Separation of Mixtures

Learning Objective

In this lesson we will learn how mixtures can be separated by a variety of methods, depending on the type of mixture and the physical properties of the component substances.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe the following separation methods, with examples: decanting, sieving, filtration, separating funnel, centrifugation, magnetic separation, evaporation, distillation and chromatography.

  • Define and identify: filtrate and residue, distillate, sediment and supernatant, mobile phase and stationary phase.

  • Draw and label diagrams of the different separation methods.

  • Determine the best method for separating common types of mixtures.

 

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Formation of Mixtures

  • Mixtures are physical combinations of two or more pure substances.
    A physical combination means substances do not change into other substances when they became part of a mixture.
    For example, when copper sulfate powder is added to water, a mixture (solution) of copper sulfate and water is created. The components look different, but no new substance is created.

 
forming a mixture

The formation of a mixture is a physical process.

(Images: Benjah-bmm27, Cjp24, LHcheM; Wikimedia Commons)

 

Separation of Mixtures

  • Since they are formed by physically combining substances, mixtures can be physically separated.
    For example, if a copper sulfate solution is boiled, the water evaporates away and the copper sulfate powder remains.

 
separation of a mixture

The formation of a mixture is a physically reversible process.

(Images: Benjah-bmm27, Cjp24, LHcheM; Wikimedia Commons)

 

Separation Techniques

  • There are many different techniques for separating mixtures into their components.
    The most appropriate technique depends on the type of mixture and the physical properties of the components.
    It also depends on whether you want to retrieve all of the components or not.
    For example, if we had a solution of salt water, the most appropriate separation technique would depend on whether we wanted to recover both the salt and water, or just the salt.

 
muddy and clean water separation

How could we separate pure water from salty, muddy water?

(Image: Meganbeckett27, Wikimedia Commons)

 

Physical Properties of Mixture Components

  • Separation techniques take advantage of differing physical properties of the components of a mixture.
    For example, by evaporating the water from a copper sulfate solution, we are taking advantage of the fact that water has a lower boiling point than copper sulfate. In fact, water boils long before copper sulfate even melts.
    However, if we heated a solution of ethanol and water, the ethanol would boil and evaporate before the water, as ethanol has a lower boiling point than water.
    We would therefore need to use a different separation technique to remove water from an ethanol-water solution.

 
melting boiling point distillation

Each pure substance within a mixture has its own specific melting and boiling point.

(Image: Aleksander Sobolewski, Wikimedia Commons)

 

  • There are other physical properties of components, besides melting and boiling point, that can be utilised when separating mixtures, including:
    • Size
    • Solubility
    • Density
    • Magnetism and other forms of attraction

 

scrap yard recycling    mining pit extraction    oil spill clean up

What are some differences in physical properties that could be used to separate these mixtures?

(Images: PublicDomainPictures, Pixabay; Jan-Mallander, *Pixabay*; Kallol Mustafa, Wikimedia Commons)

 

Separating Heterogeneous and Homogeneous Mixtures

  • Separation techniques can generally be divided into those that involve the separation of heterogeneous mixtures and those that involve the separation of homogeneous mixtures.
    Some of the main separation techniques are shown in the table below.

 

Techniques for Separating Heterogeneous Mixtures Techniques for Separating Homogeneous Mixtures
Decanting Evaporation
Sieving Distillation
Filtration Chromatography
Separating funnel
Centrifugation
Magnetic separation

 

Decanting

  • Decanting involves separating a dense, insoluble substance from a heterogeneous mixture.
    For example, if we mixed sand with water in a beaker, the sand would not dissolve, but would settle to the bottom, forming a sediment layer.
    By gently pouring off the water (the supernatant), we could separate the sand from it.
  • In the laboratory, using a glass rod can assist the decanting process, as it minimises the amount of splashing and reduces the risk of pouring off some of the sediment.
  • Decanting is only a useful method if the solid particles readily settle to the bottom.
    It would not be useful, for example, in separating fine silt particles from water, as they often remain suspended in the water.

 
decanting labelled laboratory diagram

The technique for decanting in the laboratory.

 
 
gold panning decanting

Gold panning utilises the principle of decanting; the heavier gold particles sink to the bottom of the pan, while the lighter sand and silt particles are poured off with the water.

