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Characteristics of Chemical Reaction

When a chemical reaction occurs, following observations help us to determine whether a chemical reaction has taken place.

(1) Formation of a precipitate
(2) Change in colour
(3) Evolution of a gas
(4) Change in state
(5) Change in temperature or evolution / absorption of heat
(6) Sound, light may be produced.
(7) Change in smell or a new smell can be given off

(1) Formation of a precipitate 

A precipitate is a solid insoluble product which separates out from the solution during the reaction, characterises chemical reaction.

Activity – 2

(2) Change in colour
When an apple cuts into pieces and keep it away for some times, it acquires brown colour. Similar observations can be seen in the case of potatoes and brinjals.

(3) Evolution of a gas and change in temperature

(A) Evolution of a gas and exothermic reaction 

Reactions which produce heat are called exothermic reactions. Evolution of a gas and liberation of heat characterize a chemical reaction.

Activity – 3

Other examples of exothermic reactions –

  •  Burning of natural gas
    CH4 (g) + 2O2 (g) → CO2 (g) + 2 H2O (g) + heat
  • Respiration is an exothermic reaction where glucose (food breaks into simpler substances such as carbohydrate, that further break into glucose during digestion of food) combines with oxygen in the cells of our body and release energy to work and play.
    (C6H12O6) + 6O2 → 6CO2 + 6H2O (l) + heat
  • The decomposition of vegetable matter into compost is also an exothermic reaction.

(B) Endothermic reaction

Such reactions which absorb heat during the reaction are called endothermic reactions. Absorption of heat characterizes a chemical reaction. Other example of an endothermic reaction is photosynthesis.

Activity – 4

(4) Change in state
When a wax candle is burned, its wax (solid) changes into carbondioxide (gas) and water vapour (gas). Such chemical reactions can be identified by change in state (here, solid to gas) and by formation of one or more new substances.

(5) Sound, light may be produced
Explosion of fireworks produces heat, light, sound and unpleasant gases characterize chemical reactions.

(6) Change in smell or new smell may be given off
It happens when food is spoiled, formation of curd from milk, souring of curd, cooking of food, fermentation of grapes etc.

It is important to note that a chemical reaction can show more than one observations at a time. Both physical and chemical changes can take place simultaneously. For e.g. burning of a candle.

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Chemical Equations

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