Benzoic acid identification scheme showing the benzenecarboxylic acid structure with the –COOH group circled, tests for aromaticity and the carboxylic acid group, physical tests, the sublimation and control confirmatory tests, and the benzamide and benzanilide derivatives. — Identification of salicylic acid. Structural features of 2-hydroxybenzoic acid (C₇H₆O₃, 138.12 g/mol, m.p. 159 °C) mapped to the aromaticity, phenolic group, carboxylic acid and confirmatory tests, with the immediate violet FeCl₃ result as the decisive step and the amide and anilide derivatives for final confirmation.

Benzoic Acid Identification: Key Facts at a Glance

Physical Properties: Physical Appearance · Solubility · Sublimation · Flame Test · Melting Point (121–122 °C) · Benzamide m.p. (130 °C) · Benzanilide m.p. (163 °C) · Molecular Formula (C₇H₆O₂) · MW (122.12 g/mol) · pKa (4.20)
Chemical Tests & Coverage: Litmus · NaHCO₃ Test · Ester Test · FeCl₃ Test (buff/salmon precipitate) · Bromine Water · Sublimation Test · Control Test · Benzamide · Benzanilide · Mixed Melting Point · Chemical Structure · Chemical Reactions · Applications · Safety · FAQs · MCQs · Viva Questions

Benzoic Acid — Structure, IUPAC Name, Molar Mass, Melting Point, Boiling Point, Physical State, Density, Solubility and pKa Data

What Is Benzoic Acid? — Introduction and Background

Benzoic acid is the simplest aromatic carboxylic acid, with the molecular formula C₆H₅COOH (also written C₇H₆O₂). Its IUPAC name is benzoic acid — one of the rare cases where the common name and IUPAC name are identical. The common name derives from gum benzoin, the resin from which it was first isolated in the sixteenth century. Its molar mass is 122.12 g/mol, and it presents as white needle-like or flaky crystals with a faint, pleasant aromatic odour.

Property	Value	Significance for Identification or Context
Molecular formula	C₆H₅COOH (C₇H₆O₂)	Two functional regions: benzene ring and –COOH only. No C=C, no phenolic –OH, no ester.
Molar mass	122.12 g/mol	Used in mole calculations, neutralisation equivalent determination, and calorimetric standard calibration.
IUPAC name	Benzoic acid	Preferred IUPAC name: benzoic acid (retained by IUPAC 2013 recommendations). Alternative systematic name: benzenecarboxylic acid (also accepted). Phenylmethanoic acid is a third systematic alternative.
Common name	Benzoic acid; also: benzenecarboxylic acid,	Derived from gum benzoin. Sodium salt = sodium benzoate (food preservative E211).
Colour & appearance	White, lustrous, needle-like or flaky crystals; faint pleasant aromatic odour	Colourless solid → no chromophore; aromatic odour consistent with benzene ring.
Melting point	121–122°C	Sharp m.p. = purity indicator. Used as primary standard in calorimetry.
Boiling point	249°C (atmospheric)	High b.p. due to strong –COOH H-bonding. Benzoic acid forms hydrogen-bonded dimers in the vapour phase.
Density	1.266 g/cm³ (20°C)	Denser than water; sinks when solid is placed in water.
Solubility in water	~3.4 g/L at 20°C (sparingly soluble)	Increases significantly on warming (29 g/L at 75°C). Freely soluble in NaOH (forms sodium benzoate).
Solubility in ethanol	Freely soluble	Used in FeCl₃ test and ester preparation.
pKa	4.20	Stronger acid than acetic acid (pKa 4.76). Resonance stabilisation of the carboxylate anion (C₆H₅COO⁻) through delocalization of negative charge. Reacts with NaHCO₃.
Benzoic acid vs acetic acid	Benzoic acid (pKa 4.20) is MORE acidic than acetic acid (pKa 4.76)	The electron-withdrawing benzene ring delocalises the carboxylate anion charge more effectively than the methyl group of acetic acid, stabilising the conjugate base.
Conjugate base	Benzoate anion (C₆H₅COO⁻)	Benzoate is the conjugate base of benzoic acid. Resonance stabilisation of the carboxylate anion (C₆H₅COO⁻) through delocalization of negative charge
Saturated or unsaturated	Aromatic (not simply saturated or unsaturated)	Benzoic acid’s benzene ring is aromatic. It does NOT undergo addition reactions like an alkene. It will not decolorise Bromine water.
Heat of combustion	−3226.7 kJ/mol	Used as the international calorimetric standard for bomb calorimetry.
Dimer formation	Forms H-bonded dimers in vapour and non-polar solvents	Explains anomalously high m.p. and low vapour pressure relative to its MW.
Sublimation	Sublimes readily below m.p.	Distinctive among common organic acids. White needles form on any cooler surface. Used as a qualitative test.

