Chemistery

Dissolve table salt in water and the pH stays at 7. Dissolve the same amount of ammonium chloride and the pH drops to about 5. Both are salts, both came from a neutralisation, and neither contains an obvious acid. The explanation is that "neutralisation" does not always produce something neutral. The short answer: a salt's solution is acidic if it came from a strong acid and a weak base , basic if it came from a weak acid and a strong base , and neutral if both parents were strong . The reason is hydrolysis — ions left over from a weak parent react with water and shift the pH. Quick comparison at a glance Parent acid Parent base Resulting solution Example Approx. pH of 0.10 M Strong Strong Neutral NaCl, KNO₃ 7.0 Strong Weak Acidic NH₄Cl, NH₄NO₃ 5.1 Weak Strong Basic CH₃COONa (8.9), NaF (8.1) 8–9 Weak Weak Depends on Ka vs Kb CH₃COONH₄ ≈ 7.0 Why any salt changes the pH A salt dissolves into two ions, and the question is wheth...

Ask what water is and the honest answer is "it depends what you put it with". Drop in hydrogen chloride and water behaves as a base. Bubble in ammonia and the same water behaves as an acid. Substances that can go either way have a name, and they turn up far more often than most students expect. The short answer: an amphoteric substance can act as either an acid or a base , depending on what it meets. Water is the classic example — it donates a proton to ammonia and accepts one from HCl — and so are bicarbonate, HCO₃⁻, and oxides such as Al₂O₃ and ZnO. What "amphoteric" actually means The word comes from the Greek amphoteros , "both of two". A substance is amphoteric if it has a reaction available in both directions: something it can give away to a proton-hungry partner (a base), and something it can take from a proton donor (an acid). There is a narrower term you will meet alongside it. Amphiprotic means specifically that the substance can both don...

Acetic acid, CH₃COOH, contains four hydrogen atoms and donates exactly one. Phosphoric acid, H₃PO₄, contains three and donates all three — but not equally, and not at the same pH. Counting acidic hydrogens correctly is the difference between a titration answer that works and one that is out by a factor of two or three. The short answer: a monoprotic acid can donate one proton per molecule; a polyprotic acid can donate more than one , losing them one at a time in separate steps with steadily smaller Ka values. Acidic hydrogens are the ones attached to an electronegative atom — most often O , sometimes N , S or a halogen — and never the ones on a plain C–H bond. Quick comparison at a glance Feature Monoprotic Polyprotic Protons donated Exactly 1 2 or more — diprotic (2), triprotic (3), and beyond Ionisation steps One One per proton, sequentially Ka values A single Ka Ka₁ > Ka₂ > Ka₃ Equivalence points in titration One Up to one per ionisable prot...

Pure water is supposed to be the definition of neutral. So why does a pH meter dipped into boiling distilled water read about 6.1 — and why is that water still neutral? The answer is one small constant that quietly underpins the entire pH scale. The short answer: Kw is the ion product of water , the equilibrium constant for water reacting with itself: Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C . Because that product is fixed at a given temperature, knowing either ion concentration always gives you the other. Water reacts with itself Water is amphiprotic — it can donate a proton and accept one. Given enough molecules, some of them do both to each other: 2 H₂O ⇌ H₃O⁺ + OH⁻ This is called autoionisation (or self-ionisation). It happens in every sample of water, and in every aqueous solution, all the time. It is just extremely rare: at 25 °C only about one water molecule in 555 million is ionised at any instant. The equilibrium constant for that reaction, with the solvent water fold...

Everyone learns "NaOH is strong, ammonia is weak" and then hits a question about 0.010 M NaOH versus 6 M ammonia and freezes. Which one has the higher pH? The answer needs the difference between strength and concentration — and this is the pair of words that decides it. The short answer: a strong base produces hydroxide ions essentially completely in water, so its OH⁻ concentration equals its concentration. A weak base only partly reacts, sitting at an equilibrium described by Kb , so its OH⁻ concentration is far lower than its concentration. Quick comparison at a glance Feature Strong base Weak base Behaviour in water Reacts/dissociates essentially completely Partly reacts; equilibrium Arrow used Single, → Double, ⇌ Kb Not applicable — it dissociates, it does not equilibrate with water Small and measurable [OH⁻] ≈ the base's concentration Much less than its concentration Typical pH of 0.10 M 13.0 ~11.1 Examples NaOH, KOH, Ba(OH)₂...

"Strong" and "weak" are useful words right up to the moment someone asks how weak. Is hydrofluoric acid weaker than formic acid? By how much? That is what Ka is for: it turns a vague adjective into a number you can calculate with. The short answer: Ka , the acid dissociation constant , is the equilibrium constant for an acid giving up its proton to water. A larger Ka means a stronger acid — at the same concentration, more of it is ionised at equilibrium — and because the numbers are tiny, they are usually quoted as pKa = −log Ka , where a smaller pKa means a stronger acid . What Ka actually is Every weak acid in water sets up an equilibrium: HA + H₂O ⇌ H₃O⁺ + A⁻ Write the equilibrium expression for it and you get Ka: Ka = [H₃O⁺][A⁻] ÷ [HA] Water does not appear, even though it is a reactant. It is the solvent, present in enormous excess, and its concentration barely changes — so by convention it is folded into the constant rather than written out. Read th...

