Identify representations of alloys from diagrams of structure

Cambridge IGCSE Chemistry (0620) – Complete Revision Notes

How to Use These Notes

• Learning objectives are marked with the assessment objective (AO) required:
  • AO1 – recall factual knowledge.
  • AO2 – explain concepts and processes.
  • AO3 – apply knowledge to new situations (e.g. interpret diagrams, solve calculations).
• Read each section, then attempt the Practice Questions at the end of the unit. Check the supplied answers.
• Use the Key Formulae box for quick revision before exams.

1. States of Matter

Learning Objectives

• Recall the three states of matter and their particle arrangements (AO1).
• Explain changes of state in terms of energy and particle movement (AO2).
• Use the gas‑law equations to calculate pressure, volume, temperature and moles (AO3).

Key Concepts

• Solid – particles vibrate in fixed positions; definite shape & volume.
• Liquid – particles slide past one another; definite volume, no fixed shape.
• Gas – particles move freely; no fixed shape or volume.
• Evaporation – surface molecules gain enough kinetic energy to escape.
• Boiling – vapour pressure equals external pressure; occurs throughout the liquid.
• Condensation, freezing, melting – reverse processes.
• Gas‑law equations (ideal gas approximation):
  • pV = nRT
  • p₁V₁/T₁ = p₂V₂/T₂ (combined gas law)

Practice Questions

1. A 0.250 mol sample of a gas occupies 5.60 L at 298 K. Calculate the pressure (kPa).
   Answer: p = nRT/V = (0.250 mol)(8.314 J mol⁻¹ K⁻¹)(298 K)/(5.60 L) = 111 kPa (≈ 1.1 atm).
2. Explain why evaporation occurs at any temperature, but boiling does not.
   Answer: Evaporation only requires a few high‑energy surface molecules to overcome intermolecular forces; boiling requires the vapour pressure of the whole liquid to equal the external pressure.

2. Atoms, Elements & Compounds

Learning Objectives

• Recall the structure of an atom, isotopes and ions (AO1).
• Explain ionic, covalent and metallic bonding (AO2).
• Write correct chemical formulas and balanced equations (AO3).

Key Concepts

• Atomic structure – protons (+), neutrons (0), electrons (‑); atomic number = # protons; mass number = protons + neutrons.
• Isotopes – same Z, different A; e.g. ¹²C, ¹³C, ¹⁴C.
• Ions – gain or loss of electrons; cation (+), anion (‑).
• Ionic bonding – transfer of electrons from metal to non‑metal, forming a lattice of oppositely charged ions (e.g. NaCl).
• Covalent bonding – sharing of electron pairs between non‑metals (e.g. H₂O, CH₄).
• Metallic bonding – delocalised “sea of electrons” around positive metal ions; explains conductivity, malleability.
• Formula writing – use subscripts to show the lowest whole‑number ratio of atoms; polyatomic ions retain their internal formula (e.g. Na₂SO₄).
• Balancing equations – same number of each type of atom on both sides; total charge conserved.

Practice Questions

1. Write the formula for calcium nitrate.
   Answer: Ca(NO₃)₂.
2. Balance the reaction: