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Harbinder Gill
Saint Soldier's Divine Public School, Panchkula

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www.ncerthelp.com CHEMICAL REACTIONS AND EQUATIONS GIST OF THE LESSON 1) Chemical reaction Chemical changes or chemical reactions are the changes in which one or more new substances are formed. 2 )Chemical Equations Representation of a chemical reaction in terms of symbols and formulae of the reactants and products is known as chemical equation. 3) Balanced Chemical equations The chemical equation in which the no. of atoms of different elements is same on both sides of the arrow is called balanced chemical equation. 4) The chemical reactions can be classified into different types such as a) Combination reaction The reactions in which two or more substances combine to form a new substance are called combination reaction. For example, 2Mg(s) + O2 (g) 2 MgO (s) b) Decomposition reaction - The reaction in which a single compound breaks up into two or more simpler substances are called decomposition reactions. For example, 2Pb (NO3)2 (s) 2PbO (s) + 4NO2 (g) +O2 (g) The decomposition of a substance by passing electric current through it is known as electrolysis. The decomposition of a substance on heating is known as thermal decomposition. The decomposition of a substance by absorbing light energy is called photochemical decomposition. c) Displacement reactions -The chemical reactions in which a more reactive element displaces a less reactive element from a compound are known as displacement reactions. For example, i) Zn (s) + CuSO4 (aq) ZnSO4 (aq) + Cu (s). ii) Cu (s) + 2AgNO3 (aq) Cu (NO3)2 (aq) +2Ag (s). d) Double Displacement Reactions - The chemical reactions in which compounds react to form two different compounds by mutual exchange of ions are called double displacement reactions. These reactions take place in solution two common types of this reaction are precipitation reactions and neutralization reactions i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed whereby Some salts precipitate due to mutual exchange of ions between the two salts. For example AgNO3 (aq) + NaCI (aq) -----> AgCl(s) + NaNO3. ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt and water by exchange of ions. NaOH (aq) + HCl (aq) NaCl (aq) + H2O. 7 Visit www.ncerthelp.com for Ncert Solutions in Text and Video , CBSE Sample papers, Exam tips, NCERT BOOKS, Motivational Videos, Notes for All Classes and Many More...

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