---
title: "Matter & Materials"
description: "101 topics in Science, in the order they build on each other."
canonical: https://lightmysky.com/learn/science/areas/matter-and-materials
source: https://lightmysky.com/learn/science/areas/matter-and-materials.md
retrieved: 2026-10-19
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# Matter & Materials

101 topics in Science, in the order they build on each other.

Page: https://lightmysky.com/learn/science/areas/matter-and-materials

- [Objects vs Materials](https://lightmysky.com/learn/science/objects-vs-materials-mt_5r47Pvstyn): Distinguish between an object and the material from which it is made, understanding that objects can be made from different materials
- [Naming Everyday Materials](https://lightmysky.com/learn/science/naming-everyday-materials-mt_ZAJvTcroFO): Identify and name a variety of everyday materials including wood, plastic, glass, metal, water, and rock
- [Describing Material Properties](https://lightmysky.com/learn/science/describing-material-properties-mt_auVZZEuXjs): Describe simple physical properties of everyday materials such as hard/soft, stretchy/stiff, shiny/dull, rough/smooth, waterproof/absorbent, transparent/opaque
- [Grouping Materials](https://lightmysky.com/learn/science/grouping-materials-mt_SwaNNdm_Ks): Compare and group everyday materials based on their simple physical properties
- [States of Matter Vocabulary](https://lightmysky.com/learn/science/states-of-matter-vocabulary-mt_1JFUNQDwAJ): Name and tell apart the three states of matter (solid, liquid and gas) using property words: hard, rigid, runny, flows, keeps its shape, fills its container. Use 'change of state' to describe what happens when materials are heated or cooled.
- [Changing Shapes of Solids](https://lightmysky.com/learn/science/changing-shapes-of-solids-mt_wf-SJhZ1kC): Investigate how the shapes of solid objects can be changed by squashing, bending, twisting, and stretching
- [Choosing the Right Material](https://lightmysky.com/learn/science/choosing-the-right-material-mt_rTn43s8RNX): Identify and compare the suitability of everyday materials for particular uses, explaining why specific materials are chosen for specific purposes
- [Changes & Separation Vocabulary](https://lightmysky.com/learn/science/changes-and-separation-vocabulary-mt_ahSqW_kK1b): Use process words for changes of state and separating materials (dissolve, solution, soluble, insoluble, evaporate, condense, melt, freeze, filter, sieve, mixture, separate) and understand exactly what each one describes, including the difference between dissolving and melting.
- [Classifying Materials](https://lightmysky.com/learn/science/classifying-materials-mt_zHnOGwHIEz): Plan and conduct an investigation to classify different kinds of materials by their observable properties
- [Drawing Particle Diagrams](https://lightmysky.com/learn/science/drawing-particle-diagrams-mt_htAYR-iCFF): Draw and interpret particle diagrams — dot representations showing the arrangement, spacing, and movement of particles in solids (close, regular, vibrating in place), liquids (close, random, flowing past each other), and gases (widely spaced, moving rapidly in all directions) — and use these diagrams to explain observable properties such as fixed shape, fixed volume, and compressibility
- [Taking Apart & Rebuilding](https://lightmysky.com/learn/science/taking-apart-and-rebuilding-mt_GQpqoR5YOc): Observe that an object made of a small set of pieces can be disassembled and made into a new object, understanding that the pieces still exist
- [Testing Materials for Uses](https://lightmysky.com/learn/science/testing-materials-for-uses-mt_e3-_toGuWf): Give reasons, based on evidence from comparative and fair tests, for the particular uses of everyday materials including metals, wood, and plastic
- [Solids, Liquids & Gases](https://lightmysky.com/learn/science/solids-liquids-and-gases-mt_YQkUdIHO8L): Compare and group materials as solids, liquids, or gases based on their observable properties and behaviour
- [Heating & Cooling Changes](https://lightmysky.com/learn/science/heating-and-cooling-changes-mt_Pl-nsjYGZ3): Observe and describe that some materials change state when heated or cooled, and measure the temperature at which changes occur in degrees Celsius
- [Evaporation & the Water Cycle](https://lightmysky.com/learn/science/evaporation-and-the-water-cycle-mt_Qkewo5M3_c): Identify the role of evaporation and condensation in the water cycle, and associate the rate of evaporation with temperature
- [Material Properties Vocabulary](https://lightmysky.com/learn/science/material-properties-vocabulary-mt_MVovx37Xct): Use property terms to describe and compare materials: electrical and thermal conductor or insulator, transparent, translucent, opaque, soluble, insoluble, magnetic, flexible, rigid, and density. Use these terms to explain choices of materials for particular jobs.
