In short: The Class 12 Chemistry practical exam carries 30 marks and involves salt analysis, volumetric analysis (titration), and qualitative tests. Salt analysis alone can make or break your score — getting the cation and anion wrong means losing 6-8 marks. This guide covers all experiment types, the exact marks breakdown, top 25 viva questions with concise answers, the salt analysis systematic approach, lab safety rules, and common mistakes students make.
Marks Distribution — CBSE Class 12 Chemistry Practical (30 Marks)
| Component | Marks | Details |
|---|---|---|
| Volumetric Analysis (Titration) | 8 | One titration experiment |
| Salt Analysis | 8 | Systematic qualitative analysis of a salt |
| Content-Based Experiment | 6 | Tests for functional groups / food adulterants |
| Project + Viva | 4 | Investigatory project evaluation |
| Practical Record | 4 | Year-long lab notebook |
| Total | 30 |
Experiments — Complete List
A. Volumetric Analysis (Titrations)
| # | Titration | Indicator | End Point |
|---|---|---|---|
| 1 | Mohr's salt vs KMnO₄ (Redox) | Self-indicator (KMnO₄ pink) | Permanent light pink colour |
| 2 | Oxalic acid vs KMnO₄ (Redox) | Self-indicator | Permanent light pink colour |
| 3 | NaOH vs HCl (Acid-Base) | Phenolphthalein | Colourless to pink |
| 4 | Na₂CO₃ vs HCl (Acid-Base) | Methyl orange | Yellow to orange-pink |
B. Salt Analysis (Qualitative Analysis)
Systematic Approach:
Step 1: Physical Examination
| Observation | Inference |
|---|---|
| White crystalline | May contain Na⁺, K⁺, Ca²⁺, Ba²⁺, Zn²⁺, Al³⁺ |
| Blue colour | Cu²⁺ present |
| Green colour | Fe²⁺ or Ni²⁺ present |
| Yellow/Brown | Fe³⁺ present |
| Pink | Co²⁺ or Mn²⁺ present |
Step 2: Dry Heating Test
| Observation | Inference |
|---|---|
| Colourless, odourless gas (turns lime water milky) | CO₃²⁻ (carbonate) |
| Pungent smell (turns moist red litmus blue) | NH₄⁺ (ammonium) |
| Brown fumes | NO₃⁻ (nitrate) |
| Residue yellow when hot, white when cold | Zn²⁺ (zinc) |
Step 3: Wet Tests for Anions (Acid Radicals)
| Anion | Test | Observation |
|---|---|---|
| CO₃²⁻ | Dil. HCl + lime water | Effervescence, lime water turns milky |
| SO₄²⁻ | Dil. HCl + BaCl₂ | White precipitate (insoluble in conc. HCl) |
| Cl⁻ | Dil. HNO₃ + AgNO₃ | White curdy precipitate (soluble in NH₄OH) |
| NO₃⁻ | Conc. H₂SO₄ + Cu turnings (heat) | Brown fumes |
| CH₃COO⁻ | Dil. H₂SO₄ (warm) | Vinegar-like smell |
| S²⁻ | Dil. HCl | Rotten egg smell (H₂S gas) |
| SO₃²⁻ | Dil. HCl | Gas turns acidified K₂Cr₂O₇ green |
Step 4: Wet Tests for Cations (Basic Radicals)
Group Analysis:
| Group | Reagent | Cations Precipitated |
|---|---|---|
| Group 0 | NaOH + heat | NH₄⁺ (gas turns moist red litmus blue) |
| Group I | Dil. HCl | Pb²⁺ (white ppt) |
| Group II | H₂S in acidic medium | Cu²⁺ (black), Pb²⁺ (black) |
| Group III | NH₄OH + NH₄Cl | Fe³⁺ (brown), Al³⁺ (white) |
| Group IV | H₂S in alkaline medium | Zn²⁺ (white), Ni²⁺ (black), Co²⁺ (black) |
| Group V | (NH₄)₂CO₃ | Ca²⁺ (white), Ba²⁺ (white), Sr²⁺ (white) |
| Group VI | No group reagent | Na⁺, K⁺ — flame test |
C. Tests for Functional Groups
| Functional Group | Test | Positive Result |
|---|---|---|
| -OH (Alcohol) | Sodium metal test | Hydrogen gas evolved |
| -CHO (Aldehyde) | Tollens' test (silver mirror) | Silver mirror forms |
| -CHO (Aldehyde) | Fehling's test | Red precipitate (Cu₂O) |
| -COOH (Carboxylic acid) | NaHCO₃ test | Brisk effervescence (CO₂) |
| -NH₂ (Amine) | Reaction with HNO₂ | N₂ gas evolved (1° amine) |
| -C=C- (Unsaturation) | Bromine water test | Decolourisation of Br₂ water |
| Phenolic -OH | FeCl₃ test | Violet/green/blue colour |
| Ketone | 2,4-DNP test | Yellow/orange precipitate |
D. Tests for Food Adulterants
| Food | Adulterant | Test |
|---|---|---|
| Milk | Water | Lactometer reading below 1.026 |
| Ghee | Vanaspati | Baudouin test — HCl + furfural (red colour) |
| Turmeric | Metanil yellow | Conc. HCl — turns pink/magenta |
| Chilli powder | Brick powder | Add to water — brick powder settles |
| Sugar | Chalk powder | Dissolve in water — chalk settles |
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Top 25 Viva Questions with Answers
Q1: What is the principle of titration? Titration determines the concentration of an unknown solution by reacting it with a solution of known concentration (standard solution) until the equivalence point, indicated by a colour change.