Planning the Identification: Decode the Structure First 🔍

Every competent qualitative analysis begins the same way — not at the bench, but on paper. Before a single reagent is added, the structural formula of the compound must be examined carefully. The benzoic acid structure (C₆H₅COOH) is a direct instruction manual: it tells you exactly which tests to perform and, just as importantly, which tests would be irrelevant.

Study the benzoic acid formula and structure shown in Figure 1. Two distinct regions are immediately visible, and each one maps to a specific set of confirmatory tests.

Step 1: Identify the Functional Groups in Benzoic Acid

🔵 Benzene ring → aromatic character; high carbon-to-hydrogen ratio
🟣 –COOH group → carboxylic acid; the sole acidic centre in the molecule

Notably absent from benzoic acid: there is no C=C alkene bond (unlike cinnamic acid), no phenolic –OH (unlike salicylic acid), and no ester linkage (unlike aspirin). This absence of extra functional groups simplifies test selection — fewer reactions to run, but each must be interpreted with precision.

Step 2: Match Each Structural Clue to Its Confirmatory Test

Structural Clue	Tests to Confirm It
🔵 Benzene ring (aromatic system)	Flame Test — sooty, smoky luminous yellow flame
🟣 –COOH (carboxylic acid group)	Litmus Paper Test • NaHCO₃ Test • Ester Test (Fischer esterification to ethyl benzoate)
⚪ Whole compound (benzoic acid)	FeCl₃ Test (buff/salmon precipitate) • Sublimation Test • Melting point 122°C

Step 3: Always Begin with Preliminary Observations

Before any chemical test is attempted, three preliminary observations narrow the field of possibilities: the physical appearance of the solid (white benzoic acid crystals with faint aromatic odour), its solubility behaviour in water (sparingly soluble in cold water, freely soluble in NaOH), and its melting point (122°C). A compound matching all three is already pointing strongly toward benzoic acid. The chemical tests then confirm what the physical data suggests.

The Testing Sequence at a Glance

#	Test	What It Establishes
1	Physical Appearance	White benzoic acid crystals; aromatic odour; no chromophore
2	Solubility	Hydrophobic ring + acidic –COOH ionises in NaOH → sodium benzoate
3	Flame Test	Benzene ring present (sooty flame = high C:H ratio)
4	Litmus Test	Compound is acidic
5	NaHCO₃ Test	Carboxylic acid confirmed; phenols excluded by pKa argument
6	Ester Test	Fischer esterification → ethyl benzoate (floral odour); –COOH confirmed
7	FeCl₃ Test	Buff/salmon ferric benzoate precipitate; no phenolic –OH
8	Sublimation Test	Distinctive physical property unique to benzoic acid
9	Melting Point	Purity check; identity against literature (121–122°C)
10	Control Test	Validates FeCl₃ buff result against authentic standard
11	Benzamide Derivative	Independent physical checkpoint (m.p. 130°C)
12	Benzanilide Derivative	Second independent physical checkpoint (m.p. 163°C)
13	Mixed Melting Point	Gold-standard definitive proof of identity

The skill of qualitative organic analysis lies not in running every test in the book, but in reading the molecular structure and selecting tests that carry real diagnostic weight. Benzoic acid is a beautifully simple molecule for practising this discipline: two functional regions, a clean set of tests, and a highly distinctive sublimation behaviour that sets it apart from almost every other common organic acid.

Materials & Reagents to Test the Benzoic acid

Benzoic acid sample (unknown)
Distilled water
NaHCO₃ solution (aqueous, ~5%) — for sodium bicarbonate and benzoic acid reaction
Freshly prepared lime water (Ca(OH)₂) — for CO₂ confirmation
Blue and red litmus paper
Absolute ethanol — for ester test (benzoic acid esterification to ethyl benzoate / ethyl ester)
Concentrated H₂SO₄ — for ester test
Neutral FeCl₃ solution (~1% aqueous)
Thionyl chloride (SOCl₂) or PCl₅ — for benzoic acid derivatives preparation (fume cupboard only)
Concentrated aqueous ammonia — for benzamide
Aniline + pyridine or dilute NaOH — for benzanilide
Melting point apparatus with sealed capillary tubes
Watch glass and cold water — for sublimation test
Authentic (known) benzoic acid sample — for control test and mixed melting point

