Conversion of yellow phosphorous to red phosphorous is done by heating it in covered retorts at __________ °C in absence of air.

ANS:B - 250-400

The conversion of yellow phosphorus to red phosphorus is typically done by heating it in covered retorts at temperatures within the range of 250-400°C in the absence of air. This process is known as thermal conversion or thermal disproportionation. Yellow phosphorus is heated in an inert atmosphere, such as under a blanket of nitrogen or argon, to prevent oxidation. The heating causes the yellow phosphorus molecules to break down and rearrange into a more stable form, resulting in the formation of red phosphorus. The temperatures in the range of 250-400°C are sufficient to promote the conversion while avoiding excessive decomposition or other unwanted reactions.