Science of Evaporation and Sublimation

Evaporation

• Evaporation is a physical change of state where a liquid transforms into a gas.
• It occurs at the surface of the liquid, driven by heat.
• Heat provides the energy needed for the liquid molecules to overcome their intermolecular forces and enter the gaseous phase.
• The rate of evaporation depends on several factors: temperature, surface area, wind speed, and humidity.
• Higher temperature leads to faster evaporation. A larger surface area leads to greater evaporation. Wind helps carry away the evaporated molecules, speeding up the process. Higher humidity slows evaporation because there is already a lot of water vapor in the air.
• Evaporation is a common separation technique used to separate volatile substances from non-volatile substances. For example, salt can be separated from seawater using evaporation.
• The non-volatile solute is left behind as the solvent evaporates.
• Crucial for purification of water and other solutions.

Sublimation

• Sublimation is a phase transition where a substance transitions directly from the solid phase to the gaseous phase without passing through the liquid phase.
• It occurs when the solid's vapor pressure is high enough to overcome the intermolecular forces holding the molecules in a solid structure.
• This process requires a significant amount of energy.
• Sublimation is an important separation technique. It's used to purify solids.
• An example of sublimation is the purification of iodine: solid iodine can be sublimated and then condensed to purify it.
• Impurities with lower vapor pressures remain in the original solid form during sublimation.
• Suitable for purifying compounds with high vapor pressures at relatively low temperatures. Examples include camphor and naphthalene.

Crystallization

• Crystallization is a separation technique employed for the purification of a substance from a solution.
• This technique involves inducing the formation of solid crystals of the desired substance from a solution.
• During crystallization, the solution is often slowly cooled or the solvent is slowly evaporated to decrease the solubility of the substance in question.
• The dissolved substance becomes less soluble as the solvent's temperature decreases or as solvent is removed.
• This leads to the precipitation (or 'separation') of the substance in its more solid, pure crystalline form.
• The solution's composition plays a key role. If the substance present, is already saturated, then crystallization can commence.
• The presence of impurities can affect the purity of the crystals formed via crystallization. Impurities can often be left behind during crystallization in a less concentrated form or in a more dissolved state.
• The rate of cooling or evaporation affects the size and shape of the crystals produced.
• Crystallization separates and purifies a component from a solution by leveraging the characteristic solubility differences and by influencing the rate of crystal formation.
• For example, dissolving sugar in water, then slowly evaporating the water. The sugar crystallizes out.