Qualitative Analysis of Benzoic Acid — Procedure & Observations

Test	Procedure	Observations	Inference
Physical Appearance	Examine a small sample on a white tile. Observe colour, crystal form (benzoic acid crystals), and odour.	White, lustrous, needle-like or flaky crystalline solid. Faint pleasant aromatic odour. No colour detected.	The colourless benzoic acid appearance rules out any chromophore-bearing structure and transition-metal involvement. The white benzoic acid crystals and aromatic odour are consistent with a pure aromatic organic acid.
Solubility	Add a small quantity to cold distilled water and shake. Warm gently and re-observe. Add NaOH solution to a fresh portion.	Sparingly soluble in cold water (benzoic acid solubility in water ~3.4 g/L at 20°C). Solubility increases on warming. Dissolves readily in NaOH.	Limited water solubility reflects the hydrophobic benzene ring dominating over the polar –COOH group. Ready dissolution in NaOH confirms the acidic –COOH group, which ionises to form water-soluble sodium benzoate (C₆H₅COONa). Benzoic acid is freely soluble in ethanol (benzoic acid solubility in ethanol).
Flame Test	Introduce a small quantity into the outer edge of a Bunsen flame on a spatula. Observe the flame character.	Burns with a bright, sooty yellow flame. Black carbonaceous soot visible. No clean blue flame.	A sooty, smoky flame is characteristic of a high C:H ratio — the fingerprint of an aromatic compound. The benzene ring’s incomplete combustion produces soot that glows yellow. Confirms the benzene ring is present in benzoic acid.
Litmus Test	Dissolve a small quantity in warm water. Dip blue and red litmus strips into the solution.	Blue litmus turns red. Red litmus unchanged.	Confirms the compound is acidic. Litmus alone is preliminary — does not distinguish –COOH from phenols or sulfonic acids. Proceed to NaHCO₃ and ester tests for specificity.
NaHCO₃ Test	Add compound to NaHCO₃ solution. Pass evolved gas through freshly prepared lime water via a delivery tube.	Brisk effervescence. Evolved gas turns lime water milky/white.	CO₂ evolution confirmed by lime water: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O. Sodium bicarbonate and benzoic acid reaction: C₆H₅COOH + NaHCO₃ → C₆H₅COONa + H₂O + CO₂↑. Only –COOH groups (pKa ≈4–5) are strong enough to liberate CO₂ from bicarbonate. Phenols (pKa ≈10) cannot.
Ester Test	Mix compound with absolute ethanol and a few drops of conc. H₂SO₄. Warm gently at 60–70°C for 3–4 min. Pour into cold water. Waft vapour	Sweet, pleasant floral odour of ethyl benzoate (ethyl ester of benzoic acid) detected.	Benzoic acid esterification with ethanol (Fischer esterification) produces ethyl benzoate (C₆H₅COOC₂H₅). Equation: C₆H₅COOH + C₂H₅OH ⇌ (H₂SO₄, Δ) C₆H₅COOC₂H₅ + H₂O. Phenols and sulfonic acids do not esterify under these mild conditions — both excluded.
FeCl₃ Test	Dissolve in water or dilute ethanol. Add 3–4 drops of neutral FeCl₃ solution. Run against authentic control simultaneously.	Buff/salmon-coloured precipitate forms in both tubes. No violet or purple coloration observed.	The buff precipitate is the ferric benzoate complex: 3 C₆H₅COOH + FeCl₃ → Fe(C₆H₅COO)₃↓ + 3 HCl. Absence of deep violet conclusively rules out a free phenolic –OH group. Identical result in control tube confirms reproducibility.
Sublimation Test	Heat a small quantity gently under a cold watch glass (held 2–3 cm above). Observe underside of watch glass.	White needle-like crystalline sublimate forms on the cold surface. No visible melting of bulk solid.	This distinctive physical property is not shared by most common organic acids under gentle heating conditions (citric, tartaric, and salicylic acids do not sublime under these conditions). Note: oxalic acid also sublimes but decomposes at higher temperatures to CO₂ and CO, so the sublimate is not the same compound — benzoic acid sublimes cleanly without decomposition. Confirms the molecular nature of the solid and its relatively low lattice energy.
Melting Point	Pack dry powdered sample in a sealed capillary. Heat at 1–2°C/min from ~115°C. Record onset and completion.	Sharp melting point at 121–122°C over a range of less than 1°C.	Matches the literature melting point of benzoic acid (121–122°C). A narrow melting range confirms sample purity. Note: benzoic acid boiling point is 249°C (boiling and melting point reference).
Control Test	Repeat FeCl₃ test with certified authentic benzoic acid. Compare results side-by-side.	Authentic sample gives same buff/salmon precipitate. Results visually identical.	Confirms the FeCl₃ result is a genuine, reproducible characteristic of benzoic acid — not an artefact. Essential because the buff colour is subtler than phenol’s vivid violet and requires direct comparison.
Benzamide Derivative	React with SOCl₂ → benzoyl chloride; treat with NH₃; filter, recrystallise. Determine m.p. Note: PCl₅ may be used as an alternative to SOCl₂ in Step 1: C₆H₅COOH + PCl₅ → C₆H₅COCl + POCl₃ + HCl. SOCl₂ is preferred as the by-products (SO₂ and HCl) are gases that exit the reaction mixture.	White crystalline solid (benzamide) obtained. M.p. 130°C.	Reaction: C₆H₅COCl + 2NH₃ → C₆H₅CONH₂ + NH₄Cl. Benzamide is one of the key benzoic acid derivatives. M.p. 130°C matches literature. First independent physical checkpoint.
Benzanilide Derivative	Treat benzoyl chloride with aniline in presence of mild base. Filter, recrystallise. Determine m.p.	White crystalline solid (benzanilide) obtained. M.p. 163°C.	Reaction: C₆H₅COCl + C₆H₅NH₂ → C₆H₅CONHC₆H₅ + HCl. Second independent solid derivative. M.p. 163°C matches literature. Benzanilide is among the most characteristic benzoic acid derivatives.
Mixed Melting Point	Mix equal masses of unknown sample and authentic benzoic acid. Pack in fresh capillary. Determine m.p.	Mixture melts sharply at 121–122°C. No depression or elevation observed.	No melting point depression confirms both samples are the same compound. A different compound would lower the m.p. by eutectic depression. Definitive gold-standard proof of identity in classical qualitative organic analysis