(Image: Janothird, Wikimedia Commons)

 

Sieving

  • Sieving involves separating a mixture based on different sizes of components.
    For example, small rocks can be separated from sand by sieving the mixture. The smaller sand grains will pass through the holes in the sieve, whereas the rocks will not pass through.
  • Sieving can also be used to separate solids from liquids, assuming the solid pieces are larger than the holes in the sieve.
    For example, straining cooked rice is a form of sieving.

 
sieving diagram soil mixture separation

Sieving separates mixture components based on their size.

 
 
soil sieve mixture separation

This soil sieve sits on a vibrating platform, which causes the soil components to fall through the different sized mesh dividers, separating into different fractions based on grain size.

(Image: Sibiii, Wikimedia Commons)

 

Filtration

  • Filtration is a special form of sieving that separates very fine solid particles from liquid or gas mixtures.
    Filter paper (or a similar substance with very fine pores) is used as a sieve.
    For example, air and water filters are used in a variety of applications to keep air and water free from minute dust and other particles.
  • In a laboratory, filtration is often carried out by placing filter paper in a funnel, pouring the mixture into the funnel and collecting the filtered liquid in a beaker.
    The liquid that passes through the filter is called the filtrate and the solid that gets trapped in the filter is called the residue.

 
filtration diagram mixture separation

Set-up for laboratory filtration.

 
 

air filter    mining pit extraction

Left: This air filter prevents harmful dirt and dust particles from getting inside the car engine.
Right: Water filters remove impurities from drinking water.

(Images: Tanescia, Pixabay; melvil, Wikimedia Commons)

 

Separating Funnel

  • A separating funnel can be used to separate a mixture of two non-miscible liquids – that is, liquids that do not mix together to form a homogeneous solution.
    When such a mixture is allowed to settle, the less dense liquid will form a layer on top of the more dense liquid.
    A tap attached to the separating funnel allows the bottom liquid layer to be drained, while the top liquid layer remains in the flask and can be drained separately.
  • This technique is mostly used to separate liquids that are miscible in water from liquids that are non-miscible in water.
    An oil-water mixture would be an example of this.

 

separating funnel diagram        mining pit extraction

Set-up for using a laboratory separating funnel.

(Image-right: PRHaney, Wikimedia Commons)

 

Centrifugation

  • Centrifugation involves spinning tubes of heterogeneous mixtures at very high speeds, which forces part of the mixture to settle at the bottom of the tube.
    It can be used for separating solid particles from liquids, or for separating non-miscible liquids.
  • After centrifugation, the top liquid layer can be carefully removed using a span class=”emphasis-1″>pipette.
    Examples include the separation of fat from milk and the separation of different components in blood (red blood cells, white blood cells and plasma).

 
centrifuge diagram

A centrifuge spins mixtures at very high speeds to separate the components.

 
 
laboratory centrifuge

A laboratory centrifuge.

(Image: scotth23, Pixabay)

 

  • Centrifugation can also be combined with sieving for separating liquids from solids.
    For example, washing machines and salad spinners combine centrifuging and sieving to remove excess water from clothes and salad greens.

 
salad spinner centrifuge sieve

A salad spinner combines centrifuging and sieving to clean salad greens.

(Image: Lymantria, Wikimedia Commons)

 

Magnetic Separation

  • Magnetic separation is a specialised method specifically used for separating magnetic materials, such as iron, from non-magnetic materials, such as soil and plastic.
    It is commonly used in the mining and recycling industries.

 
magnetic separation diagram iron sand salt

An example of using magnetic separation in the laboratory.

 
 
magnetic scrap metal separation

Giant electromagnets can be used to sort scrap metal from other materials.

(Image: Life-Of-Pix, Pixabay)

 

Evaporation

  • Evaporation is used for recovering dissolved solids from solutions.
    The solution is either boiled or simply left uncovered, resulting in the evaporation of water and the crystallisation of solutes.
    For example, sea salt is recovered by the evaporation of sea water.

 
evaporation Bunsen burner laboratory diagram

Laboratory set-up for evaporation using a Bunsen burner.

 
 
sea salt evaporation pond

Large man-made salt ponds are used to evaporate sea water, leaving behind sea salt.