Q2: Why is KMnO₄ a self-indicator? KMnO₄ is intensely purple/pink in colour. When added to an acidified solution of Mohr's salt, it gets decolourised as it reacts. The end point is reached when a single drop of KMnO₄ gives a permanent light pink colour, meaning all the reducing agent is consumed.
Q3: Why do we heat the solution before titrating with KMnO₄? The reaction between KMnO₄ and oxalic acid/Mohr's salt is slow at room temperature. Heating to 60-70°C increases the rate of reaction. We don't boil because oxalic acid decomposes above 70°C.
Q4: What is a primary standard? Give an example. A substance of known high purity that can be weighed accurately and used to prepare a standard solution. Examples: oxalic acid, Na₂CO₃, Mohr's salt. KMnO₄ is NOT a primary standard (it decomposes and its exact concentration is uncertain).
Q5: Why do we wash the burette with the solution to be filled? To remove water drops that would dilute the solution and change its concentration. The burette is first washed with water, then rinsed with the solution before filling.
Q6: What is concordant reading? Two consecutive titration readings that agree within 0.1 mL. The experiment requires at least two concordant readings for a reliable result.
Q7: What is the role of dilute H₂SO₄ in KMnO₄ titration? It provides an acidic medium necessary for the reduction of MnO₄⁻ to Mn²⁺ (colourless). We use H₂SO₄, not HCl (which reacts with KMnO₄) or HNO₃ (which is itself an oxidizing agent).
Q8: How do you identify the cation and anion of an unknown salt? Systematic qualitative analysis: physical examination → dry heating test → flame test → wet tests for anion (acid radical) using specific reagents → group analysis for cation (basic radical) → confirmatory tests.
Q9: What is a flame test? Which cations give characteristic colours? Dip a platinum/nichrome wire in conc. HCl and then in the salt. Hold in a non-luminous flame. Na⁺ = golden yellow, K⁺ = violet (through blue glass), Ca²⁺ = brick red, Ba²⁺ = apple green, Cu²⁺ = blue-green.
Q10: Why do we use dilute HCl instead of H₂SO₄ for testing carbonates? Dilute HCl is preferred because H₂SO₄ may form insoluble sulphates (BaSO₄, PbSO₄) that coat the salt and prevent further reaction. HCl gives soluble chlorides in most cases.
Q11: What is Tollens' reagent? How is it prepared? Ammoniacal silver nitrate — prepared by adding NH₄OH to AgNO₃ solution until the precipitate (Ag₂O) just dissolves. It oxidizes aldehydes to carboxylic acids and Ag⁺ gets reduced to metallic silver (silver mirror).
Q12: How do you distinguish between an aldehyde and a ketone? Aldehydes give a positive Tollens' test (silver mirror) and Fehling's test (red precipitate). Ketones give negative results for both. Both give a yellow-orange precipitate with 2,4-DNP (Brady's reagent).
Q13: What is the difference between a precipitate and a solution? A precipitate is an insoluble solid that forms when two solutions react. It settles at the bottom or remains suspended. A solution is a homogeneous mixture where the solute is completely dissolved.
Q14: Why should we not use concentrated acids directly on skin? Concentrated acids (H₂SO₄, HCl, HNO₃) are corrosive. They cause severe chemical burns by dehydrating or oxidizing skin tissue. Always dilute acids by adding acid to water (never water to acid).
