Some Basic Concepts of Chemistry Notes for NEET and JEE Main 2027

Some Basic Concepts of Chemistry is Unit 1 of NCERT Class 11 Chemistry Part I and Unit 1 of both the NEET UG and JEE Main chemistry syllabus. These notes cover matter, SI units, significant figures, laws of chemical combination, Dalton’s theory, atomic and molar mass, the mole, empirical formulae, stoichiometry, limiting reagent and concentration terms, with a free PDF.

Some Basic Concepts of Chemistry Notes for NEET and JEE Main

Section-wise key points from NCERT Unit 1, formula tables, high-yield facts and common mistakes for 2027 revision.

Some Basic Concepts of Chemistry at a Glance

The topic lists follow the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025 and the JEE Main 2026 syllabus released by NTA on 31 October 2025, which stay the reference until the 2027 syllabi are out. Full lists: NEET syllabus and JEE Main syllabus.

Matter and Its Classification

Matter is anything that has mass and occupies space. It exists as solid, liquid or gas, and the three states are interconvertible by changing temperature and pressure.

• Mixtures have variable composition. Homogeneous mixtures (sugar solution, air) are uniform throughout; heterogeneous mixtures (salt and sugar, grains with stones) are not.
• Components of a mixture can be separated by physical methods: hand-picking, filtration, crystallisation, distillation.
• Pure substances have fixed composition and are either elements (one type of atom, e.g. Na, Cu, H2, O2) or compounds (atoms of different elements in a fixed ratio, e.g. H2O, NH3, CO2).
• Constituents of a compound cannot be separated by physical methods, only by chemical methods, and a compound’s properties differ from those of its elements (water puts out fire; hydrogen burns and oxygen supports combustion).
• Physical properties (colour, odour, melting point, boiling point, density) are measured without changing the substance; chemical properties (acidity, basicity, combustibility) need a chemical change.

SI Units and Measurement

The International System of Units (SI) was established in 1960 by the 11th General Conference on Weights and Measures (CGPM) and has seven base units.

• Mass is the amount of matter and stays constant; weight is the force of gravity on an object and can vary from place to place.
• Volume: 1 L = 1000 mL and 1000 cm³ = 1 dm³. The litre is not an SI unit; the SI unit of volume is m³.
• Density = mass / volume; SI unit kg m⁻³, but chemists usually use g cm⁻³.
• Temperature: K = °C + 273.15 and °F = (9/5)(°C) + 32. Negative temperatures are possible on the Celsius scale but not on the Kelvin scale.
• In India, the National Physical Laboratory (NPL), New Delhi maintains the national standards of measurement.

Significant Figures, Precision and Accuracy

Significant figures are the digits known with certainty plus one estimated digit, so they show the uncertainty in a measurement.

• All non-zero digits are significant: 285 cm has three.
• Zeros before the first non-zero digit are not significant: 0.03 has one, 0.0052 has two.
• Zeros between non-zero digits are significant: 2.005 has four.
• Trailing zeros are significant only with a decimal point: 0.200 g has three; 100 has one, while 100.0 has four.
• Exact counts (2 balls, 20 eggs) have infinite significant figures. In scientific notation all digits are significant: 4.01 × 10² has three.
• Addition and subtraction: keep no more decimal places than the number with the fewest (12.11 + 18.0 + 1.012 = 31.1).
• Multiplication and division: keep no more significant figures than the least precise number (2.5 × 1.25 = 3.1).
• Rounding off when the digit removed is exactly 5: NCERT raises the preceding digit if it is odd and leaves it if it is even (6.35 becomes 6.4; 6.25 becomes 6.2).

Dimensional analysis (the unit factor or factor label method) converts units by multiplying by factors equal to 1, such as 2.54 cm / 1 in.

Laws of Chemical Combination

Five basic laws govern how elements combine to form compounds.

• Gay Lussac’s law is the law of definite proportions by volume; Avogadro’s work in 1811 explained it.
• Avogadro’s proposal gained support only after Stanislao Cannizaro presented it at the first international conference on chemistry at Karlsruhe, Germany, in 1860.

Dalton’s Atomic Theory

John Dalton published ‘A New System of Chemical Philosophy’ in 1808 with four main postulates.

• Matter consists of indivisible atoms.
• All atoms of a given element have identical properties, including identical mass; atoms of different elements differ in mass.
• Compounds form when atoms of different elements combine in a fixed ratio.
• Chemical reactions involve reorganisation of atoms; atoms are neither created nor destroyed in a chemical reaction.
• Limitations: it explained the laws of chemical combination but not the law of gaseous volumes, and it could not explain why atoms combine.

Atomic, Molecular and Formula Mass

Atomic masses are measured relative to carbon-12, which is assigned exactly 12 atomic mass units (agreed in 1961).

• One atomic mass unit is exactly one-twelfth of the mass of one carbon-12 atom; 1 amu = 1.66056 × 10⁻²⁴ g. Today ‘amu’ is replaced by ‘u’ (unified mass).
• Average atomic mass accounts for isotopes and their abundance: carbon (12C 98.892%, 13C 1.108%, 14C about 2 × 10⁻¹⁰%) averages 12.011 u. Periodic table values are average atomic masses.
• Molecular mass is the sum of the atomic masses in a molecule: CH4 = 16.043 u, H2O = 18.02 u, glucose C6H12O6 = 180.162 u.
• Formula mass is used for ionic solids that have no discrete molecules: NaCl = 23.0 + 35.5 = 58.5 u. In NaCl each Na⁺ is surrounded by six Cl⁻ and vice versa.

