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- Describe paper chromatography.
Coloured chemicals are made up of mixtures of different dyes. The method of separating pigments (coloured substances) using filter paper is called paper chromatography.
Why is it used? To identify what coloured compounds are present in a mixture, and purifying a coloured compound.
This is how it works:
On a piece of chromatography paper, a horizontal line is drawn on the bottom. This is called the base line. Then a drop of the dye mixture (ink, for example) is placed on the base line and a pencil is used to mark it’s name underneath. Then drops of some pure dyes that you think the mixture might contain, is placed on the base line, and marked as well. The chromatography paper is then put in a beaker/jar with solvent (water for example), in it. The solvent level should be below the base line and spots of dye, otherwise the dye will wash off into the solvent. As the solvent moves up the chromatography paper, the dyes in the mixture will separate themselves. You can see the solvent move up the paper and the point where it stops, is called the solvent front. The chromatography paper now, with all the different pigments filtered is called a chromatogram.
The colours will separate if the pigments have different solubilities in the solvent and/or if the pigments have different degrees of attraction for the filter paper.
- Interpret simple chromatograms.
Two of the pure dyes ( 1 and 3) have risen to the same height as the dyes in the mixture, so the mixture contains only dye 1 and dye 3, not dye 2.
To conduct chromatography on colourless mixtures, only the mixture is placed on the base line, the solvent front is marked after the chromatography process occurs, and then a chemical called a locating agent is sprayed onto the chromatogram to give the filtered compounds a colour. The compounds can then be identified by calculating the R.f (retention factor) value. Each compound has a specific R.f. value, so if you know this value you can just look up which chemical has that R.f. value to find out the different compounds in the mixture.
- Describe methods of separation and purification: filtration, crystallisation, distillation, fractional distillation.
Filtration: An undissolved/insoluble solid can be separated from a solution by passing it through a piece of filter paper in a filter funnel. The solution which passes through the filter paper is called the filtrate. The solid that stays on the filter paper is called the residue. This residue should then be washed with distilled water to remove any solution between it’s solid particles.
A crystalline solid can be extracted from a solution by heating the solution. The solvent is evaporated until the crystallisation point of the solid is reached. You can know the crystallisation point by placing a drop of the solution on a cold surface from time to time to see if crystals form. When it does, leave the solution to cool. Crystals will eventually form at the bottom. This solution can then be filtered off, leaving only the crystals to be dried.
Fractional Crystallisation: If there are two dissolved substances with different solubilities at different temperatures, then fractional crystallisation is used to separate them. Evaporate the solution containing both solutes. The one with lower solubility will form crystals first, the one with higher solubility will remain in the solution. The solution can then be filtered off to get the insoluble solute as residue.
Distillation: This method focuses on obtaining the solvent from a solution. The solution, usually salty water is heated until it boils and evaporates as steam, leaving behind the salt in the flask. The salt doesn’t evaporate and enter the condenser it has a much higher boiling point than water. Water is very volatile (has low melting/boiling points so can easily change states) so evaporates quickly. However, once this steam enters the condenser, which is cold, ti condenses and turns back into water and pours into a container at the end of the condenser.
Fractional Distillation: This method is used to separate a mixture of liquids with different boiling points. Liquids which are miscible (mix with each other) can be separated using this method. It uses a tall column in which continuous evaporation and condensation occurs. There is a range of temperatures in the column- higher at the bottom and lower at the top. The more volatile solvent (the one with the lower boiling point) evaporates first and moves up the column, into the condenser where it condenses into a liquid and is collected in a container. The other solvent which is not as volatile, won’t move up the column too far. When it reaches a certain height, the temperature in the column falls below its own boiling point and it condenses and falls back into the flask as liquid. Now you have the two solvents separated- one still in the distillation flask and the other in the collecting beaker!
- Understand the importance of purity in substances in everyday life, e.g. foodstuffs and drugs.
Purity, in chemistry, means that there is only one substance present in anything. Any unwanted substances are called impurities. It is important to ensure that there are no impurities in medical drugs and food additives as it will have harmful effects on our body and health. Even impurities in the silicon used to make computer chips will cause the chip to malfunction.
- Identify substances and assess their purity from melting point and boiling point information.
Chromatography is one method of finding out how many compounds are present in a substance, thereby seeing whether they are pure or not (more than one compound=impure). Another method to assess the purity of substances is too look at it’s melting and boiling points. Most pure substances have a distinct melting and boiling points. Melting point is the temperature at which a solid changes to liquid and the boiling point is the temperature at which a liquid changes into a gas.
If the substance does not melt or boil at the temperature it should be, then we can conclude that the substance is impure. The presence of an impurity can decrease the melting point, and increase the boiling point. Water melts at 0°C and boils at 100°C. When there are impurities present in the water, the melting point will be less than 0°C, and the boiling point will be greater than 100°C. This explains why salt is put on roads during winter- the salt added will reduce the melting point of water and ice is less likely to form.
- Suggest suitable purification techniques, given information about the substances involved.
You may have to use a combination of methods to purify a mixture. Choosing which methods for purification requires an understanding of what each method does. It is useful to know the solubility of the solutes in the mixtures.
Eg. you have a mixture of salt and sand. You need to separate the mixture into pure dry crystals of salt and sand. Salt is soluble in water but sand is not.
- You’ll need to add water to the mixture ans stir it. The sand will settle at the bottom (because it is insoluble in water) while the water will turn into salt water (solvent) as salt dissolves in it.
- You can separate the sand and solvent using filtration- the sand will be the residue and solvent will be the filtrate.
- You can then use crystallisation to extract the salt from the solvent (evaporate the solvent, filter the crystals and dry it on filter paper).
- You need to now purify the sand by rinsing it in distilled water and dry it on filter paper. Now you have dry, pure salt crystals and sand.
Notes submitted by Lintha
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