Separation Techniques
This is part of preliminary 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
Water and Solutions
Water is a versatile substance that is widely used as a solvent due to its ability to dissolve a wide variety of substances. When a substance is dissolved in water, it becomes aqueous and can move freely throughout the mixture. The combination of the dissolved substance and the liquid that dissolves it is called a solution, with the dissolved substance referred to as the solute and the liquid referred to as the solvent. The table below provides examples of typical solutions and their solute and solvent components:
|
Solution |
Solute |
Solvent |
|
Salt Water |
Salt |
Water |
|
Soft drink |
Carbon dioxide, Sugar |
Water |
|
Coffee |
Instant Coffee, Sugar |
Water |
If a substance is insoluble in water, it may form either a suspension or a colloid.
- A suspension is a heterogeneous mixture in which the 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.
- 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.
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 the pure substances or analyse the mixture's properties. In addition to the centrifugation method mentioned earlier, there are many other separation techniques available, each with its own strengths and limitations. Some common techniques include filtration, distillation, chromatography, and extraction. 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 solid from a liquid or 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.
Sedimentation and decantation.
Sedimentation and decantation are two techniques commonly used in combination to separate solid particles from a liquid. Sedimentation involves allowing the mixture to stand undisturbed, during which time the heavier solid particles settle to the bottom of the container due to gravity. The liquid component can then be carefully poured off, leaving the solid particles behind. This process is called decantation.
Sedimentation and decantation are useful for separating relatively large and heavy solid particles from liquids.
Evaporation and Crystallisation
Evaporation is a separation technique commonly used 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.
Distillation
Distillation is a separation technique commonly used to separate and purify a liquid from a mixture of liquids or from a solution containing one or more dissolved solids. The method involves heating the mixture to a temperature at which the liquid components vaporise, and then condensing the vapor 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 liquid components to vaporise. The vapor then passes through a condenser, which is cooled by cold water running through it. The cooling causes the vapor to condense back into a liquid, which is collected in a separate container, often called the "distillate."
Distillation is a powerful technique that can separate and purify liquids with different boiling points, such as water and ethanol.
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. This process, known as rectification, allows for more efficient separation of the components in the mixture. 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 and liquefaction
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.