Mole Concept and Molar Mass

One mole contains exactly 6.02214076 × 10²³ elementary entities; this fixed number is the Avogadro constant, NA (unit mol⁻¹).

• The mole is the SI unit of amount of substance, the seventh base quantity. An entity can be an atom, molecule, ion, electron or any specified group of particles.
• Molar mass is the mass of one mole in grams and is numerically equal to the atomic, molecular or formula mass in u: water 18.02 g mol⁻¹, NaCl 58.5 g mol⁻¹.
• 1 mol of water contains 6.022 × 10²³ molecules; 1 mol of NaCl contains 6.022 × 10²³ formula units.

Percentage Composition, Empirical and Molecular Formula

Mass per cent of an element = (mass of that element in the compound × 100) / molar mass of the compound.

• Water: H = 11.18%, O = 88.79%. Ethanol (C2H5OH, 46.068 g): C = 52.14%, H = 13.13%, O = 34.73%.
• The empirical formula is the simplest whole-number ratio of atoms; the molecular formula gives the exact number of each type of atom.
• Steps: take 100 g so mass per cent becomes grams; divide by atomic masses to get moles; divide by the smallest mole value; convert to whole numbers; write the empirical formula.
• Molecular formula = (empirical formula) × n, where n = molar mass / empirical formula mass.
• NCERT example: 4.07% H, 24.27% C, 71.65% Cl and molar mass 98.96 g give CH2Cl (empirical formula mass 49.48 g), n = 2, so C2H4Cl2.

Stoichiometry and Limiting Reagent

Stoichiometry (Greek stoicheion, element, and metron, measure) is the calculation of masses, and sometimes volumes, of reactants and products from a balanced equation.

• CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) means 1 mol CH4 reacts with 2 mol O2 to give 1 mol CO2 and 2 mol H2O; equally 16 g CH4 + 64 g O2 give 44 g CO2 + 36 g H2O.
• Stoichiometric coefficients give the mole (and molecule) ratio of the species taking part.
• A balanced equation has the same number of atoms of each element on both sides. Balance by changing coefficients, never subscripts (P4 + 5O2 → P4O10; C3H8 + 5O2 → 3CO2 + 4H2O).
• The limiting reagent is the reactant consumed first; it limits the amount of product. In NCERT’s example, 50.0 kg N2 and 10.0 kg H2 give 56.1 kg NH3 because H2 is limiting.

Concentration Terms for Reactions in Solutions

Most laboratory reactions are carried out in solution, so the amount of solute in a given quantity of solution is expressed in four ways.

• Dilution: M1 × V1 = M2 × V2. To make 1 L of 0.2 M NaOH from 1 M stock, take 200 mL and dilute to 1 L.
• Molarity to molality needs density: 3 M NaCl with density 1.25 g mL⁻¹ has a molality of 2.79 m.

Most-asked Facts for NEET and JEE Main: Some Basic Concepts of Chemistry

Revise these one-line facts from the NCERT unit just before the exam.

• Matter: anything that has mass and occupies space.
• SI has seven base units; the mole is the unit of amount of substance and the candela of luminous intensity.
• K = °C + 273.15; °F = (9/5)°C + 32.
• 0.0052 has two significant figures; 100.0 has four; exact counts have infinite.
• Precision is closeness of repeated readings; accuracy is closeness to the true value.
• Lavoisier: conservation of mass (1789). Proust: definite proportions. Dalton: multiple proportions (1803). Gay Lussac: gaseous volumes (1808). Avogadro’s law (1811).
• 1 amu = 1/12 of the mass of a carbon-12 atom = 1.66056 × 10⁻²⁴ g.
• Average atomic mass of carbon = 12.011 u.
• NA = 6.02214076 × 10²³ mol⁻¹ (exact).
• Molecular formula = n × empirical formula; n = molar mass / empirical formula mass.
• Limiting reagent is consumed first and fixes the amount of product.
• Molality is temperature independent; molarity is not.
• M1V1 = M2V2 for dilution.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this unit.

• Treating mass and weight as the same. Mass is constant; weight changes with gravity.
• Counting leading zeros as significant. In 0.0025 only 2 and 5 are significant.
• Rounding 6.25 to 6.3. By NCERT’s rule an even preceding digit stays, so it becomes 6.2.
• Calling a precise result accurate. Close repeated readings can still be far from the true value.
• Changing subscripts to balance an equation. Only coefficients may change.
• Picking the reactant with the smaller mass as the limiting reagent. Compare moles against the mole ratio in the balanced equation.
• Using molecular mass for ionic solids like NaCl. Use formula mass.
• Dividing by the volume of solvent for molarity or the mass of solution for molality. Molarity uses litres of solution; molality uses kg of solvent.
• Assuming the empirical and molecular formulas are always different. They are the same when n = 1.

Some Basic Concepts of Chemistry Notes PDF and Related Pages

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• Source text: NCERT Unit 1 (NCERT, Class 11 Chemistry Part I)

Chemistry Notes for NEET and JEE Main: All Chapters

• Some Basic Concepts of Chemistry
• Structure of Atom
• Chemical Kinetics
• Equilibrium
• Organic Chemistry: Some Basic Principles and Techniques
• Electrochemistry
• Amines
• Solutions
• Thermodynamics
• Coordination Compounds
• Haloalkanes and Haloarenes
• p-Block Elements
• Alcohols, Phenols and Ethers
• Hydrocarbons
• Aldehydes, Ketones and Carboxylic Acids

All chapters with key points and notes PDFs are on the Chemistry notes for NEET and JEE Main page.

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