17.5 CRYSTALLIZATION

Crystallization is a technique for purification of the substances. It is a technique that separates a solid from its solution. The process in which the solid dissolves in the liquid and arranges itself into a well-defined 3-D structure is called crystallization. When any substance undergoes crystallization, its molecule arranges itself in a fixed structure at an angle to form a 3-D structure called the crystal, and then the crystal is removed from the solution. The crystal is a pure substance, and thus the process of crystallization is widely used in the purification of material from its impure solution.

What is the principle of crystallization?

The principle of crystallization is based on the limited solubility of a compound in a solvent at a certain temperature and pressure. A change of these conditions to a state where the solubility is lower will lead to the formation of a crystalline solid.

Crystallization Process

• When a small amount of solute at a given temperature is added to a solvent, the solute dissolves very easily in the solvent.
• If the addition of solute is kept on, a stage is reached when the solvent cannot dissolve any more solute. At this stage, further added solute remains undissolved and settles down at the bottom of the container. On the particle level, a saturated solution is one in which undissolved solute is in equilibrium with dissolved solute.
• At this stage, dynamic equilibrium is established. Although dissolution and crystallization continue at any temperature, the net amount of dissolved solute remains constant.
• The solution is heated until enough of the solvent (water) has evaporated to make the solution saturated.
• We can tell when the solution is saturated by dipping a glass rod into the solution to remove a drop and seeing if the drop goes cloudy and crystals start to form as it cools.
• Once the solution is turned off and allowed to cool, solubility decreases with temperature, and the solute that can't remain dissolved forms crystals. The crystals are removed from the remaining solution by filtering.
• The crystals are dried (in a warm oven, or just left to dry in air).

Science Tidbits:

The size of the crystals formed during this process depends on the speed of cooling. Rapid cooling of the solution will cause a large number of tiny crystals to form. Slow cooling rates lead to the formation of large crystals.

17.5.1 Application of Crystallization

• Crystallization is primarily employed as a separation technique in order to obtain pure crystals of a substance from an impure mixture.
• Another important application of crystallization is its use to obtain pure salt from seawater.
• Crystallization can also be used to obtain pure alum crystals from an impure alum. In such scenarios, crystallization is known to be more effective than evaporation since it also removes the soluble impurities.

17.5.2 Application of separation and purification techniques

Filtration, crystallization, and distillation are suitable separation and purification techniques. Filtration can be a physical process or a biological process. The physical application of filtration can be seen routinely all around our daily life:

• Human kidneys are responsible for filtering both the blood and other waste materials that may enter the body, whether through food, drink, or medicine. The waste leaves the body as urine.
• Coffee filters and automotive filters are commonly used in daily routines.
• Tea staining or coffee filters are examples of filtration.
• A common example of crystallization for separation and purification is the salt we get from seawater, which can have many impurities. Hence, the process of crystallization is used to remove these impurities.
• The crystallization of water to form ice cubes and snow is commonly observed in daily life.
• The crystallization of honey when it is placed in a jar and exposed to suitable conditions.
• The formation of stalagmites and stalactites (especially in caves).
• Have you ever observed as the sugar syrup cools, sugar crystals can begin to form? This is crystallization.

17.5.3 Identification and purity of substances

Melting Point and Boiling Point are characteristic parameters of substances that help to identify them. Pure substances have a fixed boiling and melting point. For example, water boils at 100°C and melts at 0°C. Impure substances have no fixed melting and boiling points. They can melt and boil at a range of temperatures.

To determine if a substance is pure in school laboratories, we can check the substance's melting or boiling points. Impure substances tend to have a slightly lower melting point and higher boiling point than the pure substance, and a broader range.

Have you ever noticed that in chocolate milk, chocolate melts really easily in your mouth, whereas dark chocolate (which is usually 70%) takes much longer to melt? When water contains dissolved impurities (for example, table salt), it boils at a slightly higher temperature (around 100.5°C).

KEY POINTS

• The change of a liquid into its vapours is called evaporation.
• Distillation is a separation technique used to separate components of a liquid mixture by a process of heating and cooling.
• Distillation under reduced pressure is called vacuum distillation.
• Filtration separates the solid matter from a liquid mixture using a filter medium.
• The principle of crystallization is based on the limited solubility of a compound in a solvent at a certain temperature and pressure.