Sec 1 Lower Sec Science
/ Science INtegrrated Programme
Separation Techniques Notes
Name: ______( ) Class:_____ Date: ______
1) Mixtures and Pure Substances
A pure substance is a single substance not mixed with anything else. White sugar is a pure substance. The label on a packet of white sugar says `pure refined cane sugar'. This means no other substance is present. A crystal is a pure substance. White sugar is made up of crystals with the same shape. This shows that it is made up of the same single substance: it is pure.
< Pills used as medicine must be made from very pure substances
Crystals of sugar with a coin to show the scale. >
All the crystals have the same 90° angles, although many of them have been broken.
In nature, very few substances are found pure. Most substances are mixtures. A mixture contains two or more substances. Sea water is a mixture. It contains water, salt and other dissolved gases. Air is a mixture of oxygen, nitrogen and other gases.
Some solid mixtures are made up of different crystals. Most rocks are mixtures. Granite is commonly used for buildings and roads. Mixtures can easily be separated into pure substances. The process is called purification. This is done by physical methods such as filtration, crystallisation or distillation.
No chemical reactions are used. Several methods of purification are discussed in greater detail. The separation of mixtures into pure substances is important. Chemists need pure substances to study their properties. Pure sub stances are used in industry to make useful products such as food and drugs. Impurities in food and drugs can be dangerous because they can poison people. An example is a substance called tryptophan which some people consume as a health food. Some years ago, a cheap but impure quantity of tryptophan was made and sold by a chemical company for health food. Unfortunately the impurities caused irreversible damage to peoples' nervous systems and as a result several thousand healthy people became so sick that they were forced to live out the rest of their lives in wheelchairs.
Pure crystals have important uses. The great accuracy of a quartz watch is due to a little quartz crystal which vibrates at exactly 32 768 times per second. This controls the time. Very pure silicon crystals are needed to make the `chips' used in calculators, computers, watches and compact disc players.
A pure substance has definite properties. It has a fixed melting point and boiling point. Pure water, for example, melts at 0°C and boils at 100°C. All samples of pure water melt and boil at these temperatures. So rain water and water from melting ice both have the same boiling point of 100°C.
A mixture does not have definite properties. It does not have fixed melting and boiling points. An example of a mixture is coconut oil which is used for cooking. It melts over a range of temperatures. It starts melting at 14°C and completes melting at 22°C. Petrol fuel for motorcars is also a mixture. It has a boiling point range of 35°C to 75°C.
C11194-05
< Granite rock. You know it is a mixture because it is made up of different black and white crystals. The coin shows the relative sizes of the granite crystals and the coin
< Compact disc players depend on computer chips made from very pure silicon
2) Filtration
Filtration is the usual method of separating a solid from a liquid. A simple way of doing this is shown below.
The mixture is poured through a filter which is usually made of paper. The filter paper has tiny holes through which the particles of the liquid are able to pass. The particles of the solid are large. They cannot pass through the holes and are trapped by the filter paper.
The solid collected in the filter paper is called the residue. The liquid passing through the filter is called the filtrate.
When a mixture of sand and water is poured into the filter paper, the solid sand is trapped in the filter paper (the residue) and the water passes through the paper (the filtrate).
The filtrate is often a solution of, for example, a solid dissolved in water. This solution can be separated by crystallisation.
Filtration is one of the steps in making drinking water. All solid impurities are first removed by filtration. The water is then treated to kill any harmful micro-organisms.
Do you know???
Some special filters can filter biological cells. The antibiotic penicillin is made from a yeast. The penicillin is separated from the yeast by filtration. While the small penicillin molecules pass through the filter the big yeast cells are trap.
3) Crystallisation
A solid dissolves in a liquid to produce a solution. The solid that dissolves is called a solute and the liquid is called a solvent.
When the solution is heated, most of the solvent is evaporated off. The hot solution is then allowed to cool. The solution is now said to be saturated and as it cools, the dissolved solid (solute) appears as pure crystals. This process is called crystallisation. The impurities remain dissolved in the solution.
The main steps in purifying a solid by crystallisation are shown
Impure solids often contain impurities which are insoluble in the solvent. These insoluble impurities are removed by filtering the mixture before evaporating the solvent.
Question: How do you get pure salt from impure salt mixture?
The impure salt is a mixture of salt and sand - which one might obtain by evaporating seawater obtained from a beach. Salt is unusual in that the water solvent must be completely evaporated from the solution to obtain the crystals of salt.
Filtration followed by crystallisation is often used to make other salts, such as copper(II) sulphate
Crystallisation from solution is the most common method used by chemists to purify solids. Pure sugar is obtained this way. Some solids can be purified by melting them. The hot liquid is then cooled slowly. Crystals form when the liquid freezes. The first crystals to form are very pure. These are collected before the rest of the substance freezes completely. This is crystallisation without a solvent. This method is used to make the very pure silicon used in computer chips. A
pure silicon crystal is obtained by freezing molten silicon at 1410oC.
4.1) Distillation
Distillation is used to purify liquids. In this process, the liquid is changed into a gas by boiling. The gas is pure as other substances are left behind. This gas is then cooled. It condenses to a pure liquid which is called the distillate.
Distillation can be used to obtain a pure solvent from a solution of a solute. An example of a simple distillation is the distillation of sea water to obtain pure water. Below shows the experimental set-up of obtaining pure water from sea water by distillation. The seawater is boiled in the flask and the steam produced is cooled by the condenser. The pure liquid is then collected in the beaker and the salt remains in the flask.