Quick Reference Summary — Chemical Tests for Tartaric Acid Identification

#	Test	Positive Result (Benzoic Acid)	What it confirms
1	Physical Appearance	White benzoic acid crystals; aromatic odour; no colour	Purity; absence of colour rules out chromophore
2	Solubility	Sparingly soluble cold H₂O; freely soluble in hot H₂O and NaOH	Hydrophobic benzene ring + acidic –COOH group
3	Flame Test	Bright, luminous, sooty yellow flame with black smoke	Aromatic compound (benzene ring present)
4	Litmus Test	Blue litmus → red; red litmus unchanged	Acidic compound
5	NaHCO₃ Test	Brisk effervescence; lime water turns milky (CO₂ confirmed)	Carboxylic acid; phenols excluded
6	Ester Test	Sweet, floral odour of ethyl benzoate on pouring into cold water	Free –COOH confirmed; sulfonic acids excluded
7	FeCl₃ Test	Buff / pale salmon precipitate; NO violet colour	Ferric benzoate complex; phenolic –OH absent
8	Sublimation Test	White needle-like sublimate on cold watch glass; no bulk melting	Distinctive physical property of benzoic acid
9	Melting Point	Excess NH₃; evaporate; recrystallise	White solid; m.p. 195 °C
10	Benzamide Derivative	Aniline at 120 °C; recrystallise EtOH	White solid; m.p. 180 °C
11	Mixed Melting Point	Excess NH₃; evaporate; recrystallise	White solid; m.p. 195 °C

Quick Reference Summary — Chemical Tests for Tartaric Acid Identification

Reaction	Equation and Mechanism Notes
Sodium Bicarbonate and Benzoic Acid Reaction	C₆H₅COOH + NaHCO₃ → C₆H₅COONa + H₂O + CO₂↑ (Acid–base reaction: CO₂ confirmed by lime water: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O)
Benzoic Acid and NaOH Reaction (Solubility)	C₆H₅COOH + NaOH → C₆H₅COONa + H₂O (Salt formation: sodium benzoate is freely water-soluble, unlike benzoic acid itself)
Esterification — Benzoic Acid Ethyl Ester (Ester Test)	C₆H₅COOH + C₂H₅OH ⇌ (conc. H₂SO₄, Δ) C₆H₅COOC₂H₅ + H₂O (Fischer esterification; ethyl benzoate formed — sweet floral odour)
FeCl₃ Test	3 C₆H₅COOH + FeCl₃ → Fe(C₆H₅COO)₃↓ + 3 HCl (Coordination complex: ferric benzoate — buff/salmon precipitate) Ferric benzoate — buff ppt. In aqueous solution, the actual species is a polynuclear iron(III) benzoate-hydroxo complex; the simplified equation suffices for qualitative identification purposes.
Benzamide Derivative — Step 1	C₆H₅COOH + SOCl₂ → C₆H₅COCl + SO₂↑ + HCl↑ (Nucleophilic acyl substitution: –OH replaced by –Cl to give benzoyl chloride)
Benzamide Derivative — Step 2	C₆H₅COCl + 2NH₃ → C₆H₅CONH₂ + NH₄Cl (Nucleophilic acyl substitution with NH₃ → benzamide, m.p. 130°C)
Benzanilide Derivative	C₆H₅COCl + C₆H₅NH₂ → C₆H₅CONHC₆H₅ + HCl (Nucleophilic acyl substitution with aniline; base neutralises HCl → benzanilide, m.p. 163°C)
Benzoic Acid Combustion Reaction	`2C₆H₅COOH + 15 O₂ → 14CO₂ + 6H₂O ΔH° = −3226.7 kJ/mol (per mole of benzoic acid)` The fractional form (15/2 O₂) is also acceptable and commonly used in calorimetry contexts. (Complete combustion; used as calorimetric standard for bomb calorimetry; benzoic acid heat of combustion)

Result Summary

All observations are consistent with the identity of the compound as benzoic acid (C₆H₅COOH).

