Qualitative tests for carboxylic acids MCQs With Answer

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Qualitative tests for carboxylic acids MCQs With Answer

Understanding qualitative tests for carboxylic acids is essential for B. Pharm students to identify functional groups, confirm drug impurities, and interpret laboratory analyses. This concise introduction covers common reagents and observations such as the sodium bicarbonate test (CO2 effervescence), esterification (fruity odor), solubility in alkali (carboxylate formation), and reactions with thionyl chloride or PCl5 (HCl fumes/acyl chloride formation). Emphasis is placed on practical diagnostic methods, distinguishing carboxylic acids from phenols and alcohols, and the effect of substituents on reactivity and acidity. These MCQs include answers and explanations to reinforce analytical skills and exam preparation. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which qualitative test is most commonly used to distinguish a carboxylic acid from a phenol?

  • Ferric chloride gives a violet color with carboxylic acid
  • Reaction with sodium bicarbonate giving effervescence
  • Tollen’s reagent gives a silver mirror with carboxylic acid
  • 2,4-DNP gives an orange precipitate with carboxylic acid

Correct Answer: Reaction with sodium bicarbonate giving effervescence

Q2. What observation indicates a positive sodium bicarbonate test for a carboxylic acid?

  • Formation of a colored complex
  • Evolution of carbon dioxide gas (effervescence)
  • Precipitation of a white salt
  • Disappearance of smell

Correct Answer: Evolution of carbon dioxide gas (effervescence)

Q3. Why do carboxylic acids react with sodium bicarbonate but phenols generally do not?

  • Carboxylic acids are stronger acids and deprotonate to form carboxylate and CO2
  • Phenols are oxidized by bicarbonate
  • Sodium bicarbonate specifically reacts with aromatic rings
  • Phenols form esters with bicarbonate

Correct Answer: Carboxylic acids are stronger acids and deprotonate to form carboxylate and CO2

Q4. Which reagent produces a fruity odor when reacted with a carboxylic acid under acidic conditions?

  • Concentrated ammonia
  • Sodium bicarbonate
  • Primary alcohol with concentrated sulfuric acid (esterification)
  • PCl5

Correct Answer: Primary alcohol with concentrated sulfuric acid (esterification)

Q5. How does addition of sodium hydroxide help in qualitative identification of carboxylic acids?

  • Sodium hydroxide oxidizes carboxylic acids to ketones
  • Carboxylic acids dissolve forming soluble carboxylate salts
  • Sodium hydroxide causes precipitation of carboxylic acids
  • Sodium hydroxide converts acids to esters

Correct Answer: Carboxylic acids dissolve forming soluble carboxylate salts

Q6. Which gas evolution confirms neutralization of a carboxylic acid with carbonate or bicarbonate?

  • Hydrogen
  • Nitrogen
  • Carbon dioxide
  • Chlorine

Correct Answer: Carbon dioxide

Q7. Which reagent is commonly used to convert a carboxylic acid to its acyl chloride, often observed by HCl fumes?

  • Sodium borohydride
  • Sodium bicarbonate
  • Thionyl chloride (SOCl2)
  • Hydrogen peroxide

Correct Answer: Thionyl chloride (SOCl2)

Q8. The formation of dense white fumes and HCl smell on treatment of an acid indicates reaction with which reagent?

  • PCl5 or thionyl chloride
  • Sodium hydroxide
  • Ferric chloride
  • 2,4-DNP

Correct Answer: PCl5 or thionyl chloride

Q9. Which test would be negative for carboxylic acids but positive for aldehydes?

  • Sodium bicarbonate test
  • Tollen’s (silver mirror) test
  • Esters formation with alcohol and acid catalyst
  • Solubility in sodium hydroxide

Correct Answer: Tollen’s (silver mirror) test

Q10. Why does 2,4-dinitrophenylhydrazine (2,4-DNP) typically give a negative test for carboxylic acids?

  • Carboxylic acids lack carbonyl groups
  • Carboxylic acid carbonyl is not electrophilic enough to form hydrazones under standard 2,4-DNP conditions
  • 2,4-DNP reacts with carboxylate salts only
  • Carboxylic acids immediately decarboxylate with 2,4-DNP

Correct Answer: Carboxylic acid carbonyl is not electrophilic enough to form hydrazones under standard 2,4-DNP conditions

Q11. Which color change would you expect for litmus paper in presence of a carboxylic acid?

  • Blue litmus turns red
  • Red litmus turns blue
  • No change
  • Both papers turn green

Correct Answer: Blue litmus turns red

Q12. How can you qualitatively differentiate a carboxylic acid from an alcohol?

