OpenStem

@openstem · Joined Jul 2026

7420 public items8 groups

Note~536 words · 3 min

Two forces, four trends The periodic table isn't just a list of elements — moving across it in either direction changes an atom's properties in predictable ways. Every one of those changes comes down to a tug-of-war between two things: how

Chemistry

Chemistry · L3 · Periodic Trends

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Note~633 words · 3 min

Ionic and Covalent Bonding Atoms bond because a full outer shell of electrons is more stable than a partly-full one. There are two main ways atoms reach that stability together: by TRANSFERRING electrons (ionic bonding) or by SHARING electr

Chemistry

Chemistry · L3 · Ionic and Covalent Bonding

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Note~511 words · 3 min

Why molecules have shapes A molecular formula like H₂O or CH₄ tells you which atoms are present, but not how they are arranged in space. VSEPR theory (Valence Shell Electron Pair Repulsion) predicts the actual 3D shape from something you al

Chemistry

Chemistry · L3 · Molecular Geometry & VSEPR

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Note~512 words · 3 min

The mole: a counting unit for atoms A dozen always means 12, no matter whether you're counting eggs or bicycles. A mole is the same idea, scaled up for atoms and molecules: it always means 6.02 × 10²³ particles — a number called Avogadro's

Chemistry

Chemistry · L3 · Stoichiometry & the Mole

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Note~312 words · 2 min

Squeeze It, Heat It: How Gases Respond A trapped gas behaves in very predictable ways when you squeeze it or heat it. Two simple rules — Boyle's Law and Charles's Law — describe almost everything you need for calculations at this level. Boy

Chemistry

Chemistry · L3 · Gas Laws

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Note~425 words · 2 min

Watching pH climb In a titration you add a solution of known concentration (the titrant, here a strong base) drop by drop from a burette into a flask holding a fixed volume of the unknown acid. Plotting the flask's pH against the volume of

Chemistry

Chemistry · L3 · Reading a Titration Curve

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Note~451 words · 2 min

Thermochemistry basics: measuring and mapping reaction heat Every chemical reaction absorbs or releases energy, usually as heat. Thermochemistry starts with two simple ideas: we can MEASURE that heat with a thermometer, and we can DRAW it a

Chemistry

Chemistry · L3 · Thermochemistry Basics

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Note~366 words · 2 min

Reaction rates: what speeds a reaction up? The rate of a reaction is how quickly reactants turn into products — equivalently, how quickly a reactant's concentration falls. Some reactions (rusting) take years; others (an explosion) take mill

Chemistry

Chemistry · L3 · Reaction Rates & Kinetics

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Note~345 words · 2 min

Reversible reactions reach a moving balance Most reactions you meet early on run one way and stop. A reversible reaction, written with a ⇌ instead of a →, can run in BOTH directions at once under the same conditions: reactants turn into pro

Chemistry

Chemistry · L3 · Chemical Equilibrium & Le Chatelier's Principle

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Note~580 words · 3 min

Oxidation and reduction: it's all about electrons Every redox reaction is really two things happening at once: one substance loses electrons, and another substance gains them. Chemists remember the direction with a mnemonic — OIL RIG: The t

Chemistry

Chemistry · L3 · Redox Reactions & Electrochemistry

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Note~387 words · 2 min

Carbon's Special Talent Carbon forms four covalent bonds — and it can use those bonds to link to other carbon atoms, over and over, building long chains, branches, and rings. Almost no other element does this so readily. That one property i

Chemistry

Chemistry · L3 · Organic Chemistry: Functional Groups & Naming

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Note~405 words · 2 min

Unstable Nuclei Most nuclei are stable and never change. Some are not — they have too many or too few neutrons for their number of protons, and they shed particles or energy until they settle into a more stable arrangement. This process is

Chemistry

Chemistry · L3 · Nuclear Chemistry

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Note~513 words · 3 min

Molarity: putting a number on concentration Chapter L2 covered concentrated and dilute qualitatively — more solute per solvent feels stronger, tastes stronger, reacts faster. Chemistry needs an exact number for that idea, and molarity (M) i

Chemistry

Chemistry · L3 · Solutions & Molarity

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Note~704 words · 4 min

Intermolecular Forces A covalent bond is a strong force WITHIN a molecule, holding its atoms together. An intermolecular force (IMF) is a much weaker force BETWEEN separate molecules. IMFs are what a substance has to overcome to melt or boi

Chemistry

Chemistry · L3 · Intermolecular Forces

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Note~560 words · 3 min

What an oxidation number is An oxidation number (or oxidation state) is a bookkeeping charge assigned to an atom — the charge it would have if every bond to a different element were fully ionic. It isn't always a real, physical charge, but

