Class 12 Board exams can feel stressful—especially in Chemistry, where 1-mark and 2-mark questions often ask you to define a term, and case-based questions test whether you understand what that term means in application. So, instead of just memorising words, revise definitions with one supporting line (formula/example). That’s how you score consistently. If you’re revising from CBSE books these definition cards will help you write crisp, board-ready answers and score better in 1–2 mark questions.
Syllabus Note (CBSE 2025–26)
This post is updated as per the latest CBSE Class 12 Chemistry (Code 043) curriculum. Topics like The Solid State, Surface Chemistry, Metallurgy (Isolation of Elements), Polymers, Chemistry in Everyday Life, and p-Block are not part of Class 12 theory syllabus for CBSE 2025–26.
How to revise these definitions (Board-friendly method)
Use this 3-step pattern:
- Definition (1–2 lines)
- One formula / one example
- One common board use (“asked in numericals”, “asked in reasoning”, etc.)
Most Important Definitions in Chemistry (Class 12 CBSE 2025–26)
Unit 1: Solutions
1) Mole fraction (X)
Definition: Mole fraction of a component is the ratio of its moles to total moles of all components in the solution.
Formula: (X_A = \frac{n_A}{n_A+n_B})
2) Colligative properties
Definition: Properties of dilute solutions that depend only on the number of solute particles (not their nature).
Examples: Relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, osmotic pressure.
3) van’t Hoff factor (i)
Definition: Ratio of observed colligative property to the value calculated for an ideal (non-associating/non-dissociating) solute.
Use: (i>1) (dissociation), (i<1) (association).
4) Henry’s law
Definition: At constant temperature, solubility of a gas in a liquid is proportional to the partial pressure of the gas.
One form: (p = k_H x)
Unit 2: Electrochemistry
5) Conductivity / specific conductance ((\kappa))
Definition: Conductance of a solution of 1 cm length and 1 cm² cross-sectional area.
6) Molar conductivity ((\Lambda_m))
Definition: Conductivity of solution containing 1 mole of electrolyte, between electrodes 1 cm apart.
Formula: (\Lambda_m = \frac{\kappa \times 1000}{C}) (C in mol/L)
7) Kohlrausch’s law
Definition: At infinite dilution, molar conductivity equals the sum of ionic molar conductivities.
Idea: (\Lambda_m^0 = \lambda_+^0 + \lambda_-^0)
8) Faraday’s Laws of Electrolysis
First law: Mass deposited ∝ charge passed.
Second law: For the same charge, masses deposited ∝ equivalent weights.
You will also like: How to Learn Chemical Reactions Like A Pro!
Unit 3: Chemical Kinetics
9) Rate of reaction
Definition: Change in concentration of reactant/product per unit time.
Form: (\text{Rate} = -\frac{d[R]}{dt} = \frac{d[P]}{dt})
10) Order of reaction
Definition: Sum of powers of concentration terms in the experimentally determined rate law.
11) Molecularity
Definition: Number of reacting species involved in a single elementary step (always a whole number).
12) Activation energy ((E_a))
Definition: Minimum energy required for effective collisions leading to reaction.
Unit 4: d- and f-Block Elements
13) Lanthanoid contraction
Definition: Gradual decrease in atomic/ionic radii of lanthanoids with increasing atomic number due to poor shielding by 4f electrons.
Unit 5: Coordination Compounds
14) Denticity
Definition: Number of donor atoms in a ligand that can coordinate to the same metal ion.
15) Linkage isomerism
Definition: Isomerism due to ambidentate ligands binding through different donor atoms (e.g., NO₂⁻).
16) Coordination isomerism
Definition: Isomerism when both cation and anion are complex ions and ligands interchange between them.
(If you want to expand this section, you can also add: ligand, coordination number, chelation—very frequently asked.)
Unit 6: Haloalkanes and Haloarenes
17) SN1 reaction
Definition: Nucleophilic substitution unimolecular; rate depends only on substrate; proceeds via carbocation intermediate.
18) SN2 reaction
Definition: Nucleophilic substitution bimolecular; one-step backside attack; rate depends on both substrate and nucleophile.
