Monday, 11 April 2011
Law of Conservation of mass
The law of conservation of mass states that mass can neither be created nor destroyed. Thus, in a chemical reaction, the total mass of the reactants should be equal to the total mass of the products. This means that the total number of atoms of each element should be equal on both sides of a chemical equation. Such an equation is called a balanced chemical equation, and the method by which it is obtained is called balancing of chemical equations. Therefore, we need to balance every chemical equation we write. An equation having an equal number of atoms of each element on both the sides is called a balanced chemical equation.
Chemical Reactions and Equations (Class Xth)
Question 1: Why should a magnesium ribbon be cleaned before it is burnt in air? Answer : Magnesium is an extremely reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper for removing this layer so that the underlying metal can be exposed to air. Question 2: Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine → Hydrogen chloride (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (iii) Sodium + Water → Sodium hydroxide + Hydrogen Answer : i) H2(g) + Cl2(g) --------> 2HCl (g) ii ) 3BaCl2 + Al2 (SO4)3 -------> 3BaSO4 + 2AlCl3 iii) 2Na + 2H2O ---------> 2NaOH + H2 Question 3 : Write a balanced chemical equation with state symbols for the following reactions. (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride. (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water. Answer : i) BaCl2 + Na2SO4 --------> BaSO4 + 2NaCl ii) NaOH + HCl ---------> NaCl + H2O
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