Chemistery

Both sit in the same drawer in every school lab, both change colour in acid, and both get called "indicator paper". But one of them answers a yes/no question and the other gives you a number — and using the wrong one costs marks. The short answer: litmus only tells you whether a solution is acidic or basic — red in acid, blue in base — while universal indicator is a mixture of dyes that produces a whole range of colours, telling you approximately how acidic or basic it is on the pH scale. Quick comparison at a glance Feature Litmus Universal indicator What it tells you Acid or base (yes/no) Approximate pH value Number of indicators One (a lichen extract) A mixture of several Colours Red and blue only Red → orange → yellow → green → blue → purple In acid Red Red/orange/yellow In neutral No reliable colour Green In base Blue Blue/purple Precision None — just a category About ±1 pH unit Best used for A quick yes/no check Estimating pH Use in titrations Rarely No...

Add one drop of strong acid to a glass of pure water and the pH crashes from 7 to about 3. Add the same drop to your bloodstream and nothing happens — your pH shifts by maybe a hundredth of a unit. The difference is a buffer, and without it you would not survive lunch. The short answer: a buffer is a solution that resists changes in pH when small amounts of acid or base are added. It works because it contains a weak acid and its conjugate base side by side, one ready to mop up added OH⁻ and the other ready to mop up added H⁺. What a buffer actually is A buffer needs two components present in comparable amounts: a weak acid (call it HA), and its conjugate base (A⁻) — the same species minus its proton. The classic laboratory buffer is acetic acid (CH₃COOH) with sodium acetate (CH₃COONa) . The acetic acid supplies HA; the sodium acetate dissolves to supply A⁻. The pair works because a weak acid sits in equilibrium rather than committing one way: CH₃COOH ⇌ H⁺ + CH₃COO⁻ Having...

A chemistry revision sheet is most useful when you do something with it. Read the explanation, cover it, answer a question, then check where your reasoning broke down. That gives you a specific job for your next study session. Here's a five-minute routine you can try with the free Strong Acids & Bases sheet from 5 Minutes in Chemistry . The sheet has a short explanation, a reference chart, facts to remember and three self-test questions. You only need one printed page, a pen and something to cover the answers. Download the free Strong Acids & Bases sheet The download includes US Letter and A4 PDFs. Choose your paper size and print one copy. The actual free sample. The full series contains 56 revision sheets in this one-page format. 0:00–1:00 — Read one idea Start with the short explanation at the top. On this sheet, the central distinction is what “strong” means in acid–base chemistry. Read it once, then look away and explain it in your own words. If you can only ...

You've got comfortable with pH, and then a question asks for pOH and it feels like a whole new topic. It isn't. pOH is the same idea pointed at the other ion, and the two are locked together by one equation. The short answer: pH measures the concentration of hydrogen ions (H⁺) in a solution, while pOH measures the concentration of hydroxide ions (OH⁻). In water at 25 °C they are always linked by pH + pOH = 14 , so knowing either one gives you the other by subtraction. Want to try a printable revision sheet? The free Strong Acids & Bases sample gives you a one-page reference, a chart and a short self-test before you move on to pH calculations. Quick comparison at a glance Feature pH pOH Measures H⁺ concentration OH⁻ concentration Formula pH = −log[H⁺] pOH = −log[OH⁻] Low value means Acidic Basic (alkaline) High value means Basic (alkaline) Acidic Neutral water at 25 °C 7 7 Typical range 0–14 14–0 Which is quoted in daily life Almost always this one Rarely Rel...

