Why 70 percent ethanol used in sterilization?

Answer 1

Because 70% is a strong enough concentration to lead to the disintegration of the outer bacterial plasma membranes, eventually denaturing proteins and killing off bacteria. We do not tend to use 100% ethanol (one would think it to be more effective) since this could kill off the bacteria, however it may preserve the genetic material of the bacterium, which may be transferred to other bacterial cells. This is to be avoided when culturing cells, for example.