Pure Substances and Mixtures

1.1: Pure Substances and Mixture

Often we regard a pure substance as something which is clean and unpolluted and an impure substance as its opposite. However, in science, purity is more than just being clean and unpolluted. Polluted sea water is obviously impure, but to a scientist, so is clean fresh sea water. A pure substance contains only one type of substance. (i.e one type of atom or a molecule)
Pure water is a pure substance but sea water also contains dissolved salts which are referred to as impurity.


1.2 The Need for Pure Substances


Scientists need pure substances to study their properties. Food and medicines have to be tested for purity before they are sold as any impurity in them could be harmful or poisonous. In the electronics industry, microchips have to be made from pure silicon. Any impurity in the chips can greatly reduce their effectiveness.

A pure solid has a fixed and exact melting point. Presence of an impurity lowers the melting point and causes the substance to melt over a range of temperatures. An example of this is the addition of salt to ice. Pure ice melts at exactly 0 degrees celsius. When salt is added, its lowers the melting points to between -5 degree celsius to -25 degree celsius, depending upon the quantity of the salt added. That is why in cold countries, salt is spread on icy roads in winter to melt the ice. This also explains why it is very cold enough for the sea water to freeze.
With liquids, any impurity raises the boiling point. A pure liquid has a fixed and exact boiling point. For example, water boils at 100 degree celsius. However, when salt is added, it raises the boiling point. Mixtures do not have exact properties as their compositions can vary. Because of this, they do not have fixed melting and boiling points. For example, petrol is a mixture and has a boiling point within the range of 35 degrees celsius to 75 degrees celsius. Candle wax is also a mixture and has a melting point wtihin the range if 50degrees celsius to 60 degrees celsius.
Mixtures and formed by a physical change only. Because of this, they are easy to separate into pure substances. The process is called purification and can be achieved by simple physical methods like dissolving, filtering, evaporation, distillation and chromatography.

Filtration

2.1: Filtration Method

Mixtures can be easily separated into pure substances. This process is called purification. It is done by using physical methords without chemical reactions. There are several methods of purification but in this post, filtration would be the one we would be talking about.

Filtration is used to separate an insoluble solid from a liquid. A simple way of doing this is in shown in this figure.

The mixture is poured through a filter which is usually made of paper. The filter paper has tiny holes which the particles of the liquid are able to pass through. The particles of the solid are large. They cannot pass through the holes and are trapped by the filter paper. The holes allow water and dissolved substances to pass through, but not the large insoluble particles.

The solid collected in the filter paper is called the residue. The liquid passing through the filter paper is called the filtrate.

Filtration is used to purify drinking water by removing insoluble solids. Small filter systems are used in the home to filter tap water. At water purification plants, large filters are used to remove sand and mud from the water. The filters at these plants do not use filter paper, but consist of layers of sand, gravel and pebbles.

2.2: Words used

Soluble: A substance that will dissolve.

Insoluble: A subsance that will not dissolve

Solute: Normally a solid which dissolves

Solvent: Normally a liquid which does the dissolving

Solution: Solute + Solvent

Residue: Insoluble solid which is trapped in filter funnel

Filtrate: A solution that passes through the filter funnel.

Crystal: A solid that reappears from the solution.

Crsytallisation

3.1: Crystallisation Method

Crystallisation separates a dissolved Solid from a solution as well-formed crystals.
One way to carry out crystallisation is to heat a solution to evaporate off most of the solvent. The hot, saturated solution is then allowed to cool. As it cools, crystals of the dissolved solid (solute) form. The impurities remain in the solution. The main steps in purifying a solid by crystallisation are shown in this video.


Crystallisation occurs because the solutbility of most solute decreases as the temperature decreases. That is, less solute can dissolve in a solution at a lower temperature than at a higher temperature. As hot solution cools, it eventually becomes saturated, that is, it can hold on more solute. The extra solute, that cannot be dissolved, seaparates as pure crystals. Impurities, if present in small amounts, remain in the solution.

3.2: Sidenote

Salt is unusual in that the water solvent must be completely evaporated from the solution to obtain the crystals of salt. Crystallisation from a 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. Pure crystals form as the liquid freezes. This is crystallisation without a solvent. This is how crystals of rock, such as quartz, are produced from molten rock in the earth.

This is a picture of a quartz:

Sublimation, Decanting and Centrifuging

4.1 Sublimation

Sublimation separates a mixture of solids, one of which sublimes. A few substances change directly, from a solid to vapour on heating without going through the liquid state. This change is called sublimation. On cooling, water vapour changes back to solid directly.

Iodine is a solid that sublimes. if a mixture of idone and sand is heated in a beaker, the iodine changes from solid to vapour directly. This is a picture of sublimation process.

The vapour changes back to solid directly on a cold surface. The sand is not affected by the heat and remains in the beaker. Other substances that sublime include carbon dioxide, anhydrous iron (III) chloride, anhydrous aluminium chloride and some ammounium compounds. As only a few solids sublime, this method of separation is limited.

4.2: Centrifuging/Decanting

These are used as alternative techniques to filtration. Decanting means carefully pouring off the liquid and leaving the undissolved solid, for example, sand, at the bottom of the beaker. It is quicker than filtration but separation may not be so effective.

Centrifuging is again quicker than filtration and is especially useful for separating fine insoluble solid particles from a liquid. As these fine particles are light, they are held in suspension throughout the water and only sink to the botttom very slowly. So decanting would be impossible. They are called suspensions. Some examples of suspensions are clay particles in water, chalk particles in water and blood cells in plasma. In the centrifuge, an electric motor causes the test tubes to revolve at high speed. This flings the solid to the bottom of the test tube where they collect. The clear liquid can then be decanted off. A suitable appartatus for decanting is shown here.

