Some Basic Concepts of Chemistry — Class 11 Chemistry

Chemistry is the science that studies the preparation, properties, structure and reactions of material substances. Matter — anything that has mass and occupies space — exists in three interconvertible physical states (solid, liquid, gas) and is classified into pure substances (elements and compounds) and mixtures (homogeneous or heterogeneous). Properties may be physical or chemical, and are measured using the SI scheme of seven base units. Because every measurement carries uncertainty, results are expressed using scientific notation , significant figures, and the factor-label method of dimensional analysis. Five laws of chemical combination — the law of conservation of mass (Lavoisier), the law of definite proportions (Proust), the law of multiple proportions (Dalton), Gay-Lussac's law of gaseous volumes, and Avogadro's law — led John Dalton to his atomic theory. Atomic masses are referred to the carbon-12 standard, and average atomic mass accounts for isotopic abundance. The mole, containing entities, links mass to number of particles; molar mass in grams equals the atomic, molecular or formula mass in unified mass units. Percentage composition yields empirical and molecular formulae, and stoichiometry uses balanced equations, the limiting-reagent idea and concentration terms such as mass per cent, mole fraction, molarity and molality.