- [Advanced Material Properties](https://lightmysky.com/learn/science/advanced-material-properties-mt_Ge4Wtg6QMM): Compare and group everyday materials based on advanced properties: hardness, solubility, transparency, electrical and thermal conductivity, and response to magnets
- [Dissolving & Solutions](https://lightmysky.com/learn/science/dissolving-and-solutions-mt_Mf-T-fYRLX): Understand that some materials dissolve in liquid to form a solution, and describe how to recover a substance from a solution by evaporation
- [Reversible Changes](https://lightmysky.com/learn/science/reversible-changes-mt_UKmtuAsSLN): Demonstrate that dissolving, mixing, and changes of state are reversible changes where no new materials are formed
- [Irreversible Changes](https://lightmysky.com/learn/science/irreversible-changes-mt_rbPioPELM1): Explain that some changes form new materials and are not usually reversible, such as burning, rusting, and reactions with acid
- [Separating Mixtures](https://lightmysky.com/learn/science/separating-mixtures-mt_2b6CB0w3Yx): Use knowledge of solids, liquids, and gases to decide how mixtures might be separated through filtering, sieving, and evaporating
- [Matter Is Made of Particles](https://lightmysky.com/learn/science/matter-is-made-of-particles-mt_ylruY6VhOf): Develop a model to describe that matter is made of particles too small to be seen, and that this explains properties of solids, liquids, and gases
- [Conservation of Mass](https://lightmysky.com/learn/science/conservation-of-mass-mt_ehC1wsdmUz): Measure and provide evidence that the total weight of matter is conserved regardless of the type of change (heating, cooling, or mixing)
- [The Particle Model](https://lightmysky.com/learn/science/the-particle-model-mt_b6kZgqolEd): Use the particle model to explain the properties of solids, liquids, and gases, including differences in arrangement, movement, and spacing, and apply the model to explain density, compressibility, and the anomalous expansion of water
- [Atoms, Elements & Compounds](https://lightmysky.com/learn/science/atoms-elements-and-compounds-mt_Z_Wu_77ybI): Explain the differences between atoms, elements, and compounds; describe the simple Bohr model of the atom (nucleus with protons and neutrons, electrons in shells); and write and interpret chemical symbols and simple formulae
- [How Materials Change State](https://lightmysky.com/learn/science/how-materials-change-state-mt_CyV7crZ8hl): Explain melting, freezing, boiling, condensing, and sublimation using the particle model, interpreting heating and cooling curves to identify melting and boiling points
- [Physical vs Chemical Changes](https://lightmysky.com/learn/science/physical-vs-chemical-changes-mt_1hgck6ucII): Distinguish between physical changes (reversible, no new substances formed) and chemical changes (new substances formed, often irreversible), using conservation of mass to understand both types
- [The Periodic Table](https://lightmysky.com/learn/science/the-periodic-table-mt_wfEfQuHOG-): Describe the organisation of the periodic table into periods and groups, explain the contribution of Mendeleev, and use the table to identify metals, non-metals, and predict patterns in reactivity
- [Metals vs Non-Metals](https://lightmysky.com/learn/science/metals-vs-non-metals-mt_Sc_SorJhXW): Compare the physical and chemical properties of metals and non-metals, explaining metallic properties (malleability, lustre, conductivity) and how position in the periodic table predicts reactivity
- [Acids, Alkalis & pH](https://lightmysky.com/learn/science/acids-alkalis-and-ph-mt_AN2kJE6I0s): Define acids and alkalis in terms of hydrogen ion concentration, describe the pH scale (0–14), and explain how indicators are used to identify and measure acidity or alkalinity
