Chemistery

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 ...

Every solution question starts with the same small decision, and it trips up more students than the calculations that follow: which one is the solute and which is the solvent? Get it backwards and your concentration answer is upside down. The short answer: the solute is the substance that gets dissolved ; the solvent is the substance that does the dissolving . Together they make a solution , and the solvent is normally whichever one is present in the greater amount. Quick comparison at a glance Feature Solute Solvent Role Gets dissolved Does the dissolving Amount present Usually the smaller amount Usually the larger amount State after mixing Takes on the solvent's state Keeps its own state Common state Often solid, but can be any Usually liquid Everyday example Salt, sugar, CO₂ Water, ethanol, acetone In salt water Salt (NaCl) Water Changes when mixed? Broken up into particles/ions Stays as it is What a solute is A solute is wha...

Salt vanishes into water in seconds. Oil sits on top and sulks no matter how hard you stir. Sugar dissolves faster in hot tea than iced tea. All three are the same question wearing different clothes: what makes something dissolve? The short answer: solubility is the maximum amount of a substance that will dissolve in a given amount of solvent at a particular temperature. Whether something dissolves comes down to one rule — "like dissolves like" — because polar solvents pull apart polar and ionic solutes, and nonpolar solvents dissolve nonpolar solutes. What solubility actually measures Solubility is a number, not a yes/no. It's usually quoted as grams of solute per 100 g of water at a stated temperature : Table salt, NaCl: about 36 g per 100 g of water at 25 °C Sugar (sucrose): about 200 g per 100 g of water at 20 °C Chalk, CaCO₃: about 0.0013 g per 100 g of water at 25 °C Notice that chalk isn't strictly insoluble — it's just so poorly soluble that w...