Ch 1Basic Concepts of Chemistry and Chemical Calculations — Class 11 Chemistry, concept-first.

This unit opens with a line from the Greek philosopher Democritus -- "we think there is colour, we think there is sweet, we think there is bitter, but in reality there are atoms and a void" -- setting…

Chemistry is not a subject confined to a laboratory -- it is the science behind the three most basic human needs described in the classical Tamil phrase "Unna unavu, udukka udai, irukka idam" (food to…

Matter is anything that has mass and occupies space; your desk, the air you breathe, and your own body are all matter, and all matter is built from atoms.

Matter exists in three familiar physical states -- solid, liquid and gas -- depending on how tightly its particles are held together.

Classifying matter by chemical composition gives a different, and more chemically meaningful, split: mixtures versus pure substances.

Having classified matter, the next question is: how heavy is an individual atom or molecule, and how do we compare the "weights" of different atoms and compounds? This section builds the relative mass…

An atom is extraordinarily small (diameter of the order of 10⁻¹⁰ m) and extraordinarily light (mass of the order of 10⁻²⁷ kg), so its absolute mass cannot be measured directly on any balance.

Just as relative atomic mass compares an atom's mass to the unified atomic mass unit, the relative molecular mass of a compound compares the mass of one molecule of it to the same unified atomic mass…

Chemists constantly need to talk about huge numbers of atoms or molecules -- literally hundreds of billions of trillions of them in even a small sample -- and counting them one by one is obviously imp…

The number of elementary entities present in exactly one mole of any substance -- 6.022 × 10²³ -- is called the Avogadro number (or Avogadro constant), named after the Italian physicist Amedeo Avogadr…

Molar mass is defined as the mass of one mole of a substance. Numerically, the molar mass of a compound equals the sum of the relative atomic masses of its constituent atoms (i.e.

Molar volume is the volume occupied by one mole of any substance in the gaseous state, at a stated temperature and pressure.

Alongside the mole concept, chemistry -- especially analytical chemistry -- makes heavy use of a second, related counting idea: the gram equivalent concept.

The equivalence factor n -- and hence the equivalent mass formula E = (molar mass) ÷ n -- takes a different, specific meaning depending on the type of chemical entity:

Once you know the elements present in a compound and their mass percentages (from experimental elemental analysis), you can work backwards to the compound's formula.

Given the mass-percentage composition of a compound from elemental analysis, its empirical formula is found by a fixed four-step procedure:

The empirical formula only tells you the ratio of atoms -- to get the actual molecular formula, you additionally need the compound's molar mass.

The word stoichiometry comes from the Greek stoicheion (element) and metron (measure) -- it is the numerical relationship between the quantities of the substances that appear in a balanced chemical eq…

Stoichiometry is formally the quantitative mole relationship between reactants and products in a balanced chemical equation.

All of the calculations in Section 1.7.1 assumed the reactants were supplied in exactly the stoichiometric ratio, so that everything reacts completely.

A cut apple turning brown, LPG burning, iron rusting -- all of these everyday changes are oxidation reactions.

Oxidation number (also called oxidation state) is the imaginary charge left on an atom once every other atom in the compound is notionally "removed," each atom being assigned its usual oxidation state…

Once you can compute oxidation numbers, you can classify every redox reaction into one of five types, based on the pattern of oxidation-number change:

Balancing a redox equation by simple inspection quickly becomes impractical once several elements change oxidation state at once.

This unit built, step by step, the quantitative toolkit that the rest of Chemistry relies on.

The unit's concept map is rooted at "Chemistry - the centre of life" leading into "Chemical substances (matter)", which branches first into Elements and Compounds.

The chapter closes with two evaluation sets covering everything above.

Sample papers and previous-year board questions for this subject.