  • Both react with sodium bicarbonate
  • Carboxylic acids react with bicarbonate to give CO2, alcohols do not
  • Alcohols form carboxylate salts with NaOH but acids do not
  • Carboxylic acids do not change litmus color while alcohols do

Correct Answer: Carboxylic acids react with bicarbonate to give CO2, alcohols do not

Q13. A sample gives effervescence with NaHCO3 and a fruity smell upon heating with ethanol and H2SO4. These observations best indicate the presence of:

  • A phenol
  • An aldehyde
  • A carboxylic acid
  • A ketone

Correct Answer: A carboxylic acid

Q14. Which observation supports the formation of a carboxylate salt when acid is treated with aqueous NaOH?

  • Precipitation of a solid
  • Sample dissolves to give a clear solution
  • Evolution of sulfur dioxide
  • Color change to purple

Correct Answer: Sample dissolves to give a clear solution

Q15. Which analytical property is most useful for quick qualitative detection of carboxylic acids in organic mixtures?

  • Melting point determination
  • Sodium bicarbonate test for CO2 evolution
  • Mass spectrometry
  • Infrared spectroscopy only

Correct Answer: Sodium bicarbonate test for CO2 evolution

Q16. In an IR spectrum, which strong absorption band would support presence of a carboxylic acid?

  • Strong broad band around 2500–3300 cm−1 (O–H) and sharp C=O near 1700 cm−1
  • Sharp O–H at 3600 cm−1 only
  • C–H stretch at 3000 cm−1 only
  • N–H stretch at 3300 cm−1

Correct Answer: Strong broad band around 2500–3300 cm−1 (O–H) and sharp C=O near 1700 cm−1

Q17. Which pKa range is typical for simple aliphatic carboxylic acids, relevant when predicting deprotonation in qualitative tests?

  • pKa ~ 15–18
  • pKa ~ 9–11
  • pKa ~ 4–5
  • pKa ~ -1 to 0

Correct Answer: pKa ~ 4–5

Q18. Introduction of an electron-withdrawing group (e.g., –NO2) on a benzoic acid ring will qualitatively:

  • Decrease acidity
  • Increase acidity (stronger acid)
  • Have no effect on acidity
  • Make it insoluble in NaOH

Correct Answer: Increase acidity (stronger acid)

Q19. Which statement is true about the solubility of carboxylic acids in water?

  • All carboxylic acids are highly soluble in water
  • Solubility decreases with increasing alkyl chain length; short acids are water soluble
  • Solubility is independent of chain length
  • Carboxylic acids are insoluble but react violently with water

Correct Answer: Solubility decreases with increasing alkyl chain length; short acids are water soluble

Q20. Which test can help differentiate a carboxylic acid from an amide?

  • 2,4-DNP test
  • Sodium bicarbonate test (amide does not evolve CO2)
  • Tollen’s reagent (amide gives silver mirror)
  • Baeyer test with KMnO4

Correct Answer: Sodium bicarbonate test (amide does not evolve CO2)

Q21. Which reagent would convert a carboxylic acid into a more volatile derivative for smell-based qualitative testing?

  • Sodium hydroxide
  • Pyridinium chlorochromate
  • Alcohol with concentrated H2SO4 (esterification)
  • AgNO3

Correct Answer: Alcohol with concentrated H2SO4 (esterification)

Q22. On acidification of an aqueous solution containing sodium carboxylate, what qualitative change is expected?

  • Solution becomes more basic
  • Carboxylic acid precipitates or separates as an oil
  • Color changes to blue
  • Immediate evolution of oxygen

Correct Answer: Carboxylic acid precipitates or separates as an oil

Q23. Which of the following is NOT a reliable qualitative indicator for carboxylic acids?

  • Effervescence with sodium bicarbonate
  • Fruity odor after esterification
  • Immediate silver mirror with Tollen’s reagent
  • Solubility in aqueous NaOH

Correct Answer: Immediate silver mirror with Tollen’s reagent

Q24. Why is the sodium bicarbonate test often used first in qualitative screens for organic acids?

  • It requires expensive equipment
  • It is rapid, simple, and specific for carboxylic acids over phenols and alcohols
  • It detects all carbonyl compounds
  • It selectively oxidizes contaminants

Correct Answer: It is rapid, simple, and specific for carboxylic acids over phenols and alcohols

Q25. A benzoic acid derivative with strong electron-donating groups will qualitatively show:

  • Higher acidity and faster reaction with NaHCO3
  • Lower acidity and slower or no reaction with NaHCO3
  • No change in acidity
  • Immediate formation of ester without catalyst

Correct Answer: Lower acidity and slower or no reaction with NaHCO3

Q26. When testing a mixture, which procedure will help isolate carboxylic acids for qualitative tests?

