experimental techniques Flashcards
Importance of separation techniques
To obtain pure substances, remove impurities and carry out identification
Filtration
Used to separate an insoluble solid from a liquid.
The liquid passes through the filter paper and is collected as the filtrate.
The insoluble solid remains in the filter paper and is collected as the residue.
Evaporation to dryness
Used to obtain a dissolved solid (solute) from a solution.
Process: the solution of the dissolved sold is heated in an evaporating dish to evaporate off all of the solvent
Crystallization
Used to obtain a dissolute solid (solute) from A solution as pure well-formed crystals.
Crystallization occurs because solubility of most solutes decreases as temperature decreases. The mass of solute which cannot dissolve at a lower temperature will come out as crystals.
Evaporation to dryness (disadvantages)
The solid obtained by evaporation to dryness is not always pure. Any soluble impurities will be left together with the solid after heating.
Upon heating and evaporation to dryness, some solids decompose.
Crystallization (advantages)
During crystallization, the impunities remain in the solution and will not crystallize out. Thus, pure crystals are obtained.
When to use evaporation to dryness
When the solubility of the salt does not change significantly with changes in temperature
Simple distillation
Used to obtain a pure liquid (solvent) from a solution
Simple distillation- boiling chips
The boiling chips are added to the mixture to ensure a smooth boiling process.
Simple distillation- thermometer
The thermometer is positioned with its bulb beside the side-arm leading to the condenser to ensure that the thermometer measure the temperature of the vapor, which indicates the boiling point of the liquid distilling over.
Simple distillation- condenser
Water enters the condenser from the bottom and leaves the condenser at the top. This is to ensure complete condensation of all the vapor passing through the condenser.
Simple distillation - volatile liquids
Volatile liquid collected can be kept in the liquid state by placing receiver on ice
Magnetic attraction
Separate magnetic solids from non-magnetic solids
Sublimation
Separate a solid that sublimes from the other solids that do not.
Sublimation- a solid changes directly to gas without going through the liquid state
Substances that can sublime
Iodine, solid carbon dioxide (dry ice), naphthalene, ammonium chloride
Separating funnel
Used to separate a mixture of immiscible liquids.
Immiscible liquids- liquids that do not dissolve in each other
Fractional distillation
Used to separate a mixture of two or more miscible liquids which have different boiling points, with the use of a fractionating column
Fractional distillation- boiling points of liquids
The larger the difference in the boiling points of the liquids, the more complete the separation of the liquids
Fractional distillation- fractionating column
Glass beads or glass folds in the fractionating column, provide a large surface area for condensation of the vapour.
A good separation of the liquids is also achieved by using a long fractionating column with many glass beads or glass folds.
Paper chromatography
Separates a mixture of substances which are soluble in a common solvent
Paper chromatography-solvent front
Position reached by the solvent at the end of the process
Paper chromatography - locating agent
Enable the visibility of the substances/ components on the paper
Paper chromatography - ink or pencil?
The start line should be drawn in pencil and not in ink, because ink might dissolve in the solvent used, and thus interfering with the separation of the mixture
Paper chromatography - level of start line
The start line should be above the solvent level, because if it is below the solvent level, then the original spot mixture would dissolve in the solvent even before it can be separated into its components
Paper chromatography - spot size
The original spot mixture should not be too large to prevent the separated components from overlapping onto one another
Paper chromatography - length of paper
The chromatography paper should be sufficiently long to ensure complete separation of the mixture into its components
Paper chromatography - solvent front place(?)
The solvent front should reach near the top of the chromatography paper to ensure complete separation of the mixture into its components
Paper chromatography - volatile liquids
A lid should be used to cover the container to prevent evaporation of the solvent, especially when the solvent is volatile
Paper chromatography - Rf value
Ratio between the distance travelled by the substance and the distance travelled by the solvent.
Distance moved by substance/ distance moved by solvent (no units)
Paper chromatography - descending method
Carried out with the solvent running down the paper.
Solvent flows more quickly as it does not have to move against gravity.
Criteria of purity - pure solid
A pure solid has a fixed melting point. It melts at a constant temperature.
Criteria of purity - impure solid
If a solid is impure, its melting point is lowered and it melts over a range of temperatures.
Criteria of purity - pure liquid
A pure liquid has a fixed boiling point, it boils at a constant temperature.
Criteria of purity - impure liquid
If the liquid is impure, its boiling point is raised and it boils over a range of temperatures.
Criteria of purity - paper chromatography
A pure substance always gives a single spot on a chromatogram.
An impure substance gives more than one spot on a chromatogram.
Criteria of purity - heating curve
If a substance is pure, the heating curve shows a fixed and constant melting point for a solid and a fixed and constant boiling point for a liquid.