The thermometer shows a temperature of 100°C during the distillation. This is the boiling point of water. Sea water is a solution of solid salts in water. The salts, like other solids dissolved in water, cannot be distilled and remain in the flask.
4.2) Fractional Distillation
Miscible liquids are completely soluble in each other. This means two liquids mix to form one liquid. A mixture of miscible liquids can be separated by fractional distillation. This is done using a fractionating column.
A fractionating column separates liquids in order of boiling points. The liquid with the lowest boiling point is distilled first. The liquid with the highest boiling point is distilled last.
Ethanol is an important solvent. It can be made by fermenting cane sugar. A mixture of ethanol and water is obtained from the fermentation. Ethanol is then obtained from this mixture by fractional distillation. The experiment in the next page shows how this can be done in the laboratory.
The boiling points of ethanol and water are 78°C and 100°C respectively. In the experimental set-up (top right) the vapour from the boiling mixture of ethanol and water in the flask contains a larger percentage of the lower boiling point ethanol. As the vapour moves up the fractionating column, it repeatedly condenses and boils inside the column. Each time the mixture boils, the percentage of the lower boiling point ethanol increases. By the time the vapour reaches the top of the fractionating column, it has become almost pure ethanol.
This vapour then passes into the condenser where it is cooled, and condenses into liquid ethanol. The thermometer shows a constant temperature of 78°C when the ethanol is being distilled. The water, which has a higher boiling point, remains in the flask until almost all the ethanol has distilled. (See graph on top left)
In this experiment, a complete separation is not effected. The distillate contains a very large percentage of ethanol and a little water.
Fractional distillation is used to obtain pure oxygen and pure nitrogen from air. The air is first cooled to about -200°C. At this temperature air is a liquid.
The liquid air is then distilled by allowing the liquid to warm up. Nitrogen has a lower boiling point than oxygen, so it distils first. Then the oxygen (with a higher boiling point) distils. This produces nitrogen and oxygen separately.
Fractional distillation is also used in the petroleum industry. Petroleum is a mixture of liquids called hydrocarbons. Fractional distillation is used to separate the hydrocarbons into, for example, lower boiling point petrol and higher boiling point kerosene.
Large amounts of liquid chemicals are manufactured in the petroleum industry, and fractional distillation is commonly used to separate the chemicals into pure liquids.
5) Separating Liquids which are not Miscible
Many liquids do not mix together. An example is petrol and water. In a mixture of petrol and water, the lighter liquid (petrol) forms a separate layer on top of the heavier liquid (water). Such a mixture can be separated with a separating funnel. The tap is opened so that the lower water layer runs out first. The tap is closed as the last drop of water leaves the funnel. The tap is then opened again to run the petrol into another beaker.
Separating petrol and water with a separating funnel
6) Chromatography
Chromatography is a method of separating and identifying mixtures. There are several types of chromatography, one of which is paper chromatography. An example of the use of paper chromatography is in the separation of the dyes in black ink. This is shown in the experimental set-up below.
Paper chromatography of black ink
Aim: To separate and identify the dyes in black ink
1 Draw a pencil line on a piece of chromatography paper (this paper is like filter paper).
2 Place a drop of black ink on the pencil line. Also place drops of coloured dyes (red, orange, blue, green, etc.) on the pencil line. These dyes are the ones you think might be in the black ink.
3 Roll the paper into a cylinder and secure it. Then place the paper in a beaker with a suitable solvent.
4 The solvent will travel up the paper. When the solvent reaches the top of the paper, take the paper out of the beaker and allow it to dry.
A suitable solvent for this separation is a mixture of butanol (an alcohol), ethanoic acid and a little water. The solvent chosen depends on the type of substances being separated.
In the above experiment, the solvent travels up the paper. The dyes on the pencil line dissolve in the solvent and travel up the paper at different speeds. Hence, the dyes are separated. The result is called a chromatogram. A typical example is shown on the next page.
.
Chromatogram of dyes in black ink
Identical dyes travel the same distance up the paper. The unknown dyes in the black ink can be identified by comparing them with the known dyes. The result of the experiment shows that - black ink is made up of four different dyes; - three of the dyes are the known red, blue and green dyes; - there is no orange dye in the black ink; - black ink contains one dye that is different from the four known dyes.
Chromatography can also be used to separate and identify colourless substances. The chromatogram is sprayed with a locating agent to show where the substances are on the paper. The locating agent is a chemical that reacts with the substances to produce a coloured product.
An example of separating colourless substances is in identifying the sugars in a fruit juice. The same method is used as for the black ink in Experiment 2.4. A drop of fruit juice is used in place of the black ink. A different solvent is used. At the end, the chromatogram is sprayed with silver nitrate solution followed by dilute sodium hydroxide. These chemicals make the sugars appear as brown spots. A typical result is shown.
The result of the experiment shows that
- fruit juice contains three different sugars;- one of the sugars is sucrose;
- the fruit juice does not contain maltose and glucose;
- the fruit juice contains one other sugar different from the three known sugars.
Paper chromatography can also be carried out with the solvent running down the paper. The arrangement of the apparatus is shown on the left. This descending method (shown on the left) of paper chromatography works better for longer pieces of paper as the solvent does not have to move against gravity, and thus flows more quickly. This means that the solutes which are separated can travel further and thus the separation between the spots is greater.