Separating Funnel

5.1: Separating Funnel

A separating funnel is used to separate immiscible liquids.
Liquids that do not mix with each other are said to be immiscible. Two immiscible liquids, such as oil and water, can be separated by using a separating funnel. the mixture is placed in a separating funnel and allowed to stand. The oil and water form two separate layers, with the less dense liquid being ontop. The stopper is removed and the tap opened. The bottom layer is run off and collected in a container placed under the tap. The top layer remains and can be collected in a separate container.
For oil and water, the lighter liquid (oil) collects above the heavier liquid (water). When the tap is opened, the water is run out and the tap is closed before the oil reaches the bottom. Seapration is never entirely complete.

Distillation

6.1: Simple Distillation

Fractional distillation separates a pure liquid from a solution.

In distillation, the liquid is changed from a vapour by boiling. The vapour is pure as other substances are left behind. The vapour 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 exmaple of a simple distillation is the distillation of seawater to obtain pure water. The seawater in the flask is heated to boiling. Pure water vapour is produced. In the condenser, the steam is cooled to form pure liquid water. The salt remains in the flask.

6.2: Desalination?

In countries with few rivers or lakers to provide an adequate source of water, pure water is obtained from seawater. This is called desalination. The distillation of seawater is one way that pure water is obtained. Distillation plants exist in about 120 countries around the world. The disadvantage with the use of distillation is cost. The main fuel used to heat the water in the distillation process is oil, and as alot of oil is required, the pure water produced is very expensive.

6.3: Fractional Distillation

Fractional distillation separates a mixture of miscible liquids with widely differing boiling points.

Miscible liquids are completely soluble in each other. This means two liquids are mixed to form one liquid. A mixture of miscible liquids can be separated by fractional distillation. This is done with the help of a fractionating column. For example, water and alcohol are miscible and have boiling points of 100 and 78 respectively. The fractioning column used is normaly packed with glass beads or some other unreactive substances. This provides a large surface area for condesation. When the flask is heated, the vapour coming off the mixture wil lcontain both ethanol and water molecules. However, it will be richer in ethanol molecules as these have the lower boiling point of the two.

At first, this vapour just condenses on the cold fractionating column, but as this column warms up, molecules in the vapour state rise further before condensing. As we go up the column, the temperature becomes lower and so the proportion of the ethanol molecules with the lower boiling point increases. When the temperature at the top of the column reaches 78 degrees celsuis, molecules of ethanol can survive as vapour and these pass over in the Liebig condenser. Water molecules with the higher boiling point condense in the fractionating column and fall back into the flask. This continues until most of the ethanol is boiled off. When the temperature at the top of the column rises to 100 degrees celsius, water passes into the condenser. it can be collected in a different receiver.

Paper Chromatography

7.1: Paper Chromatography

The name of this method came from the Greek word khroma meaning colour. It was first used to separate colours, pigments and dyes. Nowadays, it can be used to separate colourless substances.

The principle involved depends upon the relative solubilities . In paper chromatography, the piece of paper used, like most paper, contains water loosely combined with the cellulose of the paper. If a dye is put in small spots, at the bottom of the paper, and another solvent is soaked up the paper, the solutes present in the dye dissolve to different extents. Some are more soluble in the mobile solvent moving up the paper. Other dissolve better in the water trapped in the paper and therefore do not travel very far up the paper. The difference in solubility allows the different pigments in the dye to be separated.

7.2: Applications

Chromatohraphy, although a simple technique, has many important applications besides identifying colour pigments. In medicine, proteins, which are complicated molcules, may be identified using chromatograms. Here, the building units of proteins are amino acids which, like pigments, travel different distances with solvents. Although amino acids are colourless, they can be sprayed with a liquid which reacts with them to make them visible. Other substances produced by the body, such as urine, also be analysed by chromatography. We call liquids, which, when sprayed onto chromatograms, make the separating visible, locating agents. A locating agent is a chemical which reacts with the substances to produce a visibly coloured product. One such liquid is ninhydrin spyray which can react with colourless amino acids to produce a purple stain.

7.3: Alternative Method

Paper chromatography can also be carried out with the solvent running down the paper. This descending method 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 solute which are separated can travel further and thus the separating between the spots is greater.

7.4: Rf Values

For any substance on a chromatogram,

Rf value = Distance moved by the substance/ distance moved by the solvent

Testing the Purity of Substances

Melting Point:
A pure substance has fixed and exact melting point.
The identity of a pure solid can be deduced by its melting point. For example, pure steatic acid melts at exactly 70 degrees celsius. THe presence of any impurities will lower this melting point. Also, if impurities are present, it will melt over a range of temperatures, perhaps 68-70 degrees celsius, as it is a mixture.

Boiling Point:
A pure substance has a fixed and exact boiling point.
The identity of a pure liquid can be deduced by its boiling point. For example, pure ethanol boils at exactly 78 degrees celsius. The presence of impurities will raise the boiling point, and also causes the liquid to boil over a range of temperatures.

Chromatography:
A pure substace shows only one spot on a chromatogram.
Chromatography can also be used to test for purify. It is especially useful with complicated chemicals like drugs, of food addictives which cannot be easily melted or distilled. For example, using chromatography, a public anlyst can identify the synthetic dyes many foods contain and check that they are harmless. Many green vegetables sold in markets have been treated with pesticides and herbicies. These can be harmful to the people. Chromatography can detect such cehmicals even down to very low concentrations.