12th Chemistry Important Questions – Units 1, 2, 3 & 6 for Quarterly 2026, Half‑Yearly 2026 and Public Exam 2027

12th Chemistry Important Questions – Units 1, 2, 3 and 6 (2 Marks, 3 Marks and 5 Marks)

Hello, dear students! This is KALVI MINI. A lot of students are asking 12th Standard Chemistry important questions, so I am giving the first set of important questions here.
These important questions are mainly based on the important concepts and the previous year midterm exam, quarterly exam, half-yearly exam, revision exam and public exams. Many of these questions have been asked again and again in previous year examinations.
That is why I am providing them in one place for you. All these questions are very important for your preparation up to the Public Exam 2027. So study these questions very well. I have also provided the key answer based on the textbook for you, so use this nicely for your examination.

Think of this as your final revision checklist. Once you have read through your textbook, use these answers to ensure you have the key points ready for the exam.

12th Chemistry Important Questions – Units 1, 2, 3 & 6 for Quarterly 2026, Half‑Yearly 2026 and Public Exam 2027

பன்னிரண்டாம் வகுப்பு வேதியியல்

இந்தப் பதிவில் மிக மிக முக்கியமான 2 மதிப்பெண், 3 மதிப்பெண் மற்றும் 5 மதிப்பெண் வினாக்கள் மற்றும் அவற்றிற்கான குறுகிய விடைகள் தொகுத்து வழங்கப்பட்டுள்ளன.

இங்கு இடம்பெற்றுள்ள அனைத்து வினாக்களும் இந்த இயல்களில் இருந்து பொதுத்தேர்வில் கேட்க அதிக வாய்ப்புள்ள, தேர்வுக்கு மிகவும் முக்கியமான வினாக்களாகத் தேர்ந்தெடுக்கப்பட்டுள்ளன.

எனவே, இந்த வினாக்களையும் அவற்றின் விடைகளையும் முழுமையாகப் படித்து நன்றாகப் பயிற்சி செய்து கொள்ளுங்கள். இது உங்கள் காலாண்டுத் தேர்வு, அரையாண்டுத் தேர்வு மற்றும் குறிப்பாக 2027 பொதுத்தேர்வில் சிறந்த மதிப்பெண்கள் பெற மிகவும் பயனுள்ளதாக இருக்கும்.

Unit 1: Metallurgy – Important Questions

Important 2-Mark Questions with Key Answers

1) Distinguish between minerals and ores.
Answer: Minerals are naturally occurring substances containing a metal in free or combined state. Ores are specific minerals from which a metal can be extracted profitably and easily. For example, clay is a mineral of aluminium, but bauxite is its ore.

2) What is the role of a “depressant” in froth flotation?
Answer: A depressant (like sodium cyanide) is used to selectively prevent one sulphide ore from coming to the froth when two sulphides are present. For example, in a mixture of ZnS and PbS, NaCN prevents ZnS from forming froth, allowing only PbS to be separated.

3) Define “Gangue” and “Slag”.
Answer: Gangue is the non‑metallic, earthy impurity (like silica) associated with the ore. Slag is the fusible compound formed when a flux reacts with the gangue during smelting.

Important 3-Mark Questions with Key Answers

4) Explain the principle and process of magnetic separation.

Principle: This method works on the difference in magnetic properties between the ore and the impurities.
Method: Crushed ore is dropped onto a moving belt passing over two rollers, one of which is magnetic.
Result: Magnetic particles are attracted and fall in a heap closer to the magnetic roller, while non‑magnetic particles fall further away.

5) Describe the process of roasting with an example.

Condition: Sulphide ores are heated with excess oxygen below their melting point.
Chemical change: The metal sulphide is converted into its metal oxide and impurities like sulphur escape as volatile oxides such as SO₂.
Example: 2ZnS + 3O₂ → 2ZnO + 2SO₂↑.

Important 5-Mark Questions – Answer Writing Hints

Topic: Froth Flotation Process

Subheadings: Principle (wetting properties), role of pine oil (frother), role of sodium ethyl xanthate (collector), role of the air stirrer.
Method: Explain how the ore particles preferentially stick to the froth while the gangue remains in the water. Mention how the froth is removed and the metal is recovered from it. Always include the labelled diagram showing the stirrer, tank and froth layer.

Topic: Zone Refining

Subheadings: Principle (fractional crystallisation), mobile heater setup, inert gas atmosphere.
Method: Explain that impurities are more soluble in the molten state than in the solid state. As the heater moves along the metal rod, the “molten zone” carries impurities to the end of the rod. Mention its use for obtaining ultra‑pure Ge, Si and Ga.

Unit 2: p-Block Elements – I – Important Questions

Important 2-Mark Questions with Key Answers

1) What is “inert pair effect”?
Answer: In heavier p‑block elements, the ns² electrons remain reluctant to participate in bonding due to poor shielding. This makes an oxidation state that is two units lower than the group oxidation state more stable (for example, Tl⁺ is more stable than Tl³⁺).

2) Define catenation.
Answer: Catenation is the ability of an element to form long chains or rings by bonding with its own atoms. Carbon shows maximum catenation power due to its high C–C bond energy.

3) Why are Group 18 elements called “inert gases”?
Answer: They have a completely filled valence shell configuration (ns²np⁶), which makes them highly stable and chemically unreactive under normal conditions.

