IGCSE Chemistry Interactive Study Guide

Particle model

Heating increases average kinetic energy. During a change of state, energy changes particle attractions rather than temperature.

Diffusion

Diffusion is the net movement of particles from higher to lower concentration due to random motion.

Faster when temperature is higher; lighter gas particles generally diffuse faster than heavier ones.

Choose the method

Chromatography essentials

• Use pencil for the baseline: graphite does not dissolve in the solvent.
• Keep baseline above the solvent.
• One spot suggests purity; multiple spots suggest a mixture.

Subatomic particles

Number relationships

mass number = protons + neutrons
neutrons = mass number − proton number

An ion forms when an atom gains or loses electrons. The nucleus does not change in ordinary chemical reactions.

Isotopes

Atoms of the same element with the same proton number but different neutron numbers.

Radioisotopes must be matched to an appropriate half-life and penetrating ability. Uses include tracers, medical treatment, thickness control and dating.

Electron arrangements

For the first 20 elements, fill shells 2, 8, 8, 2. Main-group position links to outer electrons.

Example: calcium, 2,8,8,2 → Ca²⁺, 2,8,8.

Patterns

• Group number indicates outer-shell electrons for Groups I–VII.
• Period number indicates occupied shells.
• Group I reactivity increases down the group.
• Group VII reactivity decreases down the group.
• Group VIII/0 gases are unreactive because their outer shells are full.

Transition elements

Typically dense, strong, high-melting metals. They often form coloured compounds, have variable oxidation states and act as catalysts.

Compared with Group I metals, they are generally less reactive and much harder.

Formula from charges

Make total positive charge equal total negative charge, then simplify.

Al³⁺ and O²⁻ → Al₂O₃
Ca²⁺ and Cl⁻ → CaCl₂

Relative formula mass tool

Ionic

Electron transfer between a metal and non-metal forms oppositely charged ions. Strong electrostatic attraction acts in a giant lattice.

• High melting and boiling points
• Conducts when molten or aqueous, not solid
• Often soluble in water

Simple molecular

Non-metal atoms share electron pairs. Strong covalent bonds occur within molecules but forces between molecules are usually weaker.

• Often low melting/boiling points
• Usually do not conduct electricity

Diamond vs graphite

Diamond: each carbon bonds to four; rigid 3D network; no mobile electrons.

Graphite: each carbon bonds to three; layers slide; one delocalised electron per carbon carries charge.

Dot-and-cross checklist

1. Show outer-shell electrons only.
2. Use different symbols for different atoms.
3. Show shared pair(s) between nuclei.
4. Include lone pairs where needed.

Reading reaction evidence

More vigorous reactions generally indicate greater reactivity. Metals reacting with cold water are usually more reactive than those requiring steam.

When comparing, keep temperature, metal surface area and reagent concentration controlled.

Rusting

Iron rusts only when both oxygen and water are present. Salt speeds the process.

• Barrier: paint, oil, plastic coating
• Galvanising: zinc barrier and sacrificial protection
• Alloying: stainless steel resists corrosion

Alloys

Different-sized atoms disrupt regular metal layers, making sliding more difficult. Alloys are often harder than pure metals.

Metal choice

Select material using a combination of density, strength, corrosion resistance, cost and conductivity—not a single property.

Air

Dry air is about 78% nitrogen, 21% oxygen, with argon, carbon dioxide and other gases making up the remainder.

Combustion of sulfur-containing fuels produces sulfur dioxide; high-temperature engines can form nitrogen oxides.

Pollutant effects

• SO₂ and nitrogen oxides: acid rain, respiratory irritation
• CO: reduces blood's oxygen-carrying capacity
• Particulates: respiratory harm and global dimming
• CO₂ and methane: enhanced greenhouse effect

Water treatment

Typical stages include screening/sedimentation, filtration and disinfection. Exact steps depend on the source.

Domestic uses include drinking and washing. Industrial uses include cooling, solvent use and steam generation.

Fertilisers

NPK labels refer to nitrogen, phosphorus and potassium nutrients. Read the actual chemical names before deciding which elements a fertiliser contains.

Gas tests

Rates graphs

• Steeper initial gradient = faster initial rate.
• Plateau = reaction has stopped.
• Same final gas volume = same amount of gas made.
• A catalyst changes rate, not theoretical yield.

Graph checklist

Planning frame

Change: one independent variable.
Measure: one dependent variable.
Control: relevant variables.
Repeat: identify anomalies and calculate a mean.
Safety: name the hazard and a matching precaution.