Fill in the blanks:
$1$. In differential extraction from two layers,the .......... layer is above because .............
$2$. The organic solvent used in differential extraction is .......... in water.
$3$. If an organic compound is ......... soluble in an organic solvent,then the continuous extraction technique is used.
$4$. ........ solution is added to produce the precipitate of halide for the estimation of halogen.