Class 10 Science

1.1 How you know a reaction has happened A reaction forms new substances. A change of state, colour or temperature, or a gas given off, points to one. 1.2 Writing reactions down From a sentence to a word equation to formulae. A skeletal equation has correct formulae but unequal atom counts. 1.3 Balancing equations Atoms are not created or destroyed, so each element must have equal counts on both sides. Change coefficients, never formulae. 1.4 Combination and decomposition Combination forms one product from several reactants; decomposition breaks one reactant into several. 1.5 Exothermic and endothermic An exothermic reaction releases heat; an endothermic one takes heat in. Bond energies decide which. 1.6 Displacement reactions A more reactive metal displaces a less reactive one from a solution of its compound. The reverse does not occur. 1.7 Double displacement and precipitation Two ionic compounds exchange ions. If one new combination is insoluble, it separates as a precipitate. 1.8 Oxidation, reduction, and everyday life Gain of oxygen is oxidation, loss is reduction, and the two occur together. Rusting and rancidity are oxidation.