  • Extract with organic solvent directly from neutral medium
  • Acidify aqueous extract of carboxylate salts to precipitate or extract the free acid
  • Add 2,4-DNP to the crude mixture
  • Boil with sodium chloride

Correct Answer: Acidify aqueous extract of carboxylate salts to precipitate or extract the free acid

Q27. Which color change indicates a positive ferric chloride test and therefore suggests phenolic rather than carboxylic functionality?

  • Violet or purple color
  • Effervescence
  • Green precipitate
  • Orange turbidity

Correct Answer: Violet or purple color

Q28. In acid–base extraction to detect carboxylic acids, which solvent pair is commonly used?

  • Water and hexane
  • Aqueous NaHCO3 and diethyl ether or dichloromethane
  • Conc. HCl and acetone
  • DMF and benzene

Correct Answer: Aqueous NaHCO3 and diethyl ether or dichloromethane

Q29. Which qualitative observation suggests formation of an acyl chloride when treating a carboxylic acid with thionyl chloride?

  • Formation of a fruity odor
  • Evolution of SO2 and HCl gases (pungent fumes)
  • Precipitation of a metal salt
  • Color change to yellow

Correct Answer: Evolution of SO2 and HCl gases (pungent fumes)

Q30. Which laboratory practice improves reliability of the bicarbonate test?

  • Use of non-aqueous sodium bicarbonate
  • Ensure sample is dissolved or finely divided so CO2 can escape visibly
  • Perform test in a closed vial to trap gas
  • Heating the bicarbonate solution strongly

Correct Answer: Ensure sample is dissolved or finely divided so CO2 can escape visibly

Q31. What happens to the pH of an aqueous solution when a carboxylic acid is dissolved and neutralized by NaOH?

  • pH decreases
  • pH remains neutral
  • pH increases (becomes basic) due to formation of carboxylate salt
  • Solution becomes acidic again immediately

Correct Answer: pH increases (becomes basic) due to formation of carboxylate salt

Q32. Which of the following is an expected qualitative test result for acetic acid?

  • No reaction with NaHCO3
  • Effervescence with NaHCO3 and characteristic vinegar odor
  • Violet color with ferric chloride
  • Silver mirror with Tollen’s reagent

Correct Answer: Effervescence with NaHCO3 and characteristic vinegar odor

Q33. A student heats a carboxylic acid with methanol and H2SO4 and detects a pleasant fruity smell. This indicates formation of:

  • An aldehyde
  • An ester
  • An amide
  • A carboxylate salt

Correct Answer: An ester

Q34. Which reagent would you use to qualitatively distinguish a carboxylic acid from an ester?

  • Sodium bicarbonate (carboxylic acid reacts, ester does not)
  • 2,4-DNP (ester reacts, acid does not)
  • Tollen’s reagent (ester gives silver mirror)
  • Ferric chloride (ester gives violet color)

Correct Answer: Sodium bicarbonate (carboxylic acid reacts, ester does not)

Q35. The presence of which functional group may interfere with the bicarbonate test by producing CO2 as well?

  • Alcohol
  • Carboxylate only
  • Carbonates or bicarbonates present as impurities
  • Aldehyde

Correct Answer: Carbonates or bicarbonates present as impurities

Q36. Which statement about aromatic carboxylic acids (e.g., benzoic acid) in qualitative testing is true?

  • They never react with NaHCO3
  • They react with NaHCO3 to give effervescence and form benzoate salts
  • They give a positive ferric chloride test
  • They give a silver mirror with Tollen’s reagent

Correct Answer: They react with NaHCO3 to give effervescence and form benzoate salts

Q37. Which pair of observations together strongly indicate carboxylic acid presence?

  • No litmus change and negative bicarbonate test
  • Blue litmus turns red and effervescence with NaHCO3
  • Violet ferric chloride test and silver mirror
  • Formation of orange 2,4-DNP precipitate and no CO2

Correct Answer: Blue litmus turns red and effervescence with NaHCO3

Q38. When performing qualitative tests, why is it important to first test a small sample before treating the whole batch?

  • To save reagents only
  • To observe dangerous reactions (e.g., vigorous gas evolution or toxic fumes) on a small scale first
  • Small tests give different chemistry than large ones
  • Large samples never react

Correct Answer: To observe dangerous reactions (e.g., vigorous gas evolution or toxic fumes) on a small scale first

Q39. Which of the following best describes qualitative acid–base extraction to separate carboxylic acids from neutral organics?