- [Types of Chemical Reaction](https://lightmysky.com/learn/science/types-of-chemical-reaction-mt_v0K6GRi4ZL): Identify and describe four types of chemical reaction: combustion (burning in oxygen), oxidation (gain of oxygen), thermal decomposition (breaking down by heat), and displacement (more reactive metal replaces less reactive one)
- [Acid Reactions & Salts](https://lightmysky.com/learn/science/acid-reactions-and-salts-mt_QKRYLwaOU8): Describe and write word equations for the reactions of acids with metals, alkalis (neutralisation), and metal oxides/hydroxides, identifying the salt produced in each case
- [Reactions That Release or Absorb Heat](https://lightmysky.com/learn/science/reactions-that-release-or-absorb-heat-mt_UoqUPI_uNz): Distinguish between exothermic reactions (release energy, temperature rises) and endothermic reactions (absorb energy, temperature falls), with everyday and industrial examples
- [The Reactivity Series](https://lightmysky.com/learn/science/the-reactivity-series-mt_zsYW61cn_q): Order common metals in the reactivity series and explain how a more reactive metal displaces a less reactive one; describe how carbon is used to extract metals from their oxides in industry
- [Ceramics, Polymers & Composites](https://lightmysky.com/learn/science/ceramics-polymers-and-composites-mt_w83U-_noVR): Describe the properties and uses of ceramics (hard, brittle, heat-resistant), polymers (flexible, lightweight, variable), and composites (combine properties of constituent materials), giving real-world examples of each
- [Inside the Atom: Protons, Neutrons and Electrons](https://lightmysky.com/learn/science/inside-the-atom-protons-neutrons-and-electrons-mt_WqDXAr2IkA): Place protons, neutrons and electrons in the atom with their relative charges and masses. Read the atomic number and mass number off the periodic table and use them to count every particle in an atom.
- [Isotopes and Relative Atomic Mass](https://lightmysky.com/learn/science/isotopes-and-relative-atomic-mass-mt_NFoRZd6UN_): Isotopes of an element differ in neutron count but behave the same chemically. Relative atomic mass is the weighted mean of an element's isotopes, which is why most table values are not whole numbers.
- [Electron Shells and the Shape of the Periodic Table](https://lightmysky.com/learn/science/electron-shells-and-the-shape-of-the-periodic-table-mt__Sdcw4RnZF): Fill electron shells for the first twenty elements and use the result to explain the table itself: the group number gives the outer-shell electrons, the period number gives how many shells are occupied.
- [Group 1, Group 7 and Group 0: Trends You Can Predict](https://lightmysky.com/learn/science/group-1-group-7-and-group-0-trends-you-can-predict-mt_cy1OrVAjIo): Use outer-shell electrons to predict how alkali metals, halogens and noble gases behave, including why group 1 gets more reactive down the group while group 7 gets less.
- [Ionic Bonding: Electron Transfer and Charged Lattices](https://lightmysky.com/learn/science/ionic-bonding-electron-transfer-and-charged-lattices-mt_sf24C4YLtN): Metal atoms hand their outer electrons to non-metal atoms, and the ions that result pull on each other in a giant lattice. Work out ionic charges and formulas from the group numbers.
- [Covalent Bonding: Shared Pairs and Simple Molecules](https://lightmysky.com/learn/science/covalent-bonding-shared-pairs-and-simple-molecules-mt_fn5Dq6qUjP): Two non-metal atoms fill their outer shells by sharing electron pairs. The shared bond is strong, but the pull between separate molecules is weak, which sets the properties of simple molecular substances.
- [Dot-and-Cross Diagrams and Lone Pairs](https://lightmysky.com/learn/science/dot-and-cross-diagrams-and-lone-pairs-mt_ghK4a25usP): A dot-and-cross diagram shows where every outer electron sits once a bond has formed, including the pairs that are not bonding anything. Those lone pairs are the part of the picture most people leave out, and they decide what the molecule does next.
