Q7.
Why do immiscible liquids form two separate layers in a separating funnel?
Answer
Because they do not dissolve in each other, and they have different densities.
Denser liquid → sinks → lower layer
Less dense liquid → floats → upper layer
Example: water (1.00 g cm⁻³) below, mustard oil (≈ 0.91 g cm⁻³) above
Less dense liquid → floats → upper layer
Example: water (1.00 g cm⁻³) below, mustard oil (≈ 0.91 g cm⁻³) above
Why it happens — two separate reasons working together:
1. They stay apart. Water molecules attract one another strongly. Oil molecules cannot join in that attraction, so instead of spreading between the water molecules they gather with their own kind. The two liquids therefore remain as two separate phases with a boundary between them.
2. Gravity stacks them. Once they are separate, gravity sorts them by density. For the same volume the denser liquid is heavier, so it sinks and pushes the lighter one up, and the arrangement with the denser liquid at the bottom is the stable one. That is why the layers always form in the same order and stay that way.
1. They stay apart. Water molecules attract one another strongly. Oil molecules cannot join in that attraction, so instead of spreading between the water molecules they gather with their own kind. The two liquids therefore remain as two separate phases with a boundary between them.
2. Gravity stacks them. Once they are separate, gravity sorts them by density. For the same volume the denser liquid is heavier, so it sinks and pushes the lighter one up, and the arrangement with the denser liquid at the bottom is the stable one. That is why the layers always form in the same order and stay that way.
Tip: This is the whole basis of the separating funnel. Open the stopcock and the lower layer runs out first; close it as the boundary reaches the tap, discard the small mixed portion, then collect the upper layer separately.