Important 3-Mark Questions with Key Answers

4) Explain the anomalous properties of nitrogen.

Size and electronegativity: Nitrogen is very small and has higher electronegativity than the other group members.
Bonding: It can form pπ–pπ multiple bonds with itself (N≡N), which the heavier elements cannot form easily.
d‑Orbitals: Nitrogen has no d‑orbitals in its valence shell, so its maximum covalency is limited to four, unlike its heavier counterparts.

5) Write a note on hydroboration.

Reaction: Diborane (B₂H₆) reacts with alkenes or alkynes in ether solvents.
Product: Organoboranes are formed as the boron atom adds across the double bond.
Utility: These organoboranes are useful intermediates in organic synthesis, especially for the preparation of alcohols.

Important 5-Mark Questions – Answer Writing Hints

Topic: Structure of Diborane (B₂H₆)

Subheadings: Terminal B–H bonds, bridge B–H–B bonds, hybridisation of boron.
Method: Explain that there are four terminal 2‑centre‑2‑electron bonds and two “banana” 3‑centre‑2‑electron bonds. Mention sp³ hybridisation of boron. Draw the structure showing the two bridge hydrogens above and below the plane.

Unit 3: p-Block Elements – II – Important Questions

Important 2-Mark Questions with Key Answers

1) What happens when PCl₅ is heated?
Answer: On heating, phosphorus pentachloride undergoes thermal dissociation to form phosphorus trichloride and chlorine gas.
PCl₅ ⇌ PCl₃ + Cl₂.

2) Why is fluorine always assigned a −1 oxidation state?
Answer: Fluorine is the most electronegative element and has no d‑orbitals. It can only gain one electron to attain a stable configuration, so it never shows positive oxidation states and is always assigned −1.

3) Give the uses of argon.
Answer: Argon is used to provide an inert atmosphere in high‑temperature metallurgical processes such as arc welding. It is also used for filling electric bulbs to prevent oxidation of the filament.

Important 3-Mark Questions with Key Answers

4) Describe the preparation of phosphine (PH₃).

Method: Phosphine is prepared by reacting white phosphorus with a concentrated solution of sodium hydroxide in an inert atmosphere of carbon dioxide.
Equation: P₄ + 3NaOH + 3H₂O → 3NaH₂PO₂ + PH₃↑.
Observation: Phosphine is a foul‑smelling gas which forms characteristic smoky rings in moist air.

Important 5-Mark Questions – Answer Writing Hints

Topic: Manufacture of Sulfuric Acid (Contact Process)

Subheadings: Production of SO₂, catalytic oxidation to SO₃, absorption in H₂SO₄ (oleum formation), dilution to get desired concentration.
Method: List and explain each step clearly. Highlight the use of V₂O₅ as the catalyst and mention the optimum temperature and pressure conditions needed for maximum yield.

Unit 6: Solid State – Important Questions

Important 2-Mark Questions with Key Answers

1) Define a unit cell.
Answer: A unit cell is the smallest repeating structural unit of a crystalline solid which, when repeated in all directions, generates the entire crystal lattice.

2) What is anisotropy?
Answer: Crystalline solids are anisotropic, which means their physical properties such as refractive index or electrical resistance have different values when measured along different directions in the same crystal.

3) State three characteristics of ionic crystals.
Answer: (i) They have high melting points. (ii) They conduct electricity only in the molten or dissolved state. (iii) They are hard but brittle.

Important 3-Mark Questions with Key Answers

4) Distinguish between Schottky and Frenkel defects.

Schottky defect: Equal numbers of cations and anions are missing from the lattice. This leads to a decrease in the density of the crystal.
Frenkel defect: An ion (usually a smaller cation) leaves its normal lattice site and occupies an interstitial position. In this case the density of the crystal remains unchanged.

A point defect in ionic crystals where an equal number of cations and anions are missing from their lattice sites, creating vacancies while maintaining electrical neutrality. This defect decreases the density of the crystal.

5) Calculate the number of atoms in a BCC unit cell.

Corner atoms: 8 corners × 1/8 atom per corner = 1 atom.
Body‑centre atom: 1 atom at the centre = 1 atom.
Total: 1 + 1 = 2 atoms per BCC unit cell.

Important 5-Mark Questions – Answer Writing Hints

Topic: Packing Efficiency in FCC (Face‑Centred Cubic Structure)

Subheadings: Geometry of the unit cell, relationship between edge length (a) and radius (r), volume of four atoms in the cell, total volume of the unit cell.
Method: Show the step‑by‑step derivation for the packing efficiency. Use the relationship a = 2√2 r for FCC, calculate the volume occupied by four spheres and then the percentage of volume occupied. Conclude that the packing efficiency of FCC is about 74%, which is the highest among simple packing types.

Final Tips for Success

Do not just memorise the lines. Try to understand the logic behind each reaction, structure and definition. Use these questions to test yourself: first cover the answer and see if you can recall the key points on your own. Then compare with the key.

Combine this list with a thorough reading of your textbook and plenty of writing practice, especially for equations and diagrams. If you revise these questions properly, you will feel more confident for your exams.

Thank you, dear students. These are very important questions. I hope this is definitely useful for you. If you want any other study material, search on our website. Definitely you can find out other materials also.
If you have any doubt, you can contact us. All these important questions are especially helpful for slow learners, because many students have asked me again and again, “Sir, please give important questions.” That is why I made these important questions for you. Please use this very well.