Miscible or Immiscible• Differences between miscible and immiscible liquids.• Techniques for separating miscible liquids using fractional distillation, and immiscible liquids using a separating funnel.

Miscible liquids mix completely with each other.

Immiscible liquids do not mix completely.

Why does water and ethanol mix but not water and oil?

Explanation In an ethanol and water mixture, the mixture

has a consistent chemical composition throughout the entire mixture, and the ethanol and water molecules are mixed on a molecular level. This makes ethanol and water a solution.

In an oil and water mixture, the top half of the mixture is pure oil and the bottom half of the mixture is pure water. The mixture does not have a consistent chemical composition throughout.

Separating Oil and Water (Immiscible Liquids)

Separating immiscible liquids is done simply using a separating funnel. The two liquids are put into the funnel and are left for a short time to settle out and form two layers. The tap of the funnel is opened and the bottom liquid is allowed to run. The two liquids are now separate.

Separating Miscible LiquidsMiscible liquids are harder to separate as

they dissolve in each other. Miscible liquids are often separated using fractional distillation. This is possible as miscible liquids have different boiling points.

Specification Link!You need to know about the

separation of liquid air

Fractional distillation of liquid airYou need to be able to explain how nitrogen

and oxygen are obtained from the air.About 78 per cent of the air is nitrogen and

21 per cent is oxygen. These two gases can be separated by fractional distillation of liquid air.

Distillation of air

Air first of all needs to be cooled down, so it becomes a liquid.

It enters a cooling unit, water solidifies at 0 and is removed.

Carbon dioxide solidifies at -79 and is removed.

This leaves nitrogen and oxygen!

Distillation of Liquid AirTop of column is colder. Nitrogen is still a gas at this temperature, any oxygen reaching this part of the tower is likely to condense and run to the bottom of the column.

-200 is below the boiling points of both nitrogen and oxygen so they both enter as liquids.

-185 is below the boling point of oxygen and alot of the oxygen stays as a liquid.

-185 is above the boiling point of nitrogen and so it the nitrogen boils and evaporates.

Extension Questions 1. (D-C) Salad dressing is a mixture of oil and

vinegar. Oil and vinegar are immiscible liquids. Explain why a bottle of salad dressing needs to be shaken before it is used. (2 Marks)

2. (B-A*)Explain why the following temperatures are used in a fractionating column used to separate nitrogen and oxygen from air:

a) -2000C for the air going into the column.(2 Marks)

b) -1850C at the bottom of the column. (2 Marks)

Answers1. Immiscible liquids will form two different

layers (1) and if not mixed the person would only get some of the top layer on their salad (1) shaking the bottle mixes the two layers temporarily (1).

2. A) nitrogen and oxygen need to be liquids as they go into the column (1) and this temperature is below the boiling points of both (1).

B) This is below the boiling point of oxygen, so it stays a liquid can be piped off (1) but above the boiling point of nitrogen so it evaporates/goes up the column (1).