The clouds consist of charged particles of water dispersed in air. Some of them are +vely charged, others are –vely charged. When +vely charged clouds come closer they cause lightening and thundering whereas when +ve and –ve charged colloids come closer they cause heavy rain by aggregation of minute particles. It is possible to cause artificial rain by throwing electrified sand or silver iodide from an aeroplane and thus coagulating the mist hanging in air. Smoke screen is a cloud of smoke used to hide military, naval police etc. it consists of fine particles of TiO 2 .

(i) When excess of AgNO 3 is treated with KI solution, AgI forms