(Image: Barni1, Pixabay)

 

Distillation

  • Distillation is similar to evaporation, except that the evaporated substance, known as the distillate, is collected.
    The evaporated substance is passed through a tube known as a condenser, which is surrounded by cold water.
    The evaporated substance is cooled, causing it to condense back to a liquid so that it can be collected.
  • Distillation can be used to separate liquids based on their differing boiling point.
    It is commonly used in producing petrol, alcohol and perfumes.
    Distillation can also be used to remove dissolved impurities from substances, such as in the purification of water.

 

distillation laboratory diagram

Set-up for laboratory distillation.

 
 
fractional distillation column crude oil

Huge distillation columns are used to separate crude oil into a variety of components, including petrol, kerosene, engine oil and natural gas.

(Image: Luigi Chiesa, Wikimedia Commons)

 

Chromatography

  • Chromatography is used to separate liquid or gas mixtures.
    It uses very small quantities as its purpose is primarily for identifying and analysing substances within a mixture, rather than separating mixtures to recover large amounts of their components.
    Some examples include drug testing of urine and blood samples, testing water samples for pollutants, and comparing a suspect’s sample with evidence found at a crime scene.
  • Chromatography uses the principle of different affinities (attraction) of substances within a mixture to two separating substances, known as the stationary phase and the mobile phase.
  • Example
    If you wanted to separate the different coloured pigments in a water-soluble marker pen, you could use a type of chromatography called paper chromatography.
    Draw a large dot on a piece of chromatography paper, then place that piece of paper in a beaker of water, with the dot slightly above the water as shown.
    The water will be absorbed by the paper and move through it. As it does, it will dissolve the pigments, causing them to move through the paper with the water. The different coloured pigments within the ink dot will move at different rates, causing them to separate out.
    Different types of ink will result in different pigment patterns, therefore different samples can be compared to see if they contain the same ink.
    In this example, the paper is the stationary phase as it doesn’t move. The water is the mobile phase as it does move.
    The pigment particles are attracted to both the paper and the water, but by different degrees. Particles that are more attracted to the water will migrate faster; particles that are more attracted to the paper will migrate slower.

 
paper chromatography ink separation

Separation of ink by paper chromatography.

 

  • The principles described in the paper chromatography example are fundamentally the same for all types of chromatography.
    Gas chromatography uses a gas as the mobile phase, which passes through a column containing the stationary phase, which is usually a non-absorbent solid or a viscous liquid.
    Liquid chromatography uses a liquid as the mobile phase, which passes through a column – usually at high pressure. The column contains the stationary phase, which is usually a porous solid.

 
hplc High-performance liquid chromatography

High-performance liquid chromatography (HPLC) can detect trace amounts of substances in a sample.

(Image: Kjaergaard, Wikimedia Commons)

 

Summary

  • Since mixtures are formed by the physical combining of substances, they can also be physically separated.
  • The most appropriate technique for separating a mixture depends on the type of mixture and the physical properties of the components.
  • Separation techniques take advantage of differing physical properties between the components of a mixture, such as: melting and boiling point, size of fragments, solubility, density, magnetism and other forms of attraction.
  • Separation techniques can be divided into those that involve the separation of heterogeneous mixtures and those that involve the separation of homogeneous mixtures.
  • Decanting involves separating a liquid (supernatant) from a solid (sediment) by gently pouring off the liquid.
  • Sieving involves separating a mixture based on different sizes of components, where smaller fragments pass through holes in the sieve but large fragments do not.
  • Filtration is a special form of sieving where filter paper is used to trap very fine solid particles (residue) from the rest of a liquid or gas mixture (filtrate).
  • Separating funnels or used for separating liquids with different densities.
  • Centrifugation separates heterogeneous mixtures by spinning them at very high speeds, which forces components to separate into layers.
  • Magnetism can be used for separating magnetic materials from non-magnetic materials.
  • Evaporation is used for recovering dissolved substances from solutions by evaporating the solvent and crystallising the solute.
  • Distillation involves the evaporation of a liquid (distillate), which is then cooled and condensed back into a liquid, to be collected separately.
  • Chromatography separates mixtures based on their differing levels of attraction to two different substances, known as the mobile phase and the stationary phase.

 
laboratory distillation set-up

(Image: Simon, Pixabay)

 

(Header image: mp_images, Adobe Stock)

 

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