O-Level Chemistry: Question on Acids, Bases & Qualitative Analysis

Today, i will be sharing with you one exam-based question that my Secondary 4 Pure Chemistry student has recently asked me.

The question is related to the following topics:

• Acids
• Bases
• Qualitative Analysis (QA) or some call it Identification of Ions & Gases

Question:

Do attempt the question first before you scroll down to find my suggested solution.

Suggested Solution:

Option A

Reaction: Zn(s) + 2HCl(aq) -> ZnCl₂(aq) + H₂(g)

Observation: H₂(g) is a neutral gas and will not have any effect on the damp blue litmus paper

Option B

Reaction: HCl(aq) + Na₂CO₃(aq) -> 2NaCl(aq) + H₂O(l) + CO₂(g)

Observation: CO₂(g) is an acidic gas and will not have any effect on the damp red litmus paper

Option C:

Reaction: NH₄Cl(aq) + NaOH(aq) -> NaCl(aq) + H₂O(l) + NH₃(g)

Observation: NH₃(g) is an alkaline gas and will not have any effect on the damp blue litmus paper

Option D:

Reaction: Upon warming Al and NaOH(aq) with nitrate, NO₃^– ion, NH₃(g) will be produced

Observation: NH₃(g) is an alkaline gas and will change the colour of damp red litmus paper to blue.

Hence,

Answer is D.

Note:

• Acidic Gas turns damp blue litmus red
• Alkaline Gas turns damp red litmus blue

Do note also that the above question is related to your Chemistry Practicals & School Practical Assessments (SPA).

Hope you find the above discussion useful. Feel free to share it with your friends.

Cheers!

Sean Chua

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• O Level Chemistry – Qualitative Analysis Cont’d