- [Naming Ionic and Simple Molecular Compounds](https://lightmysky.com/learn/science/naming-ionic-and-simple-molecular-compounds-mt_GJ267pzxCI): Chemical names are built from rules, not memorised one by one: ion charges fix the formula of an ionic compound, and Greek prefixes count atoms in a molecular one. Reading a name backwards to a formula is the skill that makes every later equation legible.
- [Redox: Oxidation and Reduction as Electron Transfer](https://lightmysky.com/learn/science/redox-oxidation-and-reduction-as-electron-transfer-mt_NMTikR0FTM): Rewrite displacement and metal-with-acid reactions as electron transfer, where oxidation is loss of electrons and reduction is gain, and split a reaction into two half equations.
- [Extracting Metals: Reduction with Carbon and Choosing a Method](https://lightmysky.com/learn/science/extracting-metals-reduction-with-carbon-and-choosing-a-method-mt_LRAEJa8OLr): A metal sits in its ore as a compound, and getting it out is a reduction. Where the metal falls in the reactivity series decides whether carbon is a strong enough reducing agent or whether electricity has to do the job, which is why aluminium stayed a precious metal long after iron was common.
- [Strong and Weak Acids and the pH Scale](https://lightmysky.com/learn/science/strong-and-weak-acids-and-the-ph-scale-mt_Y4nSn5XH56): An acid in water releases hydrogen ions. A strong acid ionises fully while a weak one only partly does, so strength and concentration are separate ideas, and pH shifts by one for every tenfold change in hydrogen ion concentration.
- [Density and How to Measure It](https://lightmysky.com/learn/science/density-and-how-to-measure-it-mt_HrKI1-UzfI): Density is mass per unit volume, found by weighing and by measuring volume, including displacement for an awkward shape. Particle spacing explains why the same substance has a much lower density as a gas.
- [Giant Covalent Structures: Diamond, Graphite and Graphene](https://lightmysky.com/learn/science/giant-covalent-structures-diamond-graphite-and-graphene-mt_YTEj9srwPG): Some covalent substances never stop bonding. Explain diamond, graphite, graphene and silicon dioxide from how many bonds each atom makes and what is left over.
- [Metallic Bonding and Alloys](https://lightmysky.com/learn/science/metallic-bonding-and-alloys-mt_yXOGOwHmZ4): Metal ions sit in a regular lattice inside a sea of delocalised electrons. That one picture explains conduction, malleability, and why an alloy is harder than the pure metal.
- [Molecular Shapes from Electron Pair Repulsion](https://lightmysky.com/learn/science/molecular-shapes-from-electron-pair-repulsion-mt_Of6ZxYjOsn): Electron pairs around a central atom push each other as far apart as they can, and that alone predicts the shape. A lone pair pushes harder than a bonding pair, which is why water is bent and not straight.
- [Electronegativity and the Polar Bond](https://lightmysky.com/learn/science/electronegativity-and-the-polar-bond-mt_TwSlh2VvUT): Unequal attraction for shared electrons gives a bond partial positive and negative charges. The strengths and directions of the bond dipoles determine whether their combined effect leaves the molecule polar.
- [Reaction Profiles and Activation Energy](https://lightmysky.com/learn/science/reaction-profiles-and-activation-energy-mt_6LtFkXn_xQ): Draw a reaction as energy against progress, marking reactants, products, the overall energy change and the activation energy hump that has to be climbed before anything happens.
- [Bond Energies and the Overall Energy Change](https://lightmysky.com/learn/science/bond-energies-and-the-overall-energy-change-mt_VI0_1ogIOy): Breaking bonds takes energy in and making bonds gives energy out. Use a table of bond energies to calculate the overall change for a reaction and check the sign against the profile.
- [Alkanes and Alkenes: The First Homologous Series](https://lightmysky.com/learn/science/alkanes-and-alkenes-the-first-homologous-series-mt_mkLJDtBanL): Alkanes and alkenes are families whose members differ by one carbon at a time and share a general formula. The double bond in an alkene makes it react in ways an alkane will not.