The sample was a white, lustrous crystalline solid with a faint aromatic odour and no detectable colour, ruling out chromophore-bearing structures. The compound was sparingly soluble in cold water (benzoic acid solubility in water ~3.4 g/L at 20°C) but dissolved freely in aqueous NaOH, confirming the ionisable –COOH group and forming sodium benzoate.

The flame test produced a bright, sooty, smoky yellow flame, confirming aromatic (benzene ring) character. The litmus, NaHCO₃, and ester tests together confirmed the –COOH group: litmus confirmed acidity; the sodium bicarbonate and benzoic acid reaction produced CO₂ (lime water turned milky), excluding phenols; and Fischer esterification with ethanol produced ethyl benzoate (benzoic acid ethyl ester) identified by its characteristic floral scent, excluding sulfonic acids.

The FeCl₃ test gave a buff/salmon-coloured precipitate — ferric benzoate complex — with no trace of violet, ruling out any phenolic –OH. The sublimation test showed the compound converting directly from solid to white needle-like crystals on a cold surface — a distinctive physical property unique to benzoic acid among common organic acids. The melting point was sharp at 122°C, matching the literature value of 121–122°C. Benzamide (m.p. 130°C) and benzanilide (m.p. 163°C) provided two independent physical confirmations. The mixed melting point test showed no depression, conclusively confirming identity.

Conclusion

Based on the combined weight of physical and chemical evidence — aromatic character (sooty flame test), carboxylic acid functionality (NaHCO₃, litmus, and ester tests), buff ferric benzoate precipitate (FeCl₃ test, no phenolic response), distinctive sublimation behaviour, sharp melting point 122°C, matching derivative melting points (benzamide 130°C; benzanilide 163°C), and absence of mixed melting point depression — the given unknown compound is conclusively identified as:

Benzoic Acid (C₆H₅COOH)

IUPAC name: benzoic acid | Molar mass: 122.12 g/mol | M.p. 121–122°C | B.p. 249°C | Density: 1.266 g/cm³

Uses and Applications of Benzoic Acid

Industry / Field	Application	Specific Role
Food Preservation	Food additive (E210) — antimicrobial preservative	Benzoic acid and its salt sodium benzoate (E211) inhibit the growth of moulds, yeasts, and certain bacteria in acidic foods. Used in soft drinks, fruit juices, pickles, and condiments. Sodium benzoate is the most commercially important derivative.
Pharmaceuticals	Active ingredient in antifungal preparations	Benzoic acid acts as the antifungal component.
Organic Synthesis	Versatile chemical building block	Starting material for benzamide, benzoyl chloride, methyl benzoate, and ethyl benzoate. Used in the synthesis of dyes, perfumes, and pharmaceuticals including ester-type local anaesthetics such as benzocaine, procaine, and tetracaine, which are para-aminobenzoic acid (PABA) or benzoate ester derivatives. Note: lidocaine is an amide-type anaesthetic derived from aniline, not from benzoic acid.
Flavour & Fragrance	Flavouring and fragrance ingredient	Imparts a balsamic, slightly sweet note. Benzyl benzoate and methyl benzoate are used in perfumery and as flavouring agents. Benzoic acid itself provides a pleasant background note in some fragrances.
Cosmetics	Preservative and pH adjuster	Used at low concentrations (≤0.5%) as a preservative in cosmetics and personal care products. Functions most effectively at acidic pH (below 4.5) where the undissociated acid predominates.
Analytical Chemistry	Primary standard for calorimetry and acid–base titrations	Benzoic acid is the international primary standard for bomb calorimetry (heat of combustion = −3226.7 kJ/mol; heat capacity standard). Also used as a primary standard for standardising NaOH solutions in acidimetry.
Industrial Chemistry	Precursor to phenol and caprolactam	Industrial route: oxidation of toluene → benzoic acid → phenol (by decarboxylation). Also a precursor in the synthesis of caprolactam, the monomer for Nylon-6.

