Gases

Traps in Gases

• Confuses tests for gases
  CO₂ turns limewater milky; H₂ gives a 'pop' with a lighted splint; O₂ relights a glowing splint; NH₃ turns damp red litmus blue; Cl₂ bleaches damp litmus.
• Picks a related substance or process with a different role
  Learn reagents and processes as one-line 'name → job' cards and contrast neighbours in a table (process | product | catalyst | conditions). Match the job in the stem, not just the topic.
• Mixes up a pair of similar chemical terms
  Keep a list of confusable pairs with one defining feature each and a hook. Underline the key word in the stem and check it against the definition, not the sound of the name.
• Over-generalises a rule beyond where it holds
  Every rule in chemistry has conditions. Before applying one, check the conditions in the stem (electrode type, concentration, moles of gas on each side, metal position).
• Puts a substance or reaction in the wrong class
  Classify from the defining feature: functional group for organic series; bond type and structure for properties; reaction with acid AND alkali for amphoteric. Check each option against the feature, not the name.
• Reverses a trend, direction or effect
  State the rule with its direction before looking at the options (down group 1 reactivity INCREASES; exothermic forward reaction: heating shifts LEFT). An option that is the mirror image of the rule is the trap.
• Uses the wrong arithmetic operation
  Write the relationship in words before substituting (crystals = solubility at high T − solubility at low T; neutrons = A − Z; three doublings = 2³). Then check the size of the answer.
• Gas-law errors
  Boyle: P₁V₁ = P₂V₂ (constant T). Charles: V₁/T₁ = V₂/T₂ (T in kelvin = °C + 273). General: P₁V₁/T₁ = P₂V₂/T₂.
• Uses everyday or macroscopic thinking instead of particles
  Ask what the particles, ions or electrons are doing. Properties of a substance come from its particles and bonds, not from how it looks or feels.
• Takes one factor as the whole answer
  Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.
• Chooses a true but irrelevant statement
  Re-read the question word (why, which, what causes). A true fact that answers a different question is still wrong.
• Properties of hydrogen/oxygen/water errors
  H₂: Zn + dilute acid; collected over water or by downward displacement of air; burns with a pop. O₂: decomposition of H₂O₂ (MnO₂ catalyst) or KClO₃; relights a glowing splint. NH₃ collected by upward delivery (less dense than air); HCl, CO₂ by downward delivery.
• Qualitative analysis errors
  NaOH(aq): Cu²⁺ blue ppt; Fe²⁺ green; Fe³⁺ reddish-brown; Zn²⁺ and Al³⁺ white ppt soluble in excess; Ca²⁺ white insoluble. Ag⁺ + Cl⁻ white ppt; Ba²⁺ + SO₄²⁻ white ppt insoluble in HCl. NH₄⁺ + NaOH warmed → NH₃.