Chemistry entrance test

• Chemical Reaction


  The transformation of chemical substance into another chemical substance
• In a chemical reaction, a new substance is formed which is completely different in properties from the original substance, so in a chemical reaction, a chemical change takes place
• Only a rearrangement of atoms takes place in a chemical reaction
• Reactants


  Substances which take part in a chemical reaction
• Products


  New substances produced as a result of a chemical reaction
• Characteristics of Chemical Reactions

  • Evolution of gas
  • Change in Colour
  • Change in state of substance
  • Change in temperature
  • Formation of precipitate
• Chemical Equation


  Representation of chemical reaction using symbols and formulae of the substances
• Types of Chemical Equations

  • Balanced Chemical Equation
  • Unbalanced Chemical Equation
• Balancing a Chemical Equation

  1. Write the number of atoms of elements present in reactants and in products in a table

  2. Balance the atom which is maximum in number on either side of a chemical equation

  3. Multiply the atoms on the side with fewer atoms to balance the equation
• Types of Chemical Reactions

  • Combination Reaction
  • Decomposition Reaction
  • Displacement Reaction
  • Double Displacement Reaction
  • Neutralization Reactions
  • Exothermic – Endothermic Reactions
  • Oxidation-Reduction Reactions
• Combination Reaction


  Reactions in which two or more reactants combine to form one product
• Decomposition Reaction


  Reactions in which one compound decomposes in two or more compounds or elements
• Thermal Decomposition


  The decomposition of a substance on heating
• Electrolytic Decomposition


  Reactions in which compounds decompose into simpler compounds because of passing of electricity
• Photolysis or Photo Decomposition Reaction

  Reactions in which a compound decomposes because of sunlight
• Electrolytic Decomposition


  Reactions in which compounds decompose into simpler compounds because of passing of electricity, also known as Electrolysis
• Photographic paper has a coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal.
• Displacement Reaction


  The chemical reactions in which a more reactive element displaces a less reactive element from a compound, also known as Substitution Reaction or Single Displacement/ replacement reactions
• Displacement Reaction


  A + BC → AC + B
• Displacement reaction takes place only when 'A' is more reactive than B. If 'B' is more reactive than 'A', then 'A' will not displace 'C' from 'BC' and reaction will not be taking place.
• Double Displacement Reaction


  Reactions in which ions are exchanged between two reactants forming new compounds
• Double Displacement Reaction


  AB + CD → AC + BD
• Double Displacement Reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.
• Precipitation Reaction


  The reaction in which precipitate is formed by the mixing of the aqueous solution of two salts
• Neutralization Reaction


  The reaction in which an acid reacts with a base to form salt and water by an exchange of ions
• Oxidation


  Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound
• Reduction


  Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound
• Oxidation and Reduction take place together.
• Oxidizing agent


  The substance which gives oxygen for oxidation or removes hydrogen
• Reducing agent


  The substance which gives hydrogen for reduction or removes oxygen
• Redox reaction


  The reaction in which oxidation and reduction both take place simultaneously
• Exothermic Reaction


  Reaction which produces energy
• Endothermic Reaction


  A chemical reaction in which heat energy is absorbed
• Effects of Oxidation Reactions in Everyday life: Corrosion and Rancidity
• Corrosion


  The process of slow conversion of metals into their undesirable compounds due to their reaction with oxygen, water, acids, gases etc. present in the atmosphere
• Corrosion (rusting) weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships etc. and cuts short their life.
• Methods to Prevent Rusting

  • Painting
  • Greasing and oiling
  • Galvanisation
• Copper objects lose their lustre and shine after some time because the surface of these objects acquires a green coating of basic copper carbonate, CuCO3.Cu(OH)2 when exposed to air.
• The surface of silver metal gets tarnished (becomes dull) on exposure to air, due to the formation of a coating of black silver sulphide(Ag2S) on its surface by the action of H2S gas present in the air.
• Rancidity


  The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time, caused due to the oxidation of fat and oil present in food materials