You've almost certainly done a neutralization on purpose without calling it that — taking an antacid for heartburn, or putting baking soda on a spill. It's one of the few reactions in the syllabus you can watch working in your own kitchen. The short answer: neutralization is the reaction between an acid and a base that produces a salt and water . The acid's H⁺ ions and the base's OH⁻ ions combine to form H₂O, which removes the ions responsible for acidity and alkalinity and moves the pH toward neutral. What neutralization actually is Every neutralization follows the same shape: acid + base → salt + water The classic example: HCl + NaOH → NaCl + H₂O Hydrochloric acid plus sodium hydroxide gives table salt and water. The Na⁺ and Cl⁻ ions never really do anything — they float around unchanged. Strip them out and you're left with the reaction that matters, the net ionic equation : H⁺ + OH⁻ → H₂O That single line is neutralization. Everything else is bookkeepi...

Here's a question that catches people out in exams every year: which is more dangerous, hydrochloric acid or hydrofluoric acid? Most students say hydrochloric, because it's the "strong" one. They're wrong — and the reason why is the whole point of this topic. The short answer: a strong acid dissociates completely in water, releasing all of its H⁺ ions, while a weak acid only dissociates partially, leaving most of its molecules intact. "Strong" describes how completely an acid ionises , not how corrosive, concentrated or dangerous it is. Quick comparison at a glance Feature Strong acid Weak acid Dissociation in water Complete (~100%) Partial (often <5%) Equation arrow Single → Reversible ⇌ Ions in solution Many H⁺ Few H⁺ pH at 0.1 mol/L About 1 About 3 (acetic acid: 2.9) Electrical conductivity High Low Ka value Very large Small (acetic acid: 1.75 × 10⁻⁵) Reaction with magnesium Fast, vigorous fizzing Slower fizzing Examples HCl, HNO₃, H₂...

Keep stirring sugar into a glass of iced tea and at some point it stops disappearing. A little pile settles at the bottom and simply refuses to go. The water hasn't got tired — it's full. The short answer: a saturated solution contains the maximum amount of solute that will dissolve in a given solvent at a given temperature. Add more and it just sits there undissolved, because dissolving and re-crystallising are happening at exactly the same rate. The three kinds of solution Type How much solute What you see Stable? Unsaturated Less than the maximum More solute still dissolves when added Yes Saturated Exactly the maximum Extra solute sits undissolved at the bottom Yes Supersaturated More than the maximum Clear, but crystallises at the slightest disturbance No — metastable What "saturated" really means The undissolved crystals at the bottom of a saturated solution aren't doing nothing. At the microscopic level, two processes are ...

A puddle on the pavement disappears by lunchtime, and it never got anywhere near 100 °C. So if water has to reach 100 °C to become a gas — how did the puddle manage it? The short answer: evaporation happens only at a liquid's surface , at any temperature , as fast-moving molecules escape one at a time. Boiling happens throughout the liquid, only at the boiling point , when bubbles of vapour can form inside it. Both are vaporisation; they just differ in where and when. Quick comparison at a glance Feature Evaporation Boiling Where it happens Surface only Throughout the whole liquid Temperature Any temperature below boiling point Only at the boiling point Speed Slow and gradual Rapid Bubbles? No Yes — vapour bubbles rise and burst Energy source Heat drawn from the surroundings Heat supplied continuously Effect on the liquid Cools it down Temperature stays constant Everyday example A puddle drying; sweat on skin A kettle at full boil ...

Put a thermometer in a pan of melting ice and watch it. You're pouring heat in, and the reading refuses to move off 0 °C until the last ice cube is gone. Where is all that energy going? The short answer: a phase change is a physical change in which a substance moves between solid, liquid and gas — melting, freezing, boiling, condensing, subliming or depositing. During a phase change the temperature stays constant, because the energy goes into breaking the attractions between particles rather than speeding them up. The six phase changes There are three states, so there are six one-way trips between them: Change From → To Everyday example Melting (fusion) Solid → Liquid Ice turning to water Freezing (solidification) Liquid → Solid Water turning to ice Vaporisation (boiling/evaporation) Liquid → Gas Water becoming steam Condensation Gas → Liquid Mist forming on a cold window Sublimation Solid → Gas Dry ice fogging without melting Deposition ...