- [Reading Structure from Properties](https://lightmysky.com/learn/science/reading-structure-from-properties-mt_yCA66GjWRw): Given melting point, conductivity and solubility for an unknown substance, decide whether it is ionic, simple molecular, giant covalent or metallic, and defend the choice.
- [Forces Between Molecules](https://lightmysky.com/learn/science/forces-between-molecules-mt_0uBZWotvZB): Melting and boiling a simple molecular substance breaks the forces between molecules, not the bonds inside them. Those forces come in a few strengths, and hydrogen bonding is the strong end of the range.
- [Nanoparticles and Why Surface Area Changes Everything](https://lightmysky.com/learn/science/nanoparticles-and-why-surface-area-changes-everything-mt_pTG4U3Hvdn): Cut a lump into small enough pieces and the fraction of atoms sitting on a surface stops being negligible. That single geometric fact is why a nanoparticle can catalyse, absorb light or pass through a membrane in ways the bulk material never does, and it is also the reason for caution about them.
- [Relative Formula Mass and the Mole](https://lightmysky.com/learn/science/relative-formula-mass-and-the-mole-mt_-SiMKJK4rH): Add relative atomic masses to get the relative formula mass of a substance, then use the mole as a fixed count of particles so a mass on the balance can be turned into a number of particles.
- [Balancing Equations and Conservation of Mass](https://lightmysky.com/learn/science/balancing-equations-and-conservation-of-mass-mt_KUaPCEIA5f): Balance a symbol equation so every element has the same atom count on both sides, add state symbols, and use the balance to explain why mass is conserved even when a reaction looks like it gained or lost some.
- [Reacting Masses, Limiting Reactants and Yield](https://lightmysky.com/learn/science/reacting-masses-limiting-reactants-and-yield-mt_C2K6kp5Chv): Use the mole ratio in a balanced equation to work out what mass of product a given mass of reactant can make, find which reactant runs out first, and compare the mass actually collected with the maximum.
- [Concentration of Solutions in Moles per Cubic Decimetre](https://lightmysky.com/learn/science/concentration-of-solutions-in-moles-per-cubic-decimetre-mt_vRK78ynnXb): Express how much solute a solution carries in g/dm3 and in mol/dm3, convert between the two, and work out what dilution does to the figure.
- [Empirical and Molecular Formulas from Composition](https://lightmysky.com/learn/science/empirical-and-molecular-formulas-from-composition-mt_4rN5MQaTf7): Masses or percentages from an analysis give the simplest whole-number ratio of atoms, and one extra measurement of relative mass turns that ratio into the real formula. It is the calculation that connects what a balance reads to what a molecule is.
- [The Gas Laws and the Ideal Gas Equation](https://lightmysky.com/learn/science/the-gas-laws-and-the-ideal-gas-equation-mt_G7DOTVl4KM): Boyle's law, Charles's law and the pressure law are three slices of one relationship, pV = nRT, once the temperature is in kelvin. The equation gives any one of the four quantities when the other three are known.
- [Titration: Finding an Unknown Concentration](https://lightmysky.com/learn/science/titration-finding-an-unknown-concentration-mt_7WLN35XxYx): Run an acid and alkali titration to a single indicator end point, then use the mole ratio in the equation to calculate the unknown concentration from concordant titres.
- [Why Some Things Dissolve and Others Do Not](https://lightmysky.com/learn/science/why-some-things-dissolve-and-others-do-not-mt__njIrlqqqL): Dissolving replaces the forces holding a solute together with forces between solute and solvent, and it only happens when that trade is worth making. This is why salt and sugar go into water but oil does not.
- [Kinetic Theory: Pressure from Molecular Motion](https://lightmysky.com/learn/science/kinetic-theory-pressure-from-molecular-motion-mt_2ARUheICqu): Treating a gas as many small particles in random motion, pressure comes from their collisions with the walls, which gives pV as one third of N m times the mean square speed. Comparing that with pV = nRT shows temperature to be a measure of mean molecular kinetic energy.
