Basic Laboratory Technique

How the real exam tests it

Questions from real CSCA papers that test this concept. Work each one out before you open the answer.

1. CSCA Chemistry April 2026 Past Paper
   Among the following gases prepared in the laboratory, the one that cannot be collected over water and can only be collected by downward displacement of air is ( ).
   • A. $\mathrm{O_{2}}$
   • B. $\mathrm{NH_{3}}$
   • C. $\mathrm{H_{2}}$
   • D. $\mathrm{CO_{2}}$
   Show answer & solution

   Answer: B

   Answer: B $\mathrm{NH_{3}}$ is extremely soluble in water and less dense than air, so it can only be collected by downward displacement of air.
   Take the full CSCA Chemistry April 2026 Past Paper — timed & graded
2. CSCA Chemistry March 2026 Past Paper
   Which gas can be collected by both water displacement and downward air displacement?
   • A．$\mathrm { H } _ { 2 }$
   • B．$\mathrm { CO } _ { 2 }$
   • C．O₂
   • D．$\mathrm { SO } _ { 2 }$
   Show answer & solution

   Answer: A

   $\mathrm { H } _ { 2 }$ : lighter than air (downward displacement) + insoluble (water displacement). Answer: A.
   Take the full CSCA Chemistry March 2026 Past Paper — timed & graded