  • Shake mixture with dilute acid to remove acids into organic layer
  • Neutralize acid with base to form water-soluble carboxylate, extract aqueous layer, then acidify to recover acid
  • Add base and discard aqueous layer
  • Directly distill the mixture

Correct Answer: Neutralize acid with base to form water-soluble carboxylate, extract aqueous layer, then acidify to recover acid

Q40. Which observation would most likely rule out a carboxylic acid during qualitative screening?

  • Evolution of CO2 with NaHCO3
  • Strong permanent basic pH unaffected by acid addition
  • Fruity ester odor upon esterification
  • Red color on litmus paper

Correct Answer: Strong permanent basic pH unaffected by acid addition

Q41. Which of the following correctly explains why tertiary carboxylic acids (if sterically hindered) might show slower esterification qualitatively?

  • Steric hindrance reduces nucleophilic attack by alcohol on the carbonyl carbon
  • They are too acidic to react
  • They immediately decarboxylate
  • They form salts preferentially with H2SO4

Correct Answer: Steric hindrance reduces nucleophilic attack by alcohol on the carbonyl carbon

Q42. Which reagent test can help identify presence of a carboxylic acid in a mixture containing phenol and alcohol?

  • Ferric chloride test alone
  • Sodium bicarbonate test to selectively deprotonate the carboxylic acid
  • Tollen’s test alone
  • Chromic acid test (only for carboxylic acids)

Correct Answer: Sodium bicarbonate test to selectively deprotonate the carboxylic acid

Q43. In a lab practical, which safety precaution is most relevant when performing a test that generates HCl or SO2 fumes?

  • Work in a fume hood and wear appropriate PPE
  • Perform test in closed room to contain fumes
  • No special precautions needed
  • Use open flames to disperse fumes

Correct Answer: Work in a fume hood and wear appropriate PPE

Q44. Which test would likely give a positive result for a β‑diketone but negative for a simple carboxylic acid?

  • Sodium bicarbonate effervescence
  • Ferric chloride color test (β‑diketones often give colored complexes)
  • Formation of esters with alcohol
  • Reaction with PCl5 producing HCl fumes

Correct Answer: Ferric chloride color test (β‑diketones often give colored complexes)

Q45. Which structural feature increases the acidity of a carboxylic acid and thus its reactivity in bicarbonate test?

  • Strong electron-donating substituents near the carboxyl group
  • Long alkyl chain only
  • Electron-withdrawing substituents (e.g., –NO2) adjacent to the carboxyl group
  • Lack of resonance stabilization

Correct Answer: Electron-withdrawing substituents (e.g., –NO2) adjacent to the carboxyl group

Q46. Which result indicates that an unknown sample contains a carboxylate salt rather than the free acid?

  • Sample dissolves readily in water and is basic to litmus
  • Effervescence with NaHCO3
  • Fruity odor on esterification
  • Formation of a silver mirror

Correct Answer: Sample dissolves readily in water and is basic to litmus

Q47. During qualitative tests, how can you confirm that effervescence was due to CO2 and not another gas?

  • Perform limewater test: CO2 turns limewater milky
  • Measure smell only
  • Assume any gas from acid is CO2
  • Use Tollen’s reagent to test the gas

Correct Answer: Perform limewater test: CO2 turns limewater milky

Q48. Which qualitative technique helps detect very small amounts of carboxylic acids in a pharmaceutical impurity screen?

  • Simple smell test
  • Thin-layer chromatography with appropriate visualization after derivatization
  • Boiling point determination only
  • Paper chromatography with water

Correct Answer: Thin-layer chromatography with appropriate visualization after derivatization

Q49. A strong carboxylic acid gives immediate effervescence with NaHCO3 while a weak acid gives slow or no effervescence. Which factor mainly causes this difference?

  • Concentration of NaHCO3 only
  • Intrinsic acidity (pKa) of the acid
  • Color of the acid
  • Molecular weight only

Correct Answer: Intrinsic acidity (pKa) of the acid

Q50. For qualitative confirmation of a carboxylic acid, which combination of observations is most conclusive?

  • No change with NaHCO3, positive ferric chloride
  • Effervescence with NaHCO3, solubilization in NaOH, and fruity odor on esterification
  • Silver mirror formation and 2,4-DNP precipitation
  • Insolubility in all solvents

Correct Answer: Effervescence with NaHCO3, solubilization in NaOH, and fruity odor on esterification

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