Factors affecting rate of evaporation:
1. Temperature: The rate of evaporation is directly proportional to the temperature of liquid. The evaporation can be accelerated by increasing the temp but it will cause decomposition of thermolabile substances.
2. Temperature and time of evaporation: Exposure to relatively high temp for short period of time may be less harmful to the active principles of a drug than a lower temp with exposure for longer time.
3. Temperature and moisture Some drug constituents decompose more readily in presence of moisture if heated at high temp.
4. Type of product required: On evaporation of the liquid, conc. liquid, semisolid, and solid are formed.
5. Effect of concentration: There is tendency of forming film on the upper layer of liquid which reduces the rate of evaporation.
6. Surface area: The rate of evaporation is directly proportional to surface area of evaporating surface.
7. Vapour pressure of the liquid to be evaporated: The rate of evaporation is directly proportional to the vapour pressure of evaporating liquid.