Chemistry
🔒 Log in to trackMatter, separation methods and atomic structure
🔒 Log in to trackStates and changes of matter
- Solid, liquid, gas; plasma is the 4th state (Sun, stars, lightning, neon signs); Bose–Einstein condensate is the 5th (predicted by S.N. Bose and Einstein).
- Sublimation — solid turns directly into vapour: camphor, naphthalene balls, iodine, ammonium chloride, dry ice (solid CO₂).
- Physical change is reversible, with no new substance (melting ice, dissolving sugar). Chemical change makes a new substance (rusting, burning, curdling of milk, digestion, cooking).
Mixtures and their separation
| Method | Used for |
|---|---|
| Chromatography | Separating dyes in ink, pigments, drugs in blood |
| Distillation | Liquids with widely different boiling points; pure water |
| Fractional distillation | Components of petroleum and of liquefied air |
| Centrifugation | Cream from milk, blood tests |
| Sublimation | Salt + camphor / ammonium chloride |
| Crystallisation | Pure salt from sea water, pure copper sulphate |
| Separating funnel | Immiscible liquids (oil + water) |
- Colloids show the Tyndall effect (scattering of a light beam) and Brownian motion. Examples: milk (emulsion — fat in water), fog, clouds (liquid in gas — aerosol), smoke (solid in gas), butter (water in fat).
- True solutions (sugar or salt in water) do not show the Tyndall effect.
Structure of the atom
| Particle | Discovered by | Charge |
|---|---|---|
| Electron | J.J. Thomson (1897, cathode rays) | −1 |
| Proton | Canal rays — E. Goldstein; identified by Rutherford | +1 |
| Neutron | James Chadwick (1932) | 0 |
| Nucleus | Rutherford (gold-foil / α-scattering) | + |
- Dalton — atomic theory (atom indivisible). Thomson — 'plum pudding' model. Rutherford — nuclear model. Bohr — fixed orbits/energy levels.
- Atomic number (Z) = number of protons; mass number (A) = protons + neutrons.
- Isotopes — same Z, different A (¹H protium, ²H deuterium, ³H tritium — the radioactive one; C-12 and C-14). Isobars — same A, different Z (e.g. ⁴⁰Ar and ⁴⁰Ca). Isotones — same number of neutrons.
- Shells hold at most 2n² electrons: K = 2, L = 8, M = 18. Valence electrons decide chemical behaviour.
- Avogadro number = 6.022 × 10²³ particles per mole.
- Gas laws: Boyle — P ∝ 1/V (T constant); Charles — V ∝ T (P constant); Avogadro — equal volumes of gases at the same T and P have equal numbers of molecules.
Detailed notes
What is matter, and what forms does it take?
Anything that has mass and occupies space is matter. Besides the familiar solid, liquid and gas, science knows two more states: plasma (ionised gas — the Sun, stars, lightning, neon signs) and the Bose–Einstein condensate (matter cooled almost to absolute zero; predicted by S.N. Bose and Einstein). Changes of state: melting, boiling, condensation, freezing, and sublimation — a solid passing straight to vapour. The sublimation set is a fixed exam list: camphor, naphthalene balls, iodine, ammonium chloride, dry ice (solid CO₂). A physical change changes only the form and is reversible (melting ice, dissolving sugar). A chemical change makes a new substance that cannot be brought back by simple means — burning, rusting, curdling of milk, cooking, digestion.
Mixtures and how we separate them
A mixture keeps the properties of its parts; a compound does not. Separation method ↔ job:
| Method | Used for |
|---|---|
| Chromatography | Dyes in ink, pigments, drugs in blood |
| Distillation | Two liquids with very different boiling points (pure water from salt water) |
| Fractional distillation | Liquids boiling close together — petroleum, liquid air |
| Centrifugation | Cream from milk, blood/urine tests |
| Sublimation | Salt mixed with camphor or ammonium chloride |
| Crystallisation | Pure salt/copper sulphate crystals from a solution |
| Separating funnel | Immiscible liquids — oil and water |
| Filtration / evaporation / magnetic separation | Insoluble solid / dissolved solid / iron filings from sand |
Solutions, suspensions, colloids
A solution (sugar in water) is clear and never settles. A suspension (muddy water) is cloudy and settles on standing. A colloid sits between the two: particles too small to see, big enough to scatter a beam of light — the Tyndall effect — and they jiggle constantly (Brownian motion). Colloid types to remember: milk = emulsion (liquid in liquid), fog/clouds = aerosol (liquid in gas), smoke = solid in gas, shaving cream = foam (gas in liquid), jelly = gel (liquid in solid).
Inside the atom
- Dalton proposed that matter is made of indivisible atoms; J.J. Thomson found the electron and suggested a plum-pudding model; Rutherford's gold-foil experiment showed a tiny dense nucleus; Bohr added fixed orbits (shells); Chadwick found the neutron.
- Atomic number Z = number of protons (the identity of the element). Mass number A = protons + neutrons.
- Isotopes — same Z, different A: hydrogen has protium (¹H), deuterium (²H), tritium (³H); chlorine is ³⁵Cl and ³⁷Cl (average mass 35.5); carbon-14 dates old remains. Isobars — same A, different Z (e.g. calcium-40 and argon-40).
- Electrons fill shells by the 2n² rule: K holds 2, L holds 8, M holds 18; the outermost electrons are valence electrons; a full outer shell (octet) is stable.
Counting particles — the mole
One mole of any substance holds 6.022 × 10²³ particles (Avogadro's number) and weighs its molecular mass in grams. So 18 g of water = 1 mole, and 36 g = 2 moles; 44 g of CO₂ = 1 mole. Boyle's law: at constant temperature, pressure × volume stays fixed (squeeze a gas, pressure rises). Charles's law: at constant pressure, volume grows in step with the kelvin temperature. Together: PV = nRT.
