Separation Techniques – HSC Chemistry

This is part of the HSC Chemistry course under the topic of Properties of Matter.

HSC Chemistry Syllabus

Explore homogeneous mixtures and heterogeneous mixtures through practical investigations:

– Using separation techniques based on physical properties (ACSCH026)

Separation Techniques in HSC Chemistry

This video will explore various common physical separation techniques for mixtures. These methods include; sieving, magnetic separation, filtration, sedimentation and decantation, evaporation and crystallisation, distillation, fractional distillation and liquefaction.

Water and Aqueous Solutions

Water is a universal solvent known for its ability to dissolve numerous substances, forming solutions where the solute (dissolved substance) and the solvent (dissolving liquid, usually water) interact closely. When a substance is dissolved in water, it is also said to be in the aqueous state.

Below are some examples of solutions, and the solute(s) and solvents which mix to make them.

Solution	Solute	Solvent
Salt Water	Salt	Water
Soft drink	Carbon dioxide, Sugar	Water
Coffee	Instant Coffee, Sugar	Water

If a substance is unable to dissolve in water, it is said to be insoluble. Insoluble substances may form a suspension or a colloid when mixed in water.

Suspension: A suspension is a heterogeneous mixture where its components can be visibly distinguished on either a microscopic or macroscopic level. Examples of suspensions include blood and muddy water. One way to separate suspensions is through the use of a centrifuge.

Colloid: A colloid is a special type of suspension in which small clusters of solute are evenly dispersed throughout the solvent. Unlike suspensions, colloids do not separate over time. Examples of colloids include milk, whipped cream, and mayonnaise.

While water is a common solvent used in chemistry, it is important to know that solutions can be created using other solvents such as different types of oil and alcohol. Aqueous solutions refer to solutions made using water as a solvent.

What are Separation Techniques?

Separation techniques are essential tools in chemistry and other fields that deal with mixtures of substances. By separating the components of a mixture, we can obtain pure substances or analyse the mixture's properties. There are many separation techniques available, and the choice of the most suitable technique depends on the nature of the mixture, the properties of its components, and the purpose of the separation.

Sieving

Sieving is a separation method commonly used to separate larger particles from smaller particles in a mixture of solids. The method involves passing the mixture through a perforated mesh with holes of a certain size, chosen based on the relative size of the impurities and the desired substance. The larger particles of the impurities are prevented from passing through the sieve, while the smaller particles of the desired substance pass through and are collected. Sieving is a simple and effective method for separating particles of different sizes, but it has limitations in separating particles that are similar in size or that differ in shape, density, or other properties.

Magnetic Separation

Magnetic separation is a technique that uses magnets to separate magnetic solids from a mixture of non-magnetic solids. This method takes advantage of the fact that magnetic materials will be attracted to a magnet, while non-magnetic materials will not. One common example of magnetic separation is separating iron filings from salt using a magnet. The mixture is passed over a magnet, which attracts the iron filings, leaving the salt behind

Filtration

Filtration is a separation method used to separate a mixture of solids and liquids. The method involves passing the mixture through a filter, which is typically made of a porous material such as filter paper. The filter paper allows the liquid to pass through while retaining the solid particles. Filtration can be performed using different types of filters, such as a funnel or a Buchner funnel, depending on the nature of the mixture and the properties of the solid particles.

The size and shape of the solid particles, as well as their solubility in the liquid, can affect the efficiency of the filtration process. Filtration cannot be used if a solid dissolves in the liquid (this would require either evaporation or distillation).

Filtration is an effective technique used to separate sand from water.

Sedimentation and Decantation

Sedimentation and decantation are two techniques commonly used in combination to separate solid, insoluble particles from a liquid or liquids in a heterogenous mixture. Sedimentation involves allowing the mixture to stand undisturbed, during which time the denser solid particles settle to the bottom of the container due to gravity. After sedimentation, the liquid component can be carefully poured off (decantation), leaving the solid particles behind. Sedimentation and decantation are useful for separating relatively large and heavy solid particles from liquids.

Sedimentation and decantation can be used to separate oil from water as these two liquids form a heterogenous mixture. Oil (lower density) typically forms a layer above water (higher density).

Evaporation and Crystallisation

Evaporation and crystallisation are separation techniques commonly used together to separate a solvent from a dissolved solid in a homogeneous mixture. The method involves heating the mixture to a temperature at which the solvent evaporates, leaving behind the solid residue. The residue may be in the form of crystals or a powder depending on the nature of the solid and the conditions of evaporation.

This method is effective in separating salt from a solution of salt water because water has a much lower boiling point than the salt (e.g. sodium chloride). When the water evaporates, the salt remains behind in its solid state.

Distillation

Distillation is a separation technique commonly used to separate and purify a liquid from a mixture of liquids (either homogenous or heterogenous) with different boiling points or solvents from a solution containing one or more dissolved solutes. The method involves heating the mixture to a temperature at which the liquid components vaporise (evaporate), and then condensing the vapour back into a liquid by cooling it down.

The process of distillation is typically carried out in a specialised apparatus called a distillation apparatus. The mixture is placed in a flask or a round-bottomed flask and heated, causing the component(s) with lower boiling points to vaporise. The vapour then passes through a condenser, which is cooled by cold water running through it. The cooling causes the vapour to condense back into a liquid, which is collected in a separate container, often called the distillate.

The main difference between distillation and evaporation & crystallisation is that the former allows for the liquid that is evaporated to be retained as the distillate.

Distillation is a powerful technique that can separate and purify liquids with different boiling points, such as water and ethanol (type of alcohol). Ethanol has a lower boiling point than water, so when heated at a temperature between the boiling points of ethanol and water, ethanol evaporates and is condensed to form the distillate; water remains behind.

Fractional Distillation

Fractional distillation is a more advanced version of distillation used to separate a mixture of liquids with different boiling points. The process is similar to simple distillation, but the apparatus used is slightly different.

In fractional distillation, the column of the apparatus contains hundreds of glass beads or plates, which provide a large surface area for repeated condensation and evaporation of the vaporised liquid. As the mixture is heated, the vaporised liquid rises up the column, and each time it comes into contact with the cooler surfaces of the beads or plates, it condenses and re-vaporises.

The process of repeated condensation and evaporation allows the different components of the mixture to be separated based on their boiling points. The liquid with the lowest boiling point will vaporise and condense more easily than the liquids with higher boiling points, so it will come out of the apparatus first.

Fractional Distillation of Crude Oil

In industry, fractional distillation is commonly used to separate the components of crude oil into different fractions based on their boiling points.

The process of fractional distillation involves heating the crude oil to vaporise the different components, and then passing the vapor through a distillation tower, which is a tall column packed with trays or plates. The temperature at each level of the column is carefully controlled, with the bottom being the hottest and the top being the coolest.

As the vapor rises through the column, it cools and condenses at different levels based on the boiling points of the components. The fractions with the lowest boiling points condense at the highest levels of the column, while the fractions with the highest boiling points condense at the lowest levels.

The different fractions collected from the distillation tower can then be further processed into useful products, such as gasoline, diesel fuel, and kerosene.

Summary

Separation Technique	Property used for Separation
Sieving	Solid particle size
Magnetic separation	Magnetic properties of substances
Filtration	Particle size and solubility
Sedimentation and decantation	Solubility and density (and particle size)
Evaporation and crystallisation	Boiling point
Distillation	Boiling point
Fractional distillation	Boiling points (closely spaced e.g. gas)

Previous Section: Homogeneous and Heterogeneous Substances

Next Section: Percentage Composition