20. The series of processes by which nitrogen and its compounds are interconverted in the environment and in living organisms, including nitrogen fixation and decomposition.21.A) The primary difference between the two is that Thermoset is a material that STRENGTHENS when heated, but cannot be remolded or heated after the initial forming, while thermoplastics can be reheated, remolded, and cooled as necessary without causing any chemical changes.B) A synthetic resin in which the polymer units are linked by ester groups, used chiefly to make synthetic textile fibres.Common uses Industrial polyester fibers, yarns and ropes are used in car tire reinforcements, fabrics for conveyor belts, safety belts, coated fabrics and plastic reinforcements with high-energy absorption. Polyester fiber is used as cushioning and insulating material in pillows, comforters,etc.22.A) Copper Vessels turn greenish black with the passage of time as the copper is oxidized to copper oxide and most of it reacts with carbon dioxide in the atmosphere to form basic copper carbonate which is green in colour.B) Copper cannot displace zinc from its salt solution because zinc is more reactive than copper. More reactive metal displaces less reactive metal from its salt solution. Zn has displaced copper, but copper cannot do so.23.A) Three things are required in proper combination before ignition and combustion can take PLACE---Heat, OXYGEN and Fuel.I. There MUST be Fuel to burn.II. There must be Air to supply oxygen.III. There must be Heat (ignition temperature) to start and continue the combustion PROCESS.B) Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work.