Chemical analysis Flashcards
What is a pure substance?
- In chemistry, the word pure has a special meaning – a pure substance contains only one type of element or one type of compound. This means that pure substances: melt and solidify at one temperature called the melting point, boil and condense at one temperature called the boiling point.
What are impure substances?
- Impure substances are mixtures. They do not melt and boil at one temperature –they change state over a range of temperatures.
What is a formulation?
- Formulations are mixtures that have been carefully designed to have specific properties. The components in a formulation are carefully controlled. Examples of formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and food.
What is chromatography?
- Chromatography involves: a stationary phase, which does not move and a mobile phase, which does move.
- In paper chromatography: the stationary phase is the absorbent paper and the mobile phase is the solvent, which is often water.
- During chromatography, mixtures are separated into their constituent components.
- The solvent dissolves the samples and carries them up the paper.
- Each components moves a different distance up the paper depending on its attraction for the paper and for the solvent.
- Chromatography can be used to identify artificial colours (e.g. food) by comparing them to the results obtained from known substances.
How do you investigate how paper chromatography can be used to separate and tell the difference between coloured substances?
Sample method:
* Draw a ‘start line’, in pencil, on a piece of absorbent paper.
* Put samples of five known food colourings (A, B, C, D and E), and the unknown substance (X) on the ‘start line’.
* Dip the paper into a solvent.
* Wait for the solvent to travel to the top of the paper.
* Identify substance X by comparing horizontal spots with the results of A, B, C, D and E.
Considerations, mistakes and errors:
* Pure substances produce a single spot in all solvents.
* Only ever use pencil to draw you start line, as ink will dissolve and affect your results.
What is the retention factor and how do you find it?
- Different components have different Rf value. Providing the same temperature and solvent are used, the Rf value for a particular components is constant and can be used to identify the component.
What are the tests for hydrogen, chlorine, oxygen and carbon dioxide?
- Hydrogen: when mixed with air, burns with squeaky pop.
- Chlorine: Turns admp indicator paper white.
- Oxygen: Relights a glowing splint.
- Carbon dioxide: when bubbled through limewater (a solution of calcium hydroxide), turns the limewater cloudy.
