40 Questions
What is filtration in the context of separating solids from liquids?
A separation technique in which very fine insoluble solids can be separated from a liquid by passing it through a filter paper
What is crystallization in the context of purification of organic solids?
The most common method for the purification of organic solids that are not heavily contaminated with other substances
Why is it necessary to separate the mixture by chromatography in some cases?
When appreciable quantities of other compounds (5 -10% or more) are present
What is the principle behind crystallization?
Solid compounds are much more soluble in hot solvents than in cold ones
What happens when a saturated hot solution of compound A is cooled?
The solution will become supersaturated and the compound will separate out as crystals
Why does impurity B stay in solution during crystallization?
Because it is present in low concentration and the solution is not supersaturated with compound B
What are the two types of recrystallization distinguished for practical purposes?
In water and in organic solvents
What is the first step in the procedure for recrystallization from water?
Dissolve the substance in the minimum amount of a hot solvent
What tool should be used when moving an Erlenmeyer flask to avoid burns?
Tongs
What is the purpose of allowing the solution to cool in the recrystallization process?
To allow the solid to crystallize out
What is done to the crystals after they have been filtered under vacuum?
They are washed with cold solvent
What is the final step in the recrystallization process?
Drying the crystals to remove traces of solvent
Why is water used as a solvent in recrystallization?
Because many organic compounds are insoluble in water at room temperature but soluble in hot water
What is added to the solution if the solid is dark in colour during recrystallization from water?
Activated carbon
Why is a reflux assembly used in recrystallization from organic solvents?
To prevent flammable volatile organic solvent vapours from causing fires
What should not be added directly to the boiling solution during recrystallization from water?
Charcoal
What is the general characteristic of most organic molecules in terms of water solubility?
Most organic molecules are not water-soluble.
What is the typical boiling point range of popular solvents for crystallization?
60-90°C
Why should a solvent with a boiling point higher than the melting point of the substance to be crystallized be avoided?
Because the compound will melt before it dissolves, and on cooling, it will probably come out as an oil rather than crystals.
What is a key consideration when selecting a solvent for extraction?
The solvent should not react with the product to be purified.
Why are low-boiling solvents like ether and dichloromethane often avoided in crystallization?
They can be difficult to work with and may not effectively dissolve the compound.
What is the main goal of liquid-liquid extraction?
To selectively separate a compound from a mixture.
What is a common characteristic of the two solvents used in liquid-liquid extraction?
They are immiscible (insoluble) in each other.
What is a common example of a solvent pair used in liquid-liquid extraction?
Water and a less polar organic solvent, such as diethyl ether or ethyl acetate.
What is the effect of impurity on the melting point of a substance?
It lowers the melting point and widens the melting range.
Why is the use of melting point as a means of identification uncertain?
Because many organic compounds may have the same melting points.
Why is it important to vent a separatory funnel when using low-boiling extraction solvents?
To prevent pressure build-up in the separatory funnel, which can cause the stopper to pop out and lead to losses of solutions and potential injury.
What is the purpose of 'mixed melting point' determination?
To confirm the identity of a substance by comparing its melting point with that of an authentic sample.
What can happen if you don't vent the separatory funnel properly?
The stopper can pop out, causing losses of solutions and potentially injuring you or your neighbour.
What is required to determine the melting point of a substance?
A capillary tube, thermometer, oil bath, and heating device.
Why is venting especially important when using a dilute sodium carbonate or bicarbonate solution?
Because carbon dioxide gas is given off in the neutralization process, which can cause pressure build-up in the separatory funnel.
How is a capillary tube prepared for melting point determination?
By sealing one end of the tube and filling it with a small amount of solid, about 2-3 mm in height.
How can you prevent pressure build-up in the separatory funnel?
By carefully inverting the stoppered separatory funnel and immediately opening the stopcock before shaking.
What is meant by boiling point?
Not mentioned in the provided content.
What is salting out, and how is it used in the extraction process?
Salting out is adding a saturated solution of NaCl or Na2SO4 to the aqueous layer to help with separation, and is used to overcome difficulties such as interface, foam, or emulsion formation.
What is the purpose of placing the capillary tube next to the thermometer?
To ensure the core material to be measured is next to the thermometer.
What can cause difficulties in the separation process during extraction?
Formation of interfaces, foams, or emulsions.
What is the significance of the melting point of a mixture of an unknown and authentic sample?
If the mixed m.p. is virtually the same as that of the two samples, it indicates they are likely the same.
How can you break interfaces, foams, or emulsions that form during separation?
By rapid rotation of the funnel.
What is added to the aqueous layer to facilitate salting out?
A saturated solution of NaCl (brine) or Na2SO4, or the salt crystals themselves.
Study Notes
Filtration
- A separation technique used to separate very fine insoluble solids from a liquid by passing it through a filter paper
Crystallization
- A common method for purifying organic solids that are not heavily contaminated with other substances
- Uses the knowledge that solid compounds are more soluble in hot solvents than in cold ones
- Involves preparing a saturated hot solution of the compound, allowing it to cool, and then filtering off the crystals that form
- Can be performed in water or organic solvents
Recrystallization
- A type of crystallization that involves dissolving the solid in a minimum amount of hot solvent, filtering, and then allowing it to cool and crystallize
- Can be performed in water or organic solvents
- Organic solvents require a reflux assembly to prevent flammable vapors from causing fires
- The choice of solvent depends on the boiling point and solubility of the compound
Recrystallization from Water
- Many organic compounds are insoluble in water at room temperature but are soluble in hot water
- A suspension of the solid is prepared in the minimum amount of water, brought to a boil, and then allowed to cool and crystallize
Recrystallization in Organic Solvents
- Uses a reflux assembly to prevent flammable vapors from causing fires
- The solvent should have a low capacity for dissolving impurities, not react with the product, and have a boiling point higher than the melting point of the substance
- Low-boiling solvents like ether and dichloromethane should be avoided, while high-boiling solvents are difficult to remove from the crystals after filtration
Extraction
- A technique used for selectively separating a compound from a mixture
- Involves the distribution of a compound between two immiscible solvents
- Often part of the workup procedure for isolating and purifying the products of organic reactions
Liquid-Liquid Extraction
- Involves the distribution of a compound between two immiscible solvents, usually water and an organic solvent
- Requires venting the separatory funnel to prevent pressure buildup
Salting Out
- A technique used to aid in the separation of the two layers in the extraction process
- Involves adding a saturated solution of NaCl or Na2SO4 to the aqueous layer
Melting Point
- A physical property that can be used to determine the purity of a compound
- The presence of impurities lowers the melting point and widens the melting range
- Can be determined using a capillary tube, thermometer, oil bath, and heating device
Learn about crystallization as a method for purifying organic solids, its applications, and importance in chemistry. Quiz covers principles and procedures of crystallization.
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