrons and holes, transport them to reactive…
Explore lesson ChemistryLessonPhotocatalytic Water Splitting
Water splitting stores light energy in hydrogen while producing oxygen from water. The net equation is 2 H₂O → 2 H₂ + O₂. Making hydrogen in an illuminated suspension is not…
Explore lesson ChemistryLessonPhotochemical [2+2] Cycloadditions — Advanced Organic Chemistry
Two alkenes can form a cyclobutane by making two new carbon–carbon σ bonds. The ordinary concerted thermal suprafacial path is symmetry-disfavored, but light changes electronic…
Explore lesson ChemistryLessonPhotochemical [2+2] Cycloadditions — Organic Synthesis and Mechanisms
Two alkenes contain exactly four carbon atoms, enough to form a cyclobutane ring if each alkene supplies one end of two new carbon–carbon bonds. The simplest concerted thermal…
Explore lesson ChemistryLessonPhotochemical Electrocyclisation: Vitamin D
Vitamin D3 formation in skin is a vivid example of photochemistry and thermal chemistry in sequence. Ultraviolet light opens a ring bond of 7-dehydrocholesterol, giving…
Explore lesson ChemistryLessonPhotochemical Isomerization
Light can turn one molecular geometry into another while leaving the chemical formula unchanged. In photoisomerization, excitation changes the forces on bonds or angles, allowing…
Explore lesson ChemistryLessonPhotochemical Pericyclic Reactions: Reversing the Rules
Heating and illuminating the same conjugated molecule can favor different electrocyclic stereochemistry. Absorbing light changes electron occupation and therefore the phase…
Explore lesson ChemistryLessonPhotochemical Reaction Coordinates
A ground-state reaction-coordinate diagram with one barrier cannot describe every light-driven transformation. After absorption, a molecule begins at a ground-state geometry on…
Explore lesson ChemistryLessonPhotochemical Reaction Rates
Some reactions require light to create reactive excited states or radicals. Their rates can depend on wavelength and the number of photons absorbed, not merely on reactant…
Explore lesson ChemistryLessonPhotochemical Smog
Photochemical smog is a reaction network, not a single smoke-like substance. Sunlight acts on emitted nitrogen oxides and volatile organic compounds to generate ozone, organic…
Explore lesson ChemistryCollectionPhotochemistry and Photophysics
Study Photochemistry and Photophysics with 30 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonPhotochemistry and Photophysics: Unit Review
Photochemistry connects four questions: which species absorbs a photon, how its excited state relaxes, which chemical pathway follows, and how product formation is measured. A…
Explore lesson ChemistryLessonPhotochemistry in Industry, Nature and Medicine
Photochemistry is useful when light can be delivered where or when a reaction is needed. The same photon absorption can also damage a material or biological molecule if…
Explore lesson ChemistryLessonPhotochemistry: From Photon to Product
Light can start a chemical process, but a lamp illuminating a flask is not by itself a mechanism. Photons must be absorbed by some species; that species enters an excited state…
Explore lesson ChemistryLessonPhotochemistry: Problem Solving
Photochemical product problems cannot be solved by one universal light-reaction rule. A photon may trigger orbital-symmetry-controlled ring opening, alkene isomerisation…
Explore lesson ChemistryLessonPhotocleavage and Radical Formation
A photon can put enough electronic energy into a molecule to weaken or break a bond. Photocleavage may produce radicals, ions or an excited fragment, so observing disappearance…
Explore lesson ChemistryLessonPhotocycloadditions
Light can enable bond-forming reactions that are difficult by ordinary thermal pathways. In a familiar [2+2] photocycloaddition, two alkene π systems form a cyclobutane framework…
Explore lesson ChemistryLessonPhotodegradation and Photostability
A molecule that captures light can also damage itself. Excited states may cleave bonds, transfer electrons to oxygen, sensitize reactive oxygen or trigger rearrangements. A…
Explore lesson ChemistryLabPhotoelectric effect
Light shines on a metal. Electrons are knocked out only if each photon has enough energy.