Unit 7: Alcohols, Phenols and Ethers
19) Reimer–Tiemann reaction
Definition: Formylation of phenol using CHCl₃/NaOH to give o-hydroxybenzaldehyde (major).
20) Friedel–Crafts reaction
Definition: Electrophilic substitution on aromatic ring using a Lewis acid (e.g., AlCl₃) for alkylation/acylation.
Read more: How to Prepare for CBSE Class 12 Exams?
Unit 8: Aldehydes, Ketones and Carboxylic Acids
21) Cannizzaro reaction
Definition: Disproportionation of aldehydes without α-H in strong base to form alcohol and carboxylate salt.
22) Tollens’ reagent
Definition: Ammoniacal silver nitrate ([Ag(NH_3)_2]^+) used to oxidise aldehydes (silver mirror test).
23) Clemmensen reduction
Definition: Reduction of aldehydes/ketones to hydrocarbons using Zn(Hg)/conc. HCl.
24) Decarboxylation
Definition: Removal of CO₂ from carboxylic acid (often via sodium salt + soda lime) to yield hydrocarbon.
25) Cross aldol condensation
Definition: Aldol condensation between two different aldehydes/ketones (with α-H).
26) Hell–Volhard–Zelinsky (HVZ) reaction
Definition: α-halogenation of carboxylic acids using halogen and red phosphorus.
Important – CBSE Sample Paper Class 12 Download
Unit 9: Amines
27) Hofmann’s bromamide reaction
Definition: Conversion of amide to primary amine with one carbon less using Br₂/NaOH.
28) Gabriel phthalimide synthesis
Definition: Method to prepare primary aliphatic amines using phthalimide via alkylation and hydrolysis.
29) Coupling reaction (Azo coupling)
Definition: Reaction of diazonium salt with phenol/aniline to form azo dye (–N=N–).
30) Carbylamine reaction
Definition: Primary amine + CHCl₃ + alcoholic KOH → isocyanide (foul smell); test for 1° amines.
Unit 10: Biomolecules
31) Glycosidic linkage
Definition: C–O–C bond joining two monosaccharides.
32) Polypeptide
Definition: Polymer of amino acids linked by peptide (–CO–NH–) bonds.
33) β-pleated sheet
Definition: Secondary structure of proteins where chains form sheet-like arrangement stabilized by H-bonds.
34) Denaturation
Definition: Loss of biological activity/structure of proteins due to heat/pH/chemicals (primary structure usually remains intact).
35) Biocatalyst (enzyme)
Definition: Biological catalyst (mostly protein) that speeds up reactions without being consumed.
Bonus (Not in CBSE 2025–26 theory, but useful for NEET/JEE)
These are excellent for competitive exams but removed from CBSE Class 12 theory for 2025–26:
Solid State: ferromagnetism, Frenkel defect, Schottky defect
Surface Chemistry: electrophoresis, Tyndall effect, peptization, aerosol, emulsion
Metallurgy: froth floatation, zone refining, electrolytic refining
Polymers: types of polymerisation, elastomer, biodegradable polymers, addition/condensation polymers, copolymers
Final tip for boards
It has been observed that most of the questions in Board exam are asked directly from problems given in the NCERT textbook Exercises or in the NCERT Exemplar book. We have come up with all the solutions to these problems in our book NCERT Solutions-Chemistry and NCERT Exemplar Problems-Solutions for Class – XI & XII. You are advised to read all the solutions provided in these cbse class 12 books that cover complete NCERT and Exemplar, text book questions answers in detail. This will not only help you to understand the concepts but also learn their applications.
Moreover, most of the questions in Board exams are asked from previous papers. MTG’s CBSE Champion-Chemistry provides chapter-wise, topic-wise practice material including previous 10 years’ CBSE Board Examination questions. Preparing for your Chemistry Board paper with the help of this book will give you comprehensive knowledge of the subject according to the latest syllabus and pattern of CBSE Board Examination.
We hope the list of definitions provided in this article will help in your exam preparation and you will be able to score high in the upcoming Board exam.
“The best preparation for tomorrow is doing your best today”
ALL THE BEST!
Also, Check out – Board Exam Writing Skills – 5 Tips to Write Perfect Answers
