Question

during decantation ,the liquid that collects on the top should be poured into another container slowly and carefully .why

Answer 1

because, if it is not done carefully, then the sediment will be disturbed and again mix up with solution. So by careful decantation, 'again mixing' problem will be avoided

Answer 2

Decantation can be used to separate immiscible liquids that have different densities. ... A separatory funnel is an alternative apparatus for separating liquid layers. It has a valve at the bottom to allow draining off the bottom layer. It can give a better separation between the two liquids.