Safety Precautions

Benzoic acid hazards and general safety

Wear a lab coat, chemical-splash goggles, and nitrile gloves throughout. Benzoic acid is a mild irritant (skin, eyes, respiratory tract).
Work in a well-ventilated fume cupboard when handling conc. H₂SO₄, SOCl₂, aniline, and during sublimation and ester tests.
Concentrated H₂SO₄ is highly corrosive: always add acid to mixture, never reverse. Flush skin/eye contact with copious water for 15 min.
SOCl₂ and PCl₅ react violently with water — all glassware must be bone-dry. Use fume cupboard only.
Aniline is toxic by skin absorption and is a suspected carcinogen. Wear double gloves; seek medical advice on any skin contact.
Benzoic acid is flammable. Keep the flame test quantity minimal. Keep ethanol away from open flames. Dispose of all waste in designated containers. Do not pour FeCl₃, acid, or organic waste down the sink.

Reagent Hazard Summary

Reagent	Hazard	Key Precaution
Benzoic acid	Mild irritant	Avoid inhalation of dust. Normal bench handling with gloves.
Conc. H₂SO₄	CORROSIVE • OXIDISING	Add to mixture slowly. Goggles + acid-resistant gloves. Fume cupboard.
SOCl₂ / PCl₅	HIGHLY CORROSIVE • TOXIC	Bone-dry glassware. Fume cupboard only. Destroy excess with dry pyridine.
Aniline	TOXIC • Suspected carcinogen	Fume cupboard. Double gloves. Wash skin contact immediately.
FeCl₃ solution	IRRITANT	Gloves and goggles. Dispose in heavy-metal waste container.
Concentrated NH₃	CORROSIVE vapour	Fume cupboard. Keep bottle sealed. Goggles + gloves.

FAQ’s

Tartaric acid is an aliphatic α-hydroxy dicarboxylic acid with the molecular formula C₄H₆O₆, molecular weight 150.09 g/mol, and IUPAC name (2R,3R)-2,3-dihydroxybutanedioic acid. It occurs naturally in grapes, tamarinds, and many other fruits, and is widely used in the food industry as an acidulant (E334), in pharmaceutical formulations, and as a chiral resolving agent in organic synthesis.

Tartaric acid (HOOC–CH(OH)–CH(OH)–COOH) contains four structural features: two –COOH carboxylic acid groups (making it diprotic), two secondary –OH groups on adjacent carbons (giving it α-hydroxy acid character), a saturated aliphatic carbon skeleton with no C=C, and two adjacent chiral centres (C-2 and C-3). See Figure 1 for the annotated structural formula.

Tartaric acid is saturated. It contains no C=C double bond, confirmed experimentally by the absence of decolourisation of bromine water. The slow decolourisation of KMnO₄ is due to oxidation of the secondary –OH groups, not a C=C bond — the slow rate distinguishes this from the instantaneous decolourisation caused by alkenes.

L-(+)-tartaric acid (the naturally occurring form) melts at 170–172 °C. The racemic dl-tartaric acid melts at 206 °C, and the meso-tartaric acid melts at 165–166 °C. The melting point alone can indicate which stereoisomeric form is present.

The IUPAC name of L-(+)-tartaric acid is (2R,3R)-2,3-dihydroxybutanedioic acid. The descriptors 2R,3R specify the absolute configurations of the two chiral centres. The common name ‘tartaric acid’ is universally used in practice.

The degree of unsaturation (DoU) of tartaric acid (C₄H₆O₆) is 2, contributed by the two C=O groups in the two –COOH moieties (one degree each). DoU = (2×4 + 2 – 6) / 2 = 2. There is no benzene ring and no C=C double bond.

The two most specific confirmatory tests for tartaric acid are the Potassium Hydrogen Tartrate (cream of tartar) test — formation of white KHC₄H₄O₆ crystals with KCl — and the Fenton’s Reagent test, which produces an intense blue colour specific to tartaric acid. The CaCl₂ test (white calcium tartrate in cold solution) also provides whole-compound fingerprint confirmation.

Tartaric acid reduces silver ions (Ag⁺) to metallic silver (Ag⁰) due to its α-hydroxy acid structure — the adjacent –OH and –COOH groups facilitate electron donation under alkaline conditions. This reduction is slower than with aldehydes — a grey precipitate rather than a perfect mirror may form, and warming for 5–10 minutes is required. This distinguishes tartaric acid from simple dicarboxylic acids (succinic, malonic) which do not reduce Tollens’ reagent. The test should always be run with freshly prepared reagent.

Tartaric acid is freely soluble in water because its two –COOH groups and two –OH groups make it highly polar and capable of extensive hydrogen bonding. This contrasts with cinnamic acid (sparingly soluble), whose benzene ring creates a dominant hydrophobic region.

