Ch. 1: Introduction to Chemistry
What chemistry is, elements vs compounds, metals and alloys, alchemy to scientific method, discovery of gases, atom models from Dalton to Bohr — with full interactive lab animations.
Chemistry HQ lab
Watch atoms build water, nails rust, air split into gases, and atom models evolve. Chemistry is a story — not just definitions.
Chemistry = scientific study of substances
What things are made of · how they look · how they behave · how they join to make new stuff.
Lemonade chemistry 🍋
Baking soda makes idli/cake fluffy 🍞
- Burning wood: carbon + oxygen → ash + heat + light + gases
- Living things: carbon with O and H
- Chemistry in food, medicine, soaps, plastics, metals…
Element vs compound — make water (realistic reaction)
In nature, hydrogen and oxygen exist as diatomic element molecules (H2, O2). Water is a compound with a bent shape (~104.5°). Reaction: 2 H2 + O2 → 2 H2O
Before: only elements (H2 and O2). Press react to join them chemically into the compound water.
One kind of atom only. Cannot be broken into simpler substances by ordinary chemistry.
H2 = only H · O2 = only O · also Fe, Au, Al…
Two or more elements chemically bonded in a fixed ratio. New properties appear.
H2O · CO2 · Fe2O3 (rust)
Also: silicates = metal+Si+O · carbonates = metal+C+O · sulphides = metal+S
Origin of chemicals · Earth’s layers & crust
Sun: mostly H + He · Earth ~4.5 billion years old
Most abundant metal in crust: aluminium. Most metals exist as compounds/ores, not free. Unreactive: Ag, Au, Pt (free form).
Metals: rust, ores, reduction & alloys
Iron + oxygen + moisture → iron oxide (rust)
- Ore — rock/mineral with metal compounds (e.g. iron oxide)
- Reduction — remove oxygen (charcoal + heat) to get metal
- Alloy — uniform mix of metals (or metal + non-metal) for better properties
- Gold barely reacts → stays shiny for millennia
From alchemy to modern chemistry
Four “elements”: fire · air · water · earth. Alchemists tried lead → gold.
Bronze = Cu + Sn (+Zn) — harder tools & weapons
Iron from ore with charcoal → steel. Delhi iron pillar (~4th c.) still barely rusts.
Challenged 4-element theory · scientific method · Boyle’s Law (gases)
Scientific method — Boyle style
Push plunger → volume ↓ (air is compressible)
Press syringe plunger → air volume decreases. Pull out → volume increases.
Boyle’s Law: At constant temperature, volume of a fixed mass of dry air varies inversely with pressure.
Discovering the gases in air
Burning candle under inverted jar → water rises. Candle dies when “nitro-aereus” (now oxygen O2) is used up. Same for breathing mouse.
Heating chalk (CaCO3) released “fixed air”. Detected with lime water — turns milky. Also from fermentation, burning, breathing.
Heated mercury → red mercuric oxide; air volume fell by 1/5. Heated oxide → mercury + oxygen back. Air = ~1/5 O2 + ~4/5 N2 (azote).
Mouse + candle under jar die; add mint plant → both survive longer. Plants restore “freshness” of air (photosynthesis: CO2 → O2). Also soda water (CO2 under pressure).
Rare/noble gases (inert, ~1%): He, Ne, Ar, Kr, Xe, Rn — Rayleigh & Ramsay.
Atom models — ideas that evolved
All matter = atoms. An atom is a hard, solid sphere that cannot be divided. Atoms of one element are identical; different elements differ. Atoms combine in fixed ratios to form compounds. In reactions they rearrange — not created or destroyed.
Tiny centre with protons (p+) and neutrons (n). Almost all mass is here. Charge of nucleus = +Z.
Electrons do not race on neat circular tracks like planets. We only know the probability of finding an electron in a region of space — drawn as a fuzzy cloud.
Cloud shapes have names: s (sphere), p (dumbbell), then d and f. Each orbital can hold up to 2 electrons (opposite spins).
Electrons still have allowed energies (like Bohr’s idea) but described by quantum numbers. Light is absorbed/emitted when electrons jump between levels.
Electrons behave as both particles and waves (wave–particle duality). The cloud is a wave-pattern of probability (Schrödinger).
| Model | Electrons look like… | Use today |
|---|---|---|
| Bohr | Planets on fixed shells | Easy intro · shell filling (2,8,8…) |
| Modern | Probability clouds / orbitals | Real chemistry, bonding, spectra |
For Class 6: Remember Bohr shells for exams. Know that scientists now use the electron-cloud / quantum picture for deeper work — electrons are not tiny balls on wires.
| Element | — |
|---|---|
| Atomic number Z | — |
| Nucleus | — |
| Electrons e− | — |
| Orbits | — |
| Key idea | — |
Rutherford (1911): tiny positive nucleus · atom mostly empty · e− revolve outside. No fixed energy shells (that is Bohr, 1913). Neutrons were not known until 1932.
| Element | — |
|---|---|
| Atomic number (Z) | — |
| Protons (p+) | — |
| Neutrons (n) | — |
| Electrons (e−) | — |
| Shells (K, L, M…) | — |
K shell max 2 · L max 8 · M max 8 (for first 20 elements) · N for outer electrons of K, Ca.
Conservation of mass: 16 (O) + 2×1 (H) = 18 (H2O) — mass of reactants = mass of products.
Chemistry in daily life
If it’s around you — natural or man-made — chemistry is in it.
Topics in this chapter
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What is chemistry?
Scientific study of substances — makeup, properties, reactions. Kitchen & life examples.…
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Elements and compounds
Element: pure, not split simply. Compound: elements chemically joined.…
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Earth’s crust & origin
O, Si, Al richest. Metals mostly as ores/compounds.…
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Metals, rust and alloys
Iron rusts. Ore → metal by reduction. Alloys improve properties.…
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Alchemy to chemistry
Four elements myth → Boyle’s method. Bronze & Iron Ages.…
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Scientific method
Observe → hypothesis → experiment → analyse → share. Boyle’s Law.…
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Discovery of gases
Mayow O₂, Black CO₂, Lavoisier air, Priestley plants, noble gases.…
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Atomic models
Dalton, Thomson, Rutherford, Bohr — ideas improved over time.…
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Chemistry in daily life
Air, water, food, medicines, fibres — all chemicals.…
Philosophy check
- What is this civilization?
- Why was it advanced?
- How did cities work?
- Where do we still see these ideas?
- Exams — what will be asked?