Explore lesson ChemistryLessonPhotoinduced Electron Transfer
An excited molecule contains stored electronic energy and may donate or accept an electron more readily than in its ground state. Photoinduced electron transfer converts an…
Explore lesson ChemistryLessonPhotolytic Decomposition
Some compounds change when they absorb light. Silver halides are a familiar example: light can lead to metallic silver forming within the material, which helps explain darkening…
Explore lesson ChemistryLessonPhotons and Energy Changes
An atomic spectral line is light at a particular wavelength, so it carries photons of a particular energy. Two compact equations connect the measured light to the atom: c = λν…
Explore lesson ChemistryLessonPhotons and the Photoelectric Effect
Light shining on some metal surfaces can eject electrons. The crucial observation is a frequency threshold: below it, merely increasing the light's intensity does not release…
Explore lesson ChemistryLessonPhotoreactor Design
A photoreaction that works in a small vial may slow when the vessel is enlarged. The central issue is often that light does not penetrate uniformly. Reactor design must deliver…
Explore lesson ChemistryLessonPhotoredox Catalysis — Advanced Organic Chemistry
Visible-light photoredox catalysis uses a light-absorbing molecule to move one electron at a time between reaction partners. This can create organic radicals under conditions…
Explore lesson ChemistryLessonPhotoredox Catalysis — Photochemistry and Photophysics
A photoredox catalyst absorbs light and uses its excited-state redox properties to move an electron. The resulting radical ions can make bonds, fragment or transfer hydrogen…
Explore lesson ChemistryLessonPhotosensitisation and Quenching
The molecule that absorbs light need not be the molecule that ultimately reacts. A sensitizer can harvest photons and pass energy or an electron to a substrate. Other molecules…
Explore lesson ChemistryLessonPhotosensitization
Many desired substrates absorb little at the available lamp wavelength. A photosensitizer can serve as an antenna: it absorbs light and passes energy or an electron to a second…
Explore lesson ChemistryLessonPhotovoltaic Absorption and Band Gaps
A solar cell needs an absorber that turns incident photons into mobile charge carriers. A semiconductor band gap helps determine which wavelengths can create electron–hole pairs…
Explore lesson ChemistryLabPhotovoltaic band alignment
A solar cell: photons above the band gap make electron–hole pairs, and the junction separates them.
Explore lesson ChemistryLessonPhotovoltaic Cells
A solar cell uses semiconductor absorption to create electron–hole pairs and selective contacts to collect them as electrical work. The p–n junction's field helps separate…
Explore lesson ChemistryLessonPhotovoltaic Degradation
A solar cell is useful only if it continues to produce electricity over its service life. Moisture, oxygen, heat, ultraviolet light and electrical bias can change absorber…
Explore lesson ChemistryLessonPhysical and Analytical Chemistry
Some chemists ask "why?" and "how fast?". Others ask "what is it?" and "how much is there?". The first group are physical chemists; the second are analytical chemists. Together…
Explore lesson ChemistryLessonPhysical and Chemical Change at the Particle Level
Ice melting and paper burning are both changes, but they are very different. Melted ice can be refrozen into ice, while burnt paper cannot be turned back into paper. The…
Explore lesson ChemistryCollectionPhysical and Chemical Changes
Study Physical and Chemical Changes with 40 free lessons, worked examples, and answered practice questions.
Explore lesson ChemistryLessonPhysical and Chemical Changes: Unit Review
This unit has explored how matter changes: melting and dissolving, fizzing and colour changes, rusting nails and roaring flames. These ideas all rest on a few big principles.…
Explore lesson ChemistryLessonPhysical and Chemical Properties
So far this unit has described physical properties: colour, density, melting point, conductivity and more. But a full description of a substance also includes what it can turn…
Explore lesson ChemistryLessonPhysical Bounds and Alternative Solutions
Algebra is indifferent to chemistry. A quadratic equation from an equilibrium table always has two roots, and the equation will not tell you which one describes the flask on the…
Explore lesson ChemistryLessonPhysical Changes
When you snap a pencil, crumple a can or leave an ice lolly in the sun, the material looks different afterwards, but it is still the same stuff. These are physical changes. They…
Explore lesson ChemistryCollectionPhysical Chemistry Problem Solving
Study Physical Chemistry Problem Solving with 80 free lessons, worked examples, and answered practice questions.
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