- [Elastic Strain Energy and Force-Extension Graphs](https://lightmysky.com/learn/science/elastic-strain-energy-and-force-extension-graphs-mt_4WnFUmR13O): The area under a force-extension graph is the work done stretching a sample, held as elastic strain energy while the material stays elastic. A loading curve that does not retrace on unloading shows energy that never came back.
- [Stress, Strain and the Young Modulus](https://lightmysky.com/learn/science/stress-strain-and-the-young-modulus-mt_uWdqhWGT7Y): Dividing force by cross-sectional area and extension by original length removes the size of the sample, leaving numbers that describe the material itself. The gradient of the straight part of a stress-strain graph is the Young modulus.
- [Pure Substances & Mixtures](https://lightmysky.com/learn/science/pure-substances-and-mixtures-mt_MBTVB-E-S7): Distinguish between pure substances and mixtures, identify formulations as useful mixtures with precise compositions, and use melting and boiling points to test for purity
- [Choosing How to Separate Mixtures](https://lightmysky.com/learn/science/choosing-how-to-separate-mixtures-mt_U_tJvy3cbB): Select and carry out appropriate separation techniques for different types of mixtures: filtration (insoluble solids), distillation (liquids by boiling point), crystallisation (dissolved solids), and chromatography (coloured substances)
- [Paper Chromatography and the Rf Value](https://lightmysky.com/learn/science/paper-chromatography-and-the-rf-value-mt_BAcCcz11ls): A mixture spotted on paper separates because each component divides differently between the paper and the moving solvent. The ratio of the distances travelled is a number that identifies a substance under fixed conditions, which is what makes the method analytical rather than merely pretty.
- [Crude Oil, Fractional Distillation and Cracking](https://lightmysky.com/learn/science/crude-oil-fractional-distillation-and-cracking-mt_Jg3O4EQ9Vz): Crude oil is a mixture of hydrocarbons separated by boiling point in a fractionating column, and the long fractions nobody wants are cracked into shorter, more useful molecules.
- [Testing for Ions and Gases](https://lightmysky.com/learn/science/testing-for-ions-and-gases-mt_PDZjzQJdVt): Identify unknown ions by flame colour and by the precipitate they form, test the common gases at the bench, and say why an instrument beats these tests on a tiny sample.
- [Electrolysis: Half Equations at the Electrodes](https://lightmysky.com/learn/science/electrolysis-half-equations-at-the-electrodes-mt_6FvxaD8wTY): A current through a molten ionic compound pulls it apart. Positive ions take electrons at the cathode and negative ions give them up at the anode, and each electrode gets its own half equation.
- [Potable Water and Treating Waste Water](https://lightmysky.com/learn/science/potable-water-and-treating-waste-water-mt_5qsFZJSmMO): Water fit to drink is not pure water, and the treatment chosen depends on what is in the source and how much energy the answer is allowed to cost. Sewage is the harder problem because most of what has to be removed is alive.
- [Earth's Atmosphere & CO2](https://lightmysky.com/learn/science/earths-atmosphere-and-co2-mt_mTpV-0rtkO): Describe the composition of Earth's atmosphere (mainly nitrogen and oxygen, with small amounts of CO₂ and other gases), explain how human activity increases CO₂, and describe the impact on global climate
- [Finite Resources & Recycling](https://lightmysky.com/learn/science/finite-resources-and-recycling-mt_0e5rZxbAeR): Explain that many raw materials (metals, fossil fuels, minerals) are finite resources, describe the environmental costs of extraction, and evaluate the benefits of recycling and the circular economy
- [Electrolysis of Solutions and Getting Reactive Metals Out of Ores](https://lightmysky.com/learn/science/electrolysis-of-solutions-and-getting-reactive-metals-out-of-ores-mt_l-t_JxRR7f): Predict products for common aqueous salt solutions using inert electrodes. Explain how reactivity affects industrial metal extraction, how molten cryolite is used in aluminium production, and why recycling aluminium saves energy.
