CAIE IGCSE Chemistry – Collecting Gases (Alt to Practical): clear guide to downward delivery (denser than air), upward delivery (less dense), collection over water (sparingly soluble), and gas syringe (measure volume).  Includes apparatus and examples (e.g., CO₂, Cl₂, SO₂, HCl; NH₃, H₂; O₂) with when-to-use tips for accurate volume collection.