Yes. Tartaric acid has two adjacent chiral centres (C-2 and C-3) and exists as three stereoisomers: L-(+)-tartaric acid (m.p. 170–172 °C, naturally occurring), D-(–)-tartaric acid (m.p. 170–172 °C, mirror image), and meso-tartaric acid (m.p. 165–166 °C, optically inactive due to an internal plane of symmetry). The racemic dl-mixture melts at 206 °C.

Fenton’s reagent in this context refers to H₂O₂ followed by excess NaOH — distinct from the industrial Fenton process (Fe²⁺/H₂O₂ used for oxidative degradation). Historically significant: Fenton’s original 1876 experiment was performed on tartaric acid itself (Fenton, H.J.H., J. Chem. Soc. Trans., 1894, 65, 899). It produces an intense blue colour with tartaric acid due to formation of a specific iron-tartrate complex under these alkaline oxidative conditions. The blue colour arises because two tartrate ligands chelate the iron(III) centre through their adjacent –OH and –COO⁻ groups, forming a chelate geometry unique to tartaric acid that citric acid and malic acid cannot replicate despite also being α-hydroxy acids. Citric acid, malic acid, and other common α-hydroxy acids do not give this specific blue response, making it one of the most discriminating single tests in the identification of tartaric acid.

Tartaric acid is used as a food acidulant (E334; wine, baking powder, confectionery), in cream of tartar (potassium hydrogen tartrate for baking), as a pharmaceutical excipient (effervescent tablets, tartrate drug salts), as a chiral resolving agent in organic synthesis, in Fehling’s solution (analytical chemistry), as an α-hydroxy acid in cosmetics, and in electroplating as Rochelle salt.

Tartaric acid itself is a low-hazard mild irritant. However, several reagents used in its identification (conc. H₂SO₄, aniline, H₂O₂, freshly prepared Tollens’ reagent) are hazardous. Wear a lab coat, safety goggles, and nitrile gloves. Never store Tollens’ reagent — explosive silver nitride forms on standing.

Tartaric acid has two ionisation steps: pKa₁ 2.98 and pKa₂ 4.34. The low pKa₁ (2.98) is significant for the NaHCO₃ test: tartaric acid is acidic enough to protonate bicarbonate (H₂CO₃ pKa₁ ≈ 6.4) and release CO₂. Phenols (pKa ~10) cannot achieve this.

The key distinguishing tests are: (1) CaCl₂ test — tartaric acid precipitates calcium tartrate in cold solution; citric acid only precipitates on heating; (2) melting point — tartaric acid 170–172 °C vs citric acid 153 °C; (3) Fenton’s reagent — tartaric acid gives intense blue; citric acid does not; (4) cream of tartar test — specific to tartaric acid only.