- [Pollutants from Burning Fuels](https://lightmysky.com/learn/science/pollutants-from-burning-fuels-mt_DDiyDBy6YW): Burning a fuel gives out more than carbon dioxide and water. Incomplete combustion produces carbon monoxide and soot, and sulfur and nitrogen end up as acidic or irritant gases.
- [Rates of Reaction: Collision Theory and What Changes a Rate](https://lightmysky.com/learn/science/rates-of-reaction-collision-theory-and-what-changes-a-rate-mt_Ma2bOotr9g): A reaction happens when particles collide often enough and hard enough. Concentration, surface area, temperature, pressure and catalysts each change one of those two things, and a rate is read as the gradient of a graph.
- [Reversible Reactions, Dynamic Equilibrium and Le Chatelier](https://lightmysky.com/learn/science/reversible-reactions-dynamic-equilibrium-and-le-chatelier-mt_Zas-V3kTiR): Some reactions run both ways at once. In a closed container the forward and backward rates settle equal, and changing temperature, pressure or a concentration shifts the position until the rates match again.
- [The Haber Process and the Industrial Compromise](https://lightmysky.com/learn/science/the-haber-process-and-the-industrial-compromise-mt_J-JkijqnY5): Making ammonia is the case where equilibrium theory meets a factory, and the chosen conditions are not the ones that give the best yield. A moderate temperature and a catalyst buy a rate that pays, and the unreacted gas is sent round again rather than wasted.
- [Inside the Atom: the Nuclear Model](https://lightmysky.com/learn/science/inside-the-atom-the-nuclear-model-mt_B_PtnvyRIo): Firing alpha particles at thin gold foil sent a few of them straight back, which no spread-out pudding of charge could do. The nuclear model that replaced it puts almost all the mass and all the positive charge in a tiny centre.
- [Radioactive Decay: Alpha, Beta and Gamma](https://lightmysky.com/learn/science/radioactive-decay-alpha-beta-and-gamma-mt_-AmjVwkR9t): An unstable nucleus throws out an alpha particle, a beta particle or a gamma ray, and each has its own range and stopping material. A nuclear equation tracks how the mass number and atomic number change.
- [Half-Life and Activity](https://lightmysky.com/learn/science/half-life-and-activity-mt_S5snTqUjyi): Activity is the number of decays each second, measured in becquerels, and it falls by half in every half-life. Reading a half-life off a decay curve turns an unpredictable single nucleus into a reliable count for a large sample.
- [Irradiation, Contamination and Dose](https://lightmysky.com/learn/science/irradiation-contamination-and-dose-mt_WyxR7ZVEhi): Irradiation is exposure to radiation. Contamination is the unwanted or uncontrolled presence of radioactive material on or inside something. Material that remains can continue causing exposure. Dose in sieverts helps assess risk but does not predict an individual's harm.
- [Nuclear Fission and Fusion](https://lightmysky.com/learn/science/nuclear-fission-and-fusion-mt_jkO89KodSb): Fission splits a heavy nucleus and can release neutrons that sustain a chain reaction. Control rods absorb neutrons; moderators slow them in thermal reactors. Fusion joins light nuclei. Useful fusion requires high temperature, suitable fuel density and enough confinement time.
- [Mass, Energy and the Unified Atomic Mass Unit](https://lightmysky.com/learn/science/mass-energy-and-the-unified-atomic-mass-unit-mt_GTfhALD90X): Mass and energy are one quantity written in two units, tied together by E = mc squared. On the nuclear scale that makes one atomic mass unit worth about 931 MeV, which is why nuclear energies dwarf chemical ones.
- [Mass Defect and Binding Energy per Nucleon](https://lightmysky.com/learn/science/mass-defect-and-binding-energy-per-nucleon-mt_UdYE8hgXyl): A nucleus weighs less than its separate nucleons, and that missing mass is the energy it would take to pull it apart. Dividing by the number of nucleons gives the curve that peaks near iron.
- [Energy Released in Fission and Fusion](https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY): Splitting a heavy nucleus and joining two light ones both move the products up the binding energy per nucleon curve, and the mass lost turns up as energy. The size of the release follows straight from E = mc squared.
