Question

Define a chemical reaction. which observations help you to determine whether a chemical reaction has taken place ?

Answer 1

CHEMICAL REACTION :


Chemical reaction is the process in which atoms of some elements rearrange that is , undergo a chemical change to form a new product with different properties.

This involves breaking of old bonds between the reacting atoms and making of new chemical bonds, between the rearranged atoms.


Following are the observations that helped you to determine whether a chemical reaction has taken place or not, they are :-


♣ Change in Temperature :

ex:- When quicklime reacts with water , slacked lime is formed with a release of large amount of heat , causing change in temperature .

CaO + H2O → Ca(OH)2 + heat


♣ Change in State :


ex :- The combustion reaction of candle wax (soild) melts initially to produce molten wax (liquid), and then produces Carbon-dioxide (gas) and water vapour (liquid) . So, in this reaction , state of matter gets changed from Solid to liquid and Gas.


♣ Change in Colour :


ex :- When Sulphur dioxide gas is passed through acidified pottasium dichromate solution, orange colour of pottasium dichromate soution changes to green colour.


3SO2 (g) + K2Cr2O7 (aq) + H2SO4 (aq) → K2SO4 (aq) + Cr2(SO4)3 (aq) + H2O (l)



♣ Evolution of Gas :


ex :- When Zinc granules react with dilute sulphuric acid in a flask, Hydrogen gas evolves and Zinc sulphate solution is formed.

Zn + H2SO4 → ZnSO4 + H2↑


♣ Formation of precipitate :

ex :- When pottasium iodide is added to lead nitrate solution ( colurless and clear ) , Yellow precipitate of lead iodide solution is formed.


Pb(NO3)2 (aq) + 2KI (aq) → pbl2 (s) + 2KNO3 (aq)

Answer 2

Hope this may help you a lot