Chemistry

Chemistry · L3 · Oxidation States & Balancing Redox Equations

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Note~508 words · 3 min

Two ways to build a polymer A polymer is a long chain of repeating monomers — that much you already know. What matters now is HOW the monomers actually join up. There are two distinct mechanisms, and knowing which one built a plastic tells

Chemistry

Chemistry · L3 · Two Ways to Build a Polymer

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Note~522 words · 3 min

Identifying the unknown Qualitative analysis asks a different question from most of chemistry so far — not 'how much?' but 'what is it?'. A handful of simple, fast tests — a flame test, a few drops of reagent turning cloudy, a gas relightin

Chemistry

Chemistry · L3 · Flame Tests & Ion Tests

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Note~644 words · 3 min

One cause, three effects "Colligative" properties are properties that depend on HOW MANY particles are dissolved in a solvent — never on what those particles actually are. A teaspoon of salt and a teaspoon of sugar, dissolved so they releas

Chemistry

Chemistry · L3 · Colligative Properties

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Note~813 words · 4 min

Why Pure Metals Are Malleable In a pure metal, every atom is the same size and packs into a regular, repeating arrangement of layers — think of identical marbles stacked neatly in rows. The outer electrons of every atom are delocalised into

Chemistry

Chemistry · L3 · Metallic Bonding and Why Alloys Are Harder

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Note~851 words · 4 min

Two industrial equilibria, one engineering compromise It's tempting to assume a chemical plant runs at whatever temperature and pressure gives the equilibrium the highest possible yield. In practice, almost none do. Real industrial conditio

Chemistry

Chemistry · L3 · Industrial Equilibrium — Haber and Contact Processes

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Note~636 words · 3 min

What a buffer does A buffer solution resists changes in pH when small amounts of acid or base are added to it. Add a splash of acid to plain water and the pH crashes; add that same splash to a buffer and the pH barely moves. Buffers do not

Chemistry

Chemistry · L3 · Buffers — Resisting pH Change

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Note~595 words · 3 min

Entropy and the second law: why things spread out Entropy is a way of measuring how spread out or disordered energy and matter are. A neat, concentrated arrangement has LOW entropy; a jumbled, spread-out arrangement has HIGH entropy. You do

Chemistry

Chemistry · L3 · Entropy and the Second Law

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Note~649 words · 3 min

Reaction mechanisms and the rate-determining step The balanced equation for a reaction only shows the starting materials and the final products — it doesn't show HOW they get there. Many reactions don't happen in a single collision at all.

Chemistry

Chemistry · L3 · Reaction Mechanisms and the Rate-Determining Step

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Note~437 words · 2 min

Transition metals: multiple oxidation states and striking color Iron, copper, chromium, manganese, and the other transition metals behave differently from main-group metals like sodium, magnesium, and potassium in two easy-to-spot ways — wo

Chemistry

Chemistry · L3 · Transition metals: variable oxidation states and vivid color

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Note~462 words · 2 min

Combustion of Hydrocarbons: Complete vs Incomplete Complete Combustion: Full Oxygen, Full Burn Hydrocarbons — molecules made only of carbon and hydrogen — are widely used as fuels because burning them releases a lot of useful energy. When t

Chemistry

Chemistry · L3 · Combustion and Cracking of Hydrocarbons

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Note~661 words · 3 min

Same monomer, very different plastic HDPE (high-density polyethylene, recycling code #2) and LDPE (low-density polyethylene, recycling code #4) are both built from exactly the same monomer, ethene. Yet HDPE is rigid enough for a hard hat, w

Chemistry

Chemistry · L3 · Polymer Structure and Properties

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Note~507 words · 3 min

What Is Hard Water? Hard water is water that contains relatively high concentrations of dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). As rainwater flows over and through rock, it dissolves small amounts of minerals out of that ro

Chemistry

Chemistry · L3 · Water Hardness

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Note~569 words · 3 min

Acid Rain: When Air Pollution Falls as Rain Acid rain is rain, snow, or fog that has become far more acidic than normal because of air pollution. It forms when two families of gases — sulfur dioxide (SO2) and nitrogen oxides (NOx) — are rel

Chemistry

Chemistry · L3 · Acid Rain

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Note~378 words · 2 min

Reading rate graphs: order from shape, rate from gradient Two different graphs answer two different questions about a reaction. A RATE-vs-CONCENTRATION graph tells you the reaction order — how strongly rate depends on how much reactant is p

Chemistry

Chemistry · L3 · Reading Rate Graphs

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Note~708 words · 4 min

Avogadro's Law: Same Volume, Same Number of Moles Boyle's Law and Charles's Law both describe a FIXED amount of gas as its pressure, volume, or temperature changes. Avogadro's Law asks a different question: what happens when you compare two

Chemistry

Chemistry · L3 · Avogadro's Law and Molar Gas Volume

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