How does a separating funnel work

We usually repeat this procedure two or three times during the practice of solvent extraction, and separate the two layers after each vigorous shake (we call this procedure running off the heavier, lower layer of liquid). Several extractions are needed because K(partition) is usually quite small, which implies that only a fraction of the solute is removed from the solution during each partition cycle. 

Immiscible solutions do not mix. The words miscible and its converse immiscible derive from the Latin word miscere, meaning to mix.  

The reason we need to shake the two solutions together when partitioning is because the solute only passes from one solvent to the other across the interface between them, i.e. across the meniscus. 

The meniscus is quite small if the funnel is kept still, and partitioning is slow. Conversely, shaking the funnel generates a large number of small globules of solvent, which gready increases the active surface area of the meniscus. Therefore, we shake the funnel to increase the rate of partitioning. 

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