Separation and Purification | Chemistry for Grade 11 (IGCSE) PDF Download

Filtration & Crystallisation

• The selection of the separation method hinges on the characteristics of the substances involved. 
• Each method relies on a distinction, typically in a physical attribute like boiling point, among the substances undergoing separation.

Separating a mixture of solids

• Differences in solubility provide a method for separating solids.
• Selecting an appropriate solvent is essential to ensure that only the desired substance dissolves, leaving impurities behind.
• For instance, water is suitable for dissolving salt in a mixture of sand and salt, while leaving the sand unaffected.

Filtration

• Separation technique used to isolate an undissolved solid from a mixture with a liquid/solution (e.g., separating sand from a sand-water mixture).
• Centrifugation is an alternative method for such separations.
• Process involves placing filter paper in a filter funnel above a receiving beaker.
• Mixture of insoluble solid and liquid is then poured onto the filter paper.
• Filter paper permits only small liquid particles to pass through as filtrate.
• Large solid particles remain trapped on the filter paper as residue.
• Filtration of a sand and water mixture:
  

Crystallisation

• Crystallisation is a technique used to separate a dissolved solid from a solution.
• It is employed when the solid is more soluble in a hot solvent than in a cold one.
• The process involves heating the solution, allowing the solvent to evaporate, and leaving a saturated solution behind.
• To check for saturation, a clean, dry, cold glass rod is dipped into the solution. If crystals form on the rod upon cooling, the solution is saturated.
• The saturated solution is then cooled slowly, causing solids to precipitate out as the solubility decreases, leading to crystal formation.
• Crystals are collected by filtering the solution to separate them from the remaining liquid.
• Subsequently, the crystals are washed with distilled water to eliminate impurities and then dried.

Distillation: Simple & Fractional

Simple Distillation

• Used for separating a liquid and soluble solid from a solution (e.g., separating water from saltwater solution) or a pure liquid from a mixture of liquids.
• The process involves heating the solution. The pure water evaporates, forming vapor that travels through the neck of the round-bottomed flask.
• The vapor then moves through the condenser, where it cools and condenses back into pure water, collected in a beaker.
• Once all the water has evaporated, only the solid solute remains behind.
• Diagram showing the distillation of a mixture of salt and water:
  
• Simple distillation is an option for separating fermentation products like alcohol and water.
• Yet, for enhanced separation, fractional distillation is preferred, especially when dealing with liquids that have similar boiling points and when higher purity levels are necessary.

Fractional Distillation

• Distillation is employed for separating multiple miscible liquids, like ethanol and water from a blend of the two.
• The mixture is heated to the boiling point of the liquid with the lowest boiling point.
• This particular substance evaporates first, rises, and then passes through a condenser.
• In the condenser, the vapors cool down and condense, turning into a liquid collected in a beaker.
• Once all of the targeted substance is evaporated and collected, the remaining components of the mixture are left behind.
• For instance, in the case of water and ethanol, the mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out, condensing into the beaker.
• Heating should be ceased when the temperature starts increasing to 100 ºC, at which point water and ethanol are separated.
• Fractional distillation of a mixture of ethanol and water:
  
• Using an electric heater is recommended for safety in the presence of flammable liquids.
• In industries, the separation of components in petroleum is achieved through fractional distillation.
• Fractional distillation of crude oil, due to its toxic components, isn't typically conducted in school labs. Instead, a synthetic crude oil can be used for simulation.

Assessing Purity

• Pure substances exhibit specific and distinct melting and boiling points. For instance, water boils at 100°C and melts at 0°C, showcasing sharp temperature transitions.
• Mixtures, on the other hand, encompass a range of melting and boiling points due to the presence of different components with varying temperature properties.
• By analyzing melting and boiling point data, one can differentiate between pure substances and mixtures effectively.
• Identification of an unknown pure substance involves experimental determination of its melting and boiling points, followed by comparison with established literature values or data tables. Boiling points are typically ascertained through distillation.
• Melting point test using an oil bath:
  
• Comparing experimental values with data tables helps determine sample purity.
• The proximity of measured values to the actual boiling or melting points indicates sample purity.
• Impurities in a sample can cause deviations:
  • Impurities may elevate the boiling point beyond the actual value.
  • Impurities may depress the melting point below the actual value.