Unit 12.3-12.4 - chemical analysis Flashcards
What is paper chromatography used for?
A technique used to assess the purity of soluble substances
Explain why and how fractional distillation works
This method is used to separate 2 or more miscible liquids and separates them according to their boiling point. It is different from simply distillation because in this case, the two components have similar boiling points.
The mixture is poured into a round-bottomed flask, and placed above a heat source. The liquids evaporate and the vapors rise into the fractionating column, which is filled with glass beads. The liquid with the higher bp condenses onto the glass beads, while the one with the lower bp will pass into the condenser, which has tubes with cold water running on either side of it, causing the vapour to condense until it drips into the conical flask as a liquid (known as a distillate).
What is paper chromatography?
A technique used to assess the purity of soluble substances by separating small quantities of unknown substances (solutes) within the solution.
How does a chromatogram work?
During the technique, the base line is drawn on a piece of filter paper in pencil, and a drop of the solution is placed on it and given some time to dry. It is then suspended above a suitable solvent in a beaker, making sure the solvent isn’t touching the base line but only the edge of the paper. The solvent rises up and dissolves the spot. Substances in the spot are carried up the paper by the solvent. The paper is removed from the solvent before it reaches the top. The graph is then analysed.
The height of the spot of a substance found within the solution determines what?
Its solubility in the solvent. The higher the spot, the more soluble it is.
Why is the baseline always drawn in pencil, not in ink?
Ink is soluble in many solvents, while graphite is insoluble. The ink dissolving in the solvent could mess up the chromatogram.
What does it mean if there is a single spot on the paper?
If it remained at the base line, it means the substance is not soluable in the solvent. If it did rise, it means that the substance was pure.
How can chromatography identify unknown substances?
A substance/solute will always rise the same distance with a specific solvent in a given time.
What is RF and how is it calculated?
Retention factor : distance moved by specific solute (spot)/distance moved by the solvent (from base line to solvent front). All Rf values are between 0 (insoluble) and 1 (100% soluble)
How would you separate 2 solids which do not have different magnetic properties?
By adding a solvent in which one of the solids is dissoluble, while the other isn’t. Filtering out the suspension would result with a filtrate of one solute dissolved in the solvent and a residue of the other solute.
How would you separate a insoluble solid from a liquid?
Filtration. By using the difference in particle size, the suspension would be poured through a filter funnel with filter paper. The insoluble solid would separate from the liquid because it would be unable to pass through the small holes of the paper. This would get us a clean sample of the filtrate (liquid) and the residue (solid).
How would you separate a soluble solid from a liquid?
Crystallization. By heating the solution, we would cause the liquid to evaporate until the solution would become saturated due to there being less solvent, and the solute would become crystallizing. The solution is then cooled down, which would reduce the solubility of the dissolved solute, causing more crystallization to occur.
In which industries is it important to know the purity of substances?
Food industry, pharmaceutical industry, etc
Formula for percentage of purity
mass of pure substance/mass of mixture x 100
Name 2 methods for assessing the purity of a substance
- Measuring physical properties such as the boiling point
- Chromatography