Q15: What is the normality and molarity of the solution you used? You should know the exact concentration. For example: 0.1N KMnO₄ means 0.1 gram equivalents per litre. Molarity = normality / n-factor. Be ready to calculate this.
Q16: What gas is evolved when Na₂CO₃ reacts with HCl? CO₂ gas is evolved. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑. The gas turns lime water milky (Ca(OH)₂ + CO₂ → CaCO₃↓).
Q17: What is the difference between a qualitative and quantitative analysis? Qualitative: identifies what substances are present (salt analysis). Quantitative: determines how much of a substance is present (titration).
Q18: Why is filter paper used in salt analysis? To separate precipitates from the solution by filtration. The precipitate is retained on the filter paper while the filtrate passes through. This allows testing the precipitate separately.
Q19: What is an indicator? Name two acid-base indicators. A substance that shows a visible change (colour) at the end point of a reaction. Phenolphthalein (colourless in acid, pink in base) and methyl orange (red in acid, yellow in base).
Q20: What is the role of NH₄Cl in Group III analysis? NH₄Cl suppresses the ionization of NH₄OH (common ion effect), ensuring only Group III cations (Fe³⁺, Al³⁺) precipitate. Without it, Group IV and V cations might also precipitate prematurely.
Q21: How do you test for the presence of sulphate ions? Add dilute HCl followed by BaCl₂ solution. A white precipitate of BaSO₄ (insoluble in conc. HCl) confirms SO₄²⁻.
Q22: What is a buffer solution? A solution that resists change in pH when small amounts of acid or base are added. Example: CH₃COOH + CH₃COONa (acidic buffer), NH₄OH + NH₄Cl (basic buffer).
Q23: What safety precautions should you take in the chemistry lab? Wear lab coat. Never taste chemicals. Never point test tubes towards anyone when heating. Add acid to water, not water to acid. Wash hands after experiments. Report spills immediately.
Q24: What is the principle of chromatography? Separation based on differential adsorption of components on a stationary phase. Components with stronger affinity for the stationary phase move slower, causing separation.
Q25: What is a hydrogen bond? Where is it important in chemistry? A weak electrostatic attraction between H bonded to an electronegative atom (O, N, F) and another electronegative atom. Important in water's properties, DNA structure, protein folding, solubility.
Common Mistakes to Avoid
| Mistake | Consequence | Solution |
|---|---|---|
| Not washing burette with solution | Dilution error, wrong titre | Rinse twice with solution before filling |
| Reading meniscus incorrectly | Wrong volume reading | Read at lower meniscus for clear liquids, eye level |
| Adding water to acid | Violent exothermic splashing | Always add acid to water slowly |
| Not heating before KMnO₄ titration | Reaction too slow, wrong endpoint | Heat to 60-70°C |
| Wrong sequence in salt analysis | Missed cation/anion | Follow systematic order strictly |
| Not recording observations immediately | Forgetting details | Write as you observe |
| Messy answer sheet | Poor impression, unclear | Use tables, neat writing |
Lab Safety Rules
- Always wear a lab coat and safety glasses during experiments
- Never pipette by mouth — use a pipette filler
- Handle concentrated acids and bases with extreme care
- Keep flammable substances away from open flames
- Know the location of the fire extinguisher, first aid kit, and eye wash station
- Clean up spills immediately — acid spills with NaHCO₃, base spills with dilute acid
- Dispose of chemicals properly — never pour them down the sink without permission
- Label all solutions clearly
- Never eat, drink, or apply cosmetics in the lab
- Wash hands thoroughly before leaving the lab
Frequently Asked Questions
What if I get the wrong salt in salt analysis?
If you identify the wrong cation or anion, you lose significant marks. However, if your procedure is correct and observations are recorded honestly, you still get marks for methodology. Always double-check with a confirmatory test before writing your final answer.
How many concordant readings are needed in titration?
At least two concordant readings (within 0.1 mL of each other). Take 3-4 readings total. Discard any anomalous reading and use the concordant ones to calculate the mean titre value.
Is the practical exam conducted by our own school teacher?
CBSE sends an external examiner along with your internal teacher. The external examiner conducts the viva and oversees the practical. Be respectful and confident.
How should I present my salt analysis answer?
Use a table format: Experiment | Observation | Inference. Write clearly. Underline the final answer (cation and anion identified). Mention the chemical formula of the salt if possible.
What should I do if I don't know an answer during viva?
Say "I'm not sure about this" honestly rather than giving a wrong answer. The examiner may give you a hint or move to the next question. Honesty is better than confidently giving incorrect information.
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