Quick revision
- 5 states: solid, liquid, gas, plasma, Bose–Einstein condensate.
- Sublimation set: camphor, naphthalene, iodine, ammonium chloride, dry ice.
- Chromatography–dyes; fractional distillation–petroleum; centrifuge–cream; funnel–oil and water.
- Colloid = Tyndall + Brownian; milk = emulsion, fog = aerosol, smoke = solid in gas.
- Electron–Thomson, nucleus–Rutherford, orbits–Bohr, neutron–Chadwick.
- 1 mole = 6.022 × 10²³ particles; 18 g water = 1 mol; PV = nRT.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Physical vs chemical change; sublimation and statesvery common3 practice Q
'Which is a chemical change?', 'which sublimes?', 4th/5th state of matter, 'dry ice is…'.
- New substance formed → chemical change (burning, rusting, curdling, cooking, digestion); only form changed → physical (melting, dissolving).
- Sublimation list: camphor, naphthalene, iodine, ammonium chloride, dry ice — nothing else sublimes at room pressure.
- Plasma = ionised gas in the Sun, lightning, neon signs; BEC = near absolute zero. Why: the two change classes differ only in whether a NEW substance appears.
Example: Burning of a candle involves —
Both changes: melting of wax is physical, burning of the vapour is chemical.
Type 2: Separation method ↔ mixturevery common3 practice Q
'Which method separates cream from milk / components of petroleum / dyes in ink?', match-the-column of methods and mixtures.
- Match by property: different boiling points → distillation (close boiling points → fractional, e.g. petroleum); different densities → centrifugation (cream, blood); different solubility/movement → chromatography (dyes).
- Immiscible liquids → separating funnel; dissolved solid → evaporation/crystallisation; magnetic material → magnetic separation; sublimable solid in a mixture → sublimation.
- Cream-from-milk is ALWAYS centrifugation in exams.
Example: The components of petroleum are separated by —
Fractional distillation — the fractions boil only a few degrees apart, so a fractionating column is needed.
Type 3: Colloids, Tyndall effect and Brownian motioncommon2 practice Q
'Milk is an example of…', 'scattering of light by colloidal particles is called…', which of these shows the Tyndall effect.
- Classify by dispersed phase and medium: milk = liquid in liquid = emulsion; fog/cloud = liquid in gas = aerosol; smoke = solid in gas; shaving cream = foam; jelly = gel.
- Light scattering by colloidal particles = Tyndall effect; zig-zag motion of colloidal particles = Brownian motion.
- True solutions do NOT show the Tyndall effect; suspensions do but settle down.
Example: Fog is a colloid in which —
Water droplets (liquid) are dispersed in air (gas) — an aerosol.
Type 4: Atomic models and discovererscommon2 practice Q
'Who proposed the plum-pudding / nuclear / planetary model?', 'which experiment discovered the nucleus?', order of models in history.
- Timeline: Dalton (atoms indivisible) → Thomson (electron; plum pudding) → Rutherford (gold foil; nucleus) → Bohr (fixed orbits) → Chadwick (neutron).
- Gold-foil: most alpha particles passed, few bounced → atom is mostly empty with a tiny dense positive nucleus.
- Bohr's orbits are the K, L, M… shells filled by the 2n² rule (2, 8, 18…).
Example: Rutherford's gold-foil experiment proved that —
The atom has a tiny, dense, positively charged nucleus — most alpha particles passed straight through the foil.
Type 5: Isotopes, isobars, atomic number and shellscommon3 practice Q
'Same atomic number but different mass number = ?' hydrogen isotopes, average mass of chlorine, electrons in a given shell for element X.
- Isotopes: same Z, different A (¹H, ²H, ³H; ³⁵Cl and ³⁷Cl → average 35.5).
- Isobars: same A, different Z (Ca-40, Ar-40).
- Shell filling 2n²: K = 2, L = 8, M = 18; for calcium (Z = 20) the arrangement is 2, 8, 8, 2.
Example: How many electrons can the M shell hold at most?
2n² with n = 3 → 2 × 9 = 18.
Type 6: Mole and gas-law numericalsoccasional3 practice Q
Moles from a given mass, number of molecules, or a Boyle/Charles volume-pressure-temperature change.
- Find molar mass first: H₂O = 18, CO₂ = 44. Moles = given mass ÷ molar mass.
- Boyle: multiply each state's P and V, equate the two products.
- Charles: convert temperatures to kelvin (add 273) before dividing. Why: a mole is just a counting unit, and the gas laws are simple proportionalities.
Example: A gas occupies 600 mL at 1 atm. What pressure will squeeze it into 400 mL at the same temperature?
P₁V₁ = P₂V₂ → 1 × 600 = P₂ × 400 → P₂ = 1.5 atm.
Formulas
M = molar mass in g/mol
Avogadro number
temperature constant
pressure constant, T in kelvin
R = 8.314 J mol⁻¹ K⁻¹
K = 2, L = 8, M = 18
Shortcut tricks
⚡ Iso-words by the letter
Isotopes — same atomic number (same protons, 'P for isoto-P-es'). Isobars — same A (mass number). Isotones — same neutrons.
Example: Deuterium and tritium are?
Isotopes of hydrogen — same atomic number 1, different mass numbers.
⚡ Sublimation set: 'Camphor, Naphthalene, Iodine, Ammonium chloride, Dry ice' — 'CNIAD'
If an option is one of these, it is the sublimating substance.
Where students lose marks
Calling milk a true solution — it is a colloid (emulsion).
Saying tritium is stable — it is the radioactive isotope of hydrogen.
Treating rusting or curdling as a physical change — both are chemical changes.
Practice sets — 18 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.