- [Particles, Antiparticles and Annihilation](https://lightmysky.com/learn/science/particles-antiparticles-and-annihilation-mt_-Ud20HRNJz): Every particle has an antiparticle of the same mass and opposite charge. A pair meeting turns entirely into photons, and a photon of enough energy can create a pair, both governed by E = mc squared.
- [Quarks, Leptons and the Conservation Laws](https://lightmysky.com/learn/science/quarks-leptons-and-the-conservation-laws-mt_kCGc5Neub_): Protons and neutrons are built from up and down quarks, while electrons and neutrinos are leptons with nothing smaller inside them. Whether a proposed reaction can happen is settled by checking charge, baryon number and lepton number.
- [Exchange Particles and Reading a Feynman Diagram](https://lightmysky.com/learn/science/exchange-particles-and-reading-a-feynman-diagram-mt_7Ka5cVpTVL): Every interaction between particles is carried by an exchanged boson, and which boson it is decides the range and the timescale. A Feynman diagram is the bookkeeping for that exchange, with the vertices doing the conserving and the internal line naming the force.
- [The Decay Constant and the Exponential Decay Law](https://lightmysky.com/learn/science/the-decay-constant-and-the-exponential-decay-law-mt_VIydSAys1W): Every unstable nucleus of one kind has the same chance of decaying each second, so the number left falls exponentially. The decay constant is that chance per nucleus per second.
- [Activity, Half-Life and Radioactive Dating](https://lightmysky.com/learn/science/activity-half-life-and-radioactive-dating-mt_LvZw4h5wlu): Activity is the decay constant times the number of nuclei, so it falls on the same curve as the nuclei do, and the half-life is ln 2 divided by the decay constant. Comparing the activity left with the activity at the start dates a sample.
- [Crystal Lattices, the Reciprocal Lattice and Bloch's Theorem](https://lightmysky.com/learn/science/crystal-lattices-the-reciprocal-lattice-and-blochs-theorem-mt_nbNnHpGnZI): A perfect crystal repeats, so its electron states can be labelled by a wavevector and written as a plane wave times a function with the lattice's own period. The reciprocal lattice is the bookkeeping that makes diffraction and band structure the same statement.
- [Band Structure: Metals, Insulators and the Gap](https://lightmysky.com/learn/science/band-structure-metals-insulators-and-the-gap-mt_9f74Vu6qnW): A periodic potential opens gaps in the energy spectrum where the Bragg condition is met, and whether the highest occupied band is full or partly filled decides whether the solid conducts. The free electron gas is the limit where those gaps are ignored.
- [Semiconductors, Doping and the p-n Junction](https://lightmysky.com/learn/science/semiconductors-doping-and-the-p-n-junction-mt_iKlnfAqDAg): A small gap lets thermal excitation put carriers into the conduction band, and impurities decide which carrier dominates. Joining two differently doped regions builds a field at the boundary that lets current pass one way.
- [Phonons and the Heat Capacity of Solids](https://lightmysky.com/learn/science/phonons-and-the-heat-capacity-of-solids-mt_WXZPdz_JTd): Lattice vibrations quantise into modes whose occupation follows the Bose distribution, so a solid's heat capacity falls away at low temperature instead of staying constant. Counting modes up to a cutoff gives the cubic law that measurements show.
- [Superconductivity: the Meissner Effect and Cooper Pairs](https://lightmysky.com/learn/science/superconductivity-the-meissner-effect-and-cooper-pairs-mt_2H9bAiAAUV): Below a critical temperature some metals expel magnetic flux completely, which is a stronger claim than zero resistance. A weak attraction carried by the lattice binds electrons into pairs that share one quantum state, and a gap opens at the Fermi level.
- [The Rock Cycle](https://lightmysky.com/learn/science/the-rock-cycle-mt_NckKLZ3uCE): Explain the rock cycle: how igneous rocks form from magma, sedimentary rocks from compressed sediment, and metamorphic rocks from heat and pressure, and how all rock types can transform into one another over geological time