Question

Give me some details about the chemical reaction that takes place with
1) White washing of walls
2) Reaction involved in black and white photography
Remember to give the detail about it!! :)

Answer 1

1) White washing is done with the compound called slaked lime (calcium hydroxide).It reacts with carbon di oxide in air and forms calcium carbonate which gives a shiny finish on walls.
Ca(OH)2 + CO2 --> CaCO3 + H2O
calcium                  calcium
hydroxide                carbonate 

2) The reaction of decomposition of light is used in this case.
When light is allowed to pass through silver chloride it decomposes to form chlorine and silver.The chlorine gas escapes and thus now the compound holds up grey color due to the presence of silver 
2AgCl ---> 2Ag + Cl2
Silver       silver   chlorine
chloride