Multiple Choice Questions

MCQ 1

1. The correct molecular formula of benzoic acid is:

A. C₆H₅OH

B. C₇H₆O₂ (C₆H₅COOH)

C. C₆H₅CH=CHCOOH

D. C₆H₄(OH)(COOH)

MCQ 2

2. The molar mass of benzoic acid (C₇H₆O₂) is:

A. 94.11 g/mol

B. 108.14 g/mol

C. 122.12 g/mol

D. 138.12 g/mol

MCQ 3

3. How many functional regions does benzoic acid have?

A. One (–COOH only)

B. Two (benzene ring and –COOH)

C. Three (benzene ring, C=C, and –COOH)

D. Three (benzene ring, phenolic –OH, and –COOH)

MCQ 4

4. Benzoic acid (pKa 4.20) is more acidic than acetic acid (pKa 4.76) because:

A. Benzoic acid has a higher molecular weight

B. The benzene ring stabilises the benzoate anion by resonance delocalisation of the negative charge, lowering pKa

C. Acetic acid has more hydrogen atoms in its molecule

D. Benzoic acid is more soluble in water

MCQ 5

5. Benzoic acid is sparingly soluble in cold water because:

A. The large hydrophobic benzene ring dominates over the polar –COOH group, limiting interaction with water molecules.

B. Benzoic acid is completely insoluble in all solvents

C. The –COOH group repels water molecules

D. Benzoic acid forms insoluble dimers in water

MCQ 6

6. When bromine water is added to tartaric acid, the result is:

A. Immediate decolourisation (C=C present)

B. White precipitate forms

C. Orange-brown colour retained — no decolourisation

D. Purple colour develops

MCQ 7

7. Tartaric acid decolourises acidified KMnO₄ slowly because:

A. It has a C=C double bond that is oxidised instantly

B. The secondary –OH groups are slowly oxidised; no C=C is present

C. The carboxyl groups reduce KMnO₄

D. KMnO₄ reacts with the tartrate ion to form a precipitate

MCQ 8

8. The cream of tartar test for tartaric acid involves:

A. Adding NaOH to form a white sodium tartrate precipitate

B. Adding CaCl₂ in cold; white precipitate forms

C. Concentrating the solution then adding saturated KCl; white KHC₄H₄O₆ precipitate forms

D. Adding H₂O₂ and excess NaOH to produce a blue colour

MCQ 9

9. The number of distinct functional group types in tartaric acid is:

A. One (–COOH only)

B. Two (–COOH and –OH)

C. Three (–COOH, –OH, and C=C)

D. Four (–COOH, –OH, C=C, and benzene ring)

MCQ 10

10. Tartaric acid burns with a clean non-sooty flame because:

A. It contains nitrogen

B. It has a high C:H ratio due to the benzene ring

C. It is a saturated aliphatic compound with no benzene ring; combustion is relatively complete

D. It contains sulfur

MCQ 11

11. The FeCl₃ test on tartaric acid gives:

A. Deep violet/purple coloration (phenol response)

B. Light yellow precipitate

C. Buff-yellow colouration; no deep violet

D. No visible change

MCQ 12

12. The degree of unsaturation (DoU) of tartaric acid (C₄H₆O₆) is:

A. 0

B. 2 (from two C=O groups in the two –COOH moieties)

C. 3

D. 6

MCQ 13

13. Which statement about stereoisomerism in tartaric acid is CORRECT?

A. Tartaric acid has no stereocentres

B. There are two stereoisomers: L and D forms only

C. There are three stereoisomers: L-(+), D-(–), and meso forms; the meso form is optically inactive

D. Tartaric acid shows geometrical isomerism due to C=C

MCQ 14

14. The key difference between the CaCl₂ test for tartaric acid and citric acid is:

A. Tartaric acid gives a blue precipitate; citric acid gives a white precipitate

B. Calcium tartrate precipitates in cold solution; calcium citrate only precipitates on heating

C. Both acids require heating to produce a precipitate

D. Only citric acid reacts with CaCl₂

MCQ 15

15. The melting point of tartramide (amide derivative of tartaric acid) is:

A. 170 °C

B. 180 °C

C. 195 °C

D. 210 °C

MCQ 16

16. The unknown sample is mixed with authentic L-tartaric acid. The mixture melts at 170–172 °C with no depression. This means:

A. The compound is meso-tartaric acid

B. The compound is dl-tartaric acid

C. Both samples are identical — the compound is conclusively identified as L-tartaric acid

D. The test has failed

MCQ 17

17. Fenton’s reagent test for tartaric acid involves:

A. Adding FeSO₄ + H₂O₂ and observing a red colour

B. Adding one drop of H₂O₂ then excess NaOH → intense blue colour

C. Adding KMnO₄ and H₂SO₄ and observing slow decolourisation

D. Adding AgNO₃ in ammonia and observing a silver mirror

MCQ 18

18. The cream of tartar used in baking is:

A. Calcium tartrate (CaC₄H₄O₆)

B. Potassium hydrogen tartrate (KHC₄H₄O₆)

C. Sodium potassium tartrate (Rochelle salt)

D. dl-Tartaric acid

MCQ 19

19. On heating in a dry test tube, tartaric acid gives:

A. Clean sublimation with no residue

B. Sweet fruity ester odour

C. Charring and odour of burnt sugar

D. White fumes of SO₂

MCQ 20

20. The molecular weight of tartaric acid (C₄H₆O₆) is:

A. 132.07 g/mol

B. 150.09 g/mol

C. 192.12 g/mol

D. 168.06 g/mol

What Is Benzoic Acid? — Introduction and Background

Planning the Identification: Decode the Structure First 🔍

Step 1: Identify the Functional Groups in Benzoic Acid

Step 2: Match Each Structural Clue to Its Confirmatory Test

Step 3: Always Begin with Preliminary Observations

The Testing Sequence at a Glance

Materials & Reagents to Test the Benzoic acid

Qualitative Analysis of Benzoic Acid — Procedure & Observations

Quick Reference Summary — Chemical Tests for Tartaric Acid Identification

Quick Reference Summary — Chemical Tests for Tartaric Acid Identification

Result Summary

Conclusion

Benzoic Acid (C₆H₅COOH)

Uses and Applications of Benzoic Acid

Safety Precautions

Reagent Hazard Summary

FAQ’s

Multiple Choice Questions

MCQ 1

MCQ 2

MCQ 3

MCQ 4

MCQ 5

MCQ 6

MCQ 7

MCQ 8

MCQ 9

MCQ 10

MCQ 11

MCQ 12

MCQ 13

MCQ 14

MCQ 15

MCQ 16

MCQ 17

MCQ 18

MCQ 19

MCQ 20

Similar Posts

Leave a Reply Cancel reply

Other Posts

Chemical Tests

Resources

Learn More