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Science: Class 10 — Chapter 1: Chemical Reactions and Equations

1.1 Chemical Equations

1.1.1 Writing a Chemical Equation
  • Chemical Equation: A symbolic representation of a chemical reaction using formulae of reactants and products.
  • Reactants: Substances undergoing change in a reaction, written on the Left-Hand Side (LHS) with a plus sign (+).
  • Products: New substances formed in a reaction, written on the Right-Hand Side (RHS) with a plus sign (+).
  • Skeletal Equation: An unbalanced chemical equation where the count of atoms of each element on LHS is not equal to that on RHS.

1.1.2 Balanced Chemical Equations
  • Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction; total mass of elements in products must equal that in reactants.
  • Balancing Equations: Equalizing the number of atoms of each element on both sides of the arrow using the hit-and-trial method.
  • Physical States: Notations added for clarity: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solutions.
1.2 Types of Chemical Reactions

1.2.1 Combination Reaction
  • Combination Reaction: A reaction where two or more reactants combine to form a single product.
  • Exothermic Reactions: Chemical reactions accompanied by the release of heat (e.g., respiration, burning of natural gas).

1.2.2 Decomposition Reaction
  • Decomposition Reaction: A reaction where a single reactant breaks down into two or more simpler products.
  • Thermal Decomposition: Decomposition carried out by supplying heat energy.
  • Electrolytic Decomposition: Decomposition carried out using electric current (e.g., electrolysis of water).
  • Photolytic Decomposition: Decomposition triggered by light energy (e.g., silver chloride turning grey in sunlight).

1.2.3 Displacement Reaction
  • Displacement Reaction: A chemical reaction in which a more reactive element displaces a less reactive element from its compound.

1.2.4 Double Displacement Reaction
  • Double Displacement Reaction: A reaction where there is an exchange of ions between reactants to form new compounds.
  • Precipitation Reaction: Any reaction that produces an insoluble solid (precipitate) in water.

1.2.5 Oxidation and Reduction
  • Oxidation: The gain of oxygen or loss of hydrogen by a substance during a reaction.
  • Reduction: The loss of oxygen or gain of hydrogen by a substance during a reaction.
  • Redox Reaction: A reaction where oxidation and reduction occur simultaneously.
1.3 Effects of Oxidation Reactions in Everyday Life

1.3.1 Corrosion
  • Corrosion: The gradual deterioration of metals due to interaction with moisture, air, acids, or chemicals (e.g., rusting of iron, black silver tarnish, green copper coating).

1.3.2 Rancidity
  • Rancidity: The oxidation of fats and oils in food items, resulting in an unpleasant smell and taste.
  • Prevention: Retarded by adding antioxidants, storing food in airtight containers, or flushing packaging with nitrogen gas.