You write out a reaction, the mark scheme says "identify the conjugate base", and suddenly every species on the page looks like a candidate. The good news is that the answer is never a judgement call. There is a mechanical test, and it takes about three seconds. The short answer: a conjugate base is what is left of an acid after it donates a proton (H⁺); a conjugate acid is what a base becomes after it accepts one. The two members of a conjugate pair always differ by exactly one H⁺ — one hydrogen and one unit of charge — and nothing else. Quick comparison at a glance Feature Conjugate acid Conjugate base Formed by A base gaining H⁺ An acid losing H⁺ Change in formula One H added One H removed Change in charge More positive by 1 More negative by 1 Where to find it Opposite the base it came from Opposite the acid it came from Example NH₃ → NH₄⁺ CH₃COOH → CH₃COO⁻ Its own role Can donate that H⁺ back Can accept an H⁺ back What ...

Before 1892 the same compound might be called four different things in four different countries, and chemists genuinely could not be sure they were discussing the same substance. IUPAC nomenclature fixed that: a set of rules that turns any structure into exactly one name, and any name back into exactly one structure. The short answer: IUPAC nomenclature is the international system for naming chemical compounds unambiguously. For organic molecules it builds every name from three parts — a prefix listing the substituents, a root giving the longest carbon chain, and a suffix naming the main functional group — so the name is a set of assembly instructions. The anatomy of a name Take 2-methylbutan-1-ol and cut it apart: Piece What it says 2-methyl Prefix — a CH₃ branch on carbon 2 but Root — the main chain is 4 carbons long an All single bonds 1-ol Suffix — an –OH group on carbon 1 Read backwards and you can draw it without ever having seen the compound. That is the en...

Once you accept that one molecular formula can describe several compounds, the next question arrives immediately: how many kinds of difference are there? Two molecules can be built from the same atoms and differ in the wiring, or they can have identical wiring and differ only in how they sit in space. That split is the whole of isomer classification. The short answer: structural isomers have their atoms bonded together in a different order , while stereoisomers have exactly the same bonding order but a different arrangement in three-dimensional space. The quick test: list which atom is bonded to which for both molecules — if the lists differ it's structural, if they match it's stereo. Quick comparison at a glance Feature Structural isomers Stereoisomers Also called Constitutional isomers Configurational isomers What differs Which atom is bonded to which Spatial arrangement only Connectivity Different Identical Subtypes Chain, position, functional group Geometric...

Here is a fact that quietly breaks a rule you learned early on. Two bottles on a shelf: one holds a gas that boils at −0.5 °C, the other a gas that boils at −11.7 °C. They are different compounds with different properties. And both have the molecular formula C₄H₁₀ . The short answer: isomers are compounds with the same molecular formula but different arrangements of atoms . Because chemical behaviour depends on how atoms are connected and positioned in space — not merely on how many of each there are — isomers are genuinely different substances with different physical and chemical properties. What a molecular formula leaves out A molecular formula is an inventory: C₄H₁₀ says four carbons and ten hydrogens. It says nothing about which carbon is joined to which. Once a molecule has four or more carbons, there is usually more than one legal way to wire them up. For C₄H₁₀ there are exactly two: Butane — a straight chain, CH₃–CH₂–CH₂–CH₃. Boils at −0.5 °C. 2-methylpropane — a th...

Cyclohexane and benzene look almost like the same molecule: both are six-carbon rings, differing by six hydrogens. Yet cyclohexane is a forgettable solvent and benzene is a whole separate branch of organic chemistry with its own rules and its own reactions. The word that separates them is aromatic . The short answer: aromatic compounds contain a flat, fully conjugated ring whose π electrons are delocalised around the whole ring, giving unusual stability; aliphatic compounds are everything else organic — chains, branches and non-aromatic rings. The practical consequence is that aromatic rings undergo substitution to preserve their delocalisation, while unsaturated aliphatic compounds undergo addition . Quick comparison at a glance Feature Aliphatic Aromatic Structure Open chains, branches, non-aromatic rings At least one aromatic ring Includes Alkanes, alkenes, alkynes, cycloalkanes and cycloalkenes Benzene, toluene, naphthalene, pyridine π electrons Localised in individua...

There are more than a hundred million known organic compounds. Nobody memorises how each one behaves — and nobody has to, because the chemistry of an organic molecule almost never depends on the whole molecule. It depends on one small reactive patch. That patch is the functional group . The short answer: a functional group is a specific arrangement of atoms within a molecule that is responsible for that molecule's characteristic chemical reactions. Molecules sharing the same functional group react in the same ways, which is why organic chemistry can be learned as a few dozen groups instead of millions of compounds. What "functional" really means Think of a hydrocarbon chain as scaffolding. It is nonpolar, unreactive toward most reagents, and chemically boring — which is exactly what makes it useful as a backbone. Now swap one hydrogen for an –OH group and everything changes: the molecule can hydrogen-bond, its boiling point jumps, a short-chain one dissolves in wate...