Purity And Chromatography
Overzicht
A carton in a supermarket promises pure orange juice. A bottle on a bathroom shelf promises pure spring water. A chemist reading either label would tell you, without opening anything, that both claims are false. Not dishonest, just written in a different language. In chemistry the word pure has a single narrow meaning, and almost nothing you eat or drink meets it.
This lesson gives you two ways to hold a substance up to the light and find out what is really in it. The first is a thermometer: a pure solid melts at one sharp temperature and an impure one smears that melting over a range. The second is a strip of paper, a shallow pool of solvent and a pencil line, which will pull a drop of black ink apart into the separate dyes that were hiding inside it. You will learn to run that separation yourself, to calculate the Rf value that turns a coloured smudge into a number, and to use that number to name a substance you have never seen before.
Doelstellingen
- A pure element or compound contains only one substance, with no other substances mixed in. Students should be able to identify substances and assess their purity from melting point and boiling point information.
- Measures of purity are important in everyday substances such as foodstuffs and drugs.
- A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged. It is possible to separate the substances in a mixture by physical methods, including distillation, filtration and crystallisation.
- Paper chromatography can be used to analyse substances present in a solution, eg food colourings and inks/dyes. Students should be able to describe how to carry out paper chromatography separations and how the components of a mixture can be identified using Rf values. They have to be aware that solvents other than water can be used.
- Chromatography involves a stationary and a mobile phase and separation depends on the relative solubility of the components. Students should be able to suggest chromatographic methods for distinguishing pure from impure substances.
Mindmap
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- Which statement describes a pure substance in chemistry? A. A substance that has had nothing added to it B. A substance that contains only one element or one compound C. A substance that is clear and colourless D. A substance that has been filtered Answer: B
- A solid melts between 74 degrees Celsius and 81 degrees Celsius. What does this show? A. The solid is a pure element B. The solid is a pure compound C. The solid is impure D. The solid has a high boiling point Answer: C
- In paper chromatography, why is the baseline drawn in pencil rather than in ink? A. Pencil lines are easier to see on wet paper B. Pencil does not dissolve in the solvent, so the line stays in place C. Ink reacts chemically with the chromatography paper D. Pencil makes the samples travel further up the paper Answer: B
- In paper chromatography, which is the mobile phase? A. The chromatography paper B. The solvent C. The spots of dye D. The lid on the container Answer: B
- A spot on a chromatogram has moved 6.0 cm from the baseline. The solvent front has moved 15.0 cm from the baseline. What is the Rf value of the spot? A. 0.25 B. 0.40 C. 2.50 D. 9.00 Answer: B
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