Class 11 Chemistry – Chapter 1 Notes
Importance of Chemistry
Chemistry plays a central role in science and technology. It is important in daily life, medicine,
industry, agriculture, and environmental protection. Applications include development of drugs,
fertilizers, polymers, food preservatives, and green chemistry innovations.
Properties of Matter
Physical properties: Measured without changing chemical composition (e.g., colour, density,
melting point). Chemical properties: Observed during chemical changes (e.g., flammability,
reactivity).
States of Matter
Solid: Definite shape and volume, strong intermolecular forces. Liquid: Definite volume, no fixed
shape. Gas: No fixed shape/volume, highly compressible.
Classification of Matter
Mixtures – Physical combination of substances. • Homogeneous: Uniform (air, salt solution). •
Heterogeneous: Non-uniform (sand in water). Pure substances – Elements (H, O, Fe) and
Compounds (H2O, CO2).
Laws of Chemical Combination
1. Law of Conservation of Mass (Lavoisier) – Mass can neither be created nor destroyed. 2. Law of
Definite Proportions (Proust) – Compound has same elements in fixed ratio by mass. 3. Law of
Multiple Proportions (Dalton) – Mass ratios of elements in compounds are simple whole numbers.
4. Law of Gaseous Volumes (Gay Lussac) – Volumes of gases combine in simple ratios. 5.
Avogadro’s Law – Equal volumes of gases at same T & P contain equal number of molecules.
Dalton’s Atomic Theory
Matter is made of indivisible atoms. Atoms of same element are identical. Atoms combine in simple
ratios to form compounds. Atoms cannot be created or destroyed. Limitations: Does not explain
isotopes, isobars, or atomic structure.
Atomic and Molecular Mass
Atomic mass: Mass of atom in unified mass unit (1 u = 1/12 of mass of C-12 atom). Molecular
mass: Sum of atomic masses of atoms in a molecule.
Mole Concept
1 mole = 6.022 × 10^23 particles (Avogadro number). Moles = Given mass / Molar mass. Moles =
Number of particles / Avogadro’s number.
Molar Volume of Gases
At STP (0°C, 1 atm), 1 mole of gas occupies 22.4 L. Moles = Volume of gas at STP / 22.4 L.
Empirical and Molecular Formula
Empirical formula – simplest ratio of atoms. Molecular formula – actual ratio of atoms. Molecular
formula = (Empirical formula)n, where n = molar mass / empirical formula mass.
Importance of Chemistry
Chemistry plays a central role in science and technology. It is important in daily life, medicine,
industry, agriculture, and environmental protection. Applications include development of drugs,
fertilizers, polymers, food preservatives, and green chemistry innovations.
Properties of Matter
Physical properties: Measured without changing chemical composition (e.g., colour, density,
melting point). Chemical properties: Observed during chemical changes (e.g., flammability,
reactivity).
States of Matter
Solid: Definite shape and volume, strong intermolecular forces. Liquid: Definite volume, no fixed
shape. Gas: No fixed shape/volume, highly compressible.
Classification of Matter
Mixtures – Physical combination of substances. • Homogeneous: Uniform (air, salt solution). •
Heterogeneous: Non-uniform (sand in water). Pure substances – Elements (H, O, Fe) and
Compounds (H2O, CO2).
Laws of Chemical Combination
1. Law of Conservation of Mass (Lavoisier) – Mass can neither be created nor destroyed. 2. Law of
Definite Proportions (Proust) – Compound has same elements in fixed ratio by mass. 3. Law of
Multiple Proportions (Dalton) – Mass ratios of elements in compounds are simple whole numbers.
4. Law of Gaseous Volumes (Gay Lussac) – Volumes of gases combine in simple ratios. 5.
Avogadro’s Law – Equal volumes of gases at same T & P contain equal number of molecules.
Dalton’s Atomic Theory
Matter is made of indivisible atoms. Atoms of same element are identical. Atoms combine in simple
ratios to form compounds. Atoms cannot be created or destroyed. Limitations: Does not explain
isotopes, isobars, or atomic structure.
Atomic and Molecular Mass
Atomic mass: Mass of atom in unified mass unit (1 u = 1/12 of mass of C-12 atom). Molecular
mass: Sum of atomic masses of atoms in a molecule.
Mole Concept
1 mole = 6.022 × 10^23 particles (Avogadro number). Moles = Given mass / Molar mass. Moles =
Number of particles / Avogadro’s number.
Molar Volume of Gases
At STP (0°C, 1 atm), 1 mole of gas occupies 22.4 L. Moles = Volume of gas at STP / 22.4 L.
Empirical and Molecular Formula
Empirical formula – simplest ratio of atoms. Molecular formula – actual ratio of atoms. Molecular
formula = (Empirical formula)n, where n = molar mass / empirical formula mass.
