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Understand Separation Techniques

Worksheet Overview

QUESTION 1 of 10

It's a classic scenario - you are trapped on a desert island with no hope of rescue and the only thing to drink is water that has a load of salt and sand in it. How do you get clean and pure drinking water? You use some of these separation techniques - that's how. Remember these, they might save your life if you get trapped with scientific equipment on a desert island with no hope of rescue! It's a common occurrence - honest!

 

Oil and water do not mix together because they cannot dissolve in each other, as you can see in the picture. They are immiscible liquids. 

 

Image of oil and water.

 

Even if we shake and mix them, they will soon separate when the mixture is left to stand still for a while. It is easy to separate them using a separating funnel. If the tap at the bottom is opened, water will run out because it sets below oil.

 

Miscible liquids, on the other hand, are much harder to separate because they dissolve in each other. One example is crude oil which is separated into different products by fractional distillation (this is covered in another worksheet). Fractional distillation is also used to obtain nitrogen and oxygen from air. The diagram below illustrates how it happens. Air is filtered to remove dust and then cooled to -200°C. As the air is liquefied, water vapour and carbon dioxide are removed and oxygen and nitrogen are collected from separate exits at the temperatures shown in the diagram.

 

 Image of a distillation column

 

Chromatography is another technique which is used to separate mixtures of colour compounds like inks and dyes. A spot of the mixture is placed on a special piece of paper (see diagram below) and it is allowed to soak up a solvent. As it soaks up, the solvent separates the different components because they travel different distances at the same time.

 

Image of chromatography (several chromatograms)

 

Components have set Rf values when chromatography is conducted in exactly the same way, so the substances can be identified by scientists. It is calculated with this formula:

 

How to work out Rf vaules

 

Are you ready to try some questions now?

Oil and water are immiscible liquids.

 

What does this mean?

They dissolve in each other

They do not mix together

They mix together

An example of a mixture of miscible liquids is:

Olive oil

Crude oil

Argon oil

What can be used to separate a mixture of immiscible liquids?

A filter

Fractional distillation

A separating funnel

How is air treated before fractional distillation begins?

It is liquefied

It is chlorinated to kill bacteria

It is filtered to remove dust

What is the purpose of fractional distillation of liquid air?

To purify it

To obtain nitrogen and oxygen

To obtain carbon dioxide

What temperature is air cooled to after the dust is removed?

-2°C

-20°C

-200°C

-200°F

What technique is used to separate colour compounds?

Fractional distillation

Separation funnel

Chromatography

What is the first step in chromatography?

You let a special piece of paper soak up a solvent

You measure the Rf values

You place a spot of the mixture on a special piece of paper

How are the different components of the mixture separated in chromatography?

They travel the same distances

The Rf value is known

They travel different distances

How do Rf values help in the identification of substances separated in chromatography?

Rf values are set when chromatography is conducted in exactly the same way

Rf values are calculated when chromatography is conducted using different solvents

The distances change every time chromatography is conducted

  • Question 1

Oil and water are immiscible liquids.

 

What does this mean?

CORRECT ANSWER
They do not mix together
EDDIE SAYS
What a horrid word to start with! It's a hard one to remember but it might help if you think of the word 'miscible' being like 'mixable' and the prefix 'im' makes the word mean the opposite - so 'unmixable'. Is that any help at all? Oil and water are immiscible liquids. This means that they do not mix together because they do not dissolve in each other.
  • Question 2

An example of a mixture of miscible liquids is:

CORRECT ANSWER
Crude oil
EDDIE SAYS
Did you get this one? Remember that 'miscible' is like the word 'mixable' and it refers to liquids that can mix together. An example of a mixture of miscible liquids is crude oil, as its components must be separated using fractional distillation.
  • Question 3

What can be used to separate a mixture of immiscible liquids?

CORRECT ANSWER
A separating funnel
EDDIE SAYS
A separating funnel is used to separate a mixture of immiscible liquids. This is because one of the liquids will settle below the other and can be separated by opening the tap at the bottom of the funnel. Fractional distillation is used to separate miscible substances such as the components of crude oil, as well as to obtain nitrogen and oxygen from air.
  • Question 4

How is air treated before fractional distillation begins?

CORRECT ANSWER
It is filtered to remove dust
EDDIE SAYS
The first thing to happen is that the air is filtered to remove dust and then cooled to -200 °C. As the air is liquefied, water vapour and carbon dioxide are removed, and oxygen and nitrogen are collected from separate exits at different temperatures.
  • Question 5

What is the purpose of fractional distillation of liquid air?

CORRECT ANSWER
To obtain nitrogen and oxygen
EDDIE SAYS
Did you get this one? Remember what air is made from and this question becomes easier. Nitrogen and oxygen are obtained from fractional distillation of liquid air. They each form a solid at different temperatures, and so you can separate them out by cooling them.
  • Question 6

What temperature is air cooled to after the dust is removed?

CORRECT ANSWER
-200°C
EDDIE SAYS
After dust is removed, the air is cooled to -200°Celsius. This is the temperature at which air becomes liquid.
  • Question 7

What technique is used to separate colour compounds?

CORRECT ANSWER
Chromatography
EDDIE SAYS
Have you ever done this experiment - it's a lot of fun and you can create some beautiful patterns. Chromatography is used to separate coloured compounds. A spot of the mixture is placed on a special piece of paper and it is allowed to soak up a solvent. As it soaks up, the solvent separates the different components.
  • Question 8

What is the first step in chromatography?

CORRECT ANSWER
You place a spot of the mixture on a special piece of paper
EDDIE SAYS
First, you place a spot of the mixture on a special piece of paper. It has to be on the paper, otherwise you are not going to be able to separate it!
  • Question 9

How are the different components of the mixture separated in chromatography?

CORRECT ANSWER
They travel different distances
EDDIE SAYS
Different components travel different distances along the piece of paper. The larger molecules will not travel as far as the smaller molecules will, so the smaller molecules will end up further along the piece of paper.
  • Question 10

How do Rf values help in the identification of substances separated in chromatography?

CORRECT ANSWER
Rf values are set when chromatography is conducted in exactly the same way
EDDIE SAYS
A hard one to finish with - but at least it is the final question! Rf values are set when chromatography is conducted in exactly the same way. It is then possible to work out what different components are from working out their Rf values using the formula below: Distance moved by the compound ÷ distance moved by the solvent Well done - that's another activity completed.
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