Define roasting.

Hint:
-Roasting is one of the processes of metallurgy which involves conversion of ores into oxides.
-Various types of reactions take place in roasting. Roasting is mostly used for the sulphide ores.

Complete step by step answer:
Roasting is defined as the process in which ore is converted into its oxide in the presence of excess air.
This process is mostly used for the sulphide ores.
In the process of roasting, various impurities and moisture is released in the form of volatile gas.
As this process involves sulphide ores, it gets released in the environment causing pollution .
The gases that are released are metallic as well as toxic also some acidic compounds.
The reaction that is involved during roasting is as follows:
$2ZnS + 3{O_2} \to 2ZnO + C{O_2}$ .
In this reaction zinc sulphide is converted into zinc oxide.
Roasting process does not involve dehydrating ores.

Additional information:
-Along with roasting there is another process that takes place in metallurgy that is calcination.
-Calcination is the process in which ore is converted to oxide in the absence of air or in the limited supply of air.
-In this process there is thermal decomposition of carbonates.
-This process is mostly used in the decomposition of limestone to lime and carbon dioxide.
-The reaction of calcination is given below as follows:
$CaC{O_3} \to CaO + C{O_2}$

Note:Roasting is done in the presence of air whereas calcination takes place in the absence of air. During roasting a large amount of toxic gas is released whereas in case of calcination large amounts of carbon dioxide gas is released.