A good (suitable) recrystallization solvent will dissolve a large amount of the impure compound at temperatures near the boiling point of the solvent. Small amount of compound being purified should remain in solution at low temperatures, between approximately 25 and –5 ºC.

Crystallization is a technique which chemists use to purify solid compounds. It is one of the fundamental procedures each chemist must master to become proficient in the laboratory. If a saturated hot solution is allowed to cool, the solute is no longer soluble in the solvent and forms crystals of pure compound.

Recrystallization is a purification technique for solid compounds. To perform recrystallization, an impure solid compound is mixed with hot solvent to form a saturated solution. As this solution cools, the solubility of the compound decreases, and pure crystals grow from solution.

In theory, insoluble impurities can be removed from a compound fairly easily. The compound is dissolved in a solvent, the solution is filtered to remove the insoluble impurities, and the solvent evaporated to produce the solid compound. The insoluble impurities are left behind in the filter paper.

Summary of Recrystallization Steps

1. Add a small quantity of appropriate solvent to an impure solid.
2. Apply heat to dissolve the solid.
3. Cool the solution to crystallize the product.
4. Use vacuum filtration to isolate and dry the purified solid.

Why is water a good solvent for the recrystallization of acetanilide? Acetanilide readily dissolves in hot water, but is insoluble at low temps. Thus, it dissolves in hot water but crystalizes easily when cool. Because water is not a good solvent, ethanol must be added.

Recrystallization is the process in which deformed grains of the crystal structure are replaced by a new set of stress-free grains that nucleate and grow until all the original grains have been consumed. The process is accomplished by heating the material to temperatures above that of crystallization.

Crystallization is defined as a process by which a chemical is converted from a liquid solution into a solid crystalline state.

Annealing -A heat treatment used to eliminate part or all of the effects of cold working. Cold working – Deformation of a metal below the recrystallization temperature. During cold working, the number of dislocations increases, causing the metal to be strengthened as its shape is changed.

LIMITATIONS, ADVANTAGES AND DISADVANTAGES OF COLD WORKING

• The cold worked process possesses less ductility.
• Imparted directional properties may be detrimental.
• Strain hardening occurs.
• Metal surfaces must be clean and scale free before cold working.
• Hot worked metal has to be pickled in acid to remove scale, etc.

The application of hot metal working includes hot rolling, forging, extrusion and hot drawing. The carbon steel and stainless steel products are rolled to form thin plates and extruded to produce desired shapes. Hot working is used for altering the form of iron and steel without fracture and use of excessive force.

Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases. These changes result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working.

Indicate some of the advantages of cold working relative to warm and hot working. Advantages of cold working are: (1) better accuracy, (2) better surface finish, (3) increased strength due to work hardening, (4) possible directional properties due to grain flow, and (5) no heating of work required.

Warm working, (or warm forming), is a metal forming process carried out above the temperature range of cold working, but below the recrystallization temperature of the metal. Warm working may be preferred over cold forming because it will reduce the force required to perform the operation.

Cold working is the process of strengthening metals through plastic deformation. This is made possible through the dislocation movements that are produced within a material’s crystal structure. This is a technique commonly used in non-brittle metals that have remarkably elevated melting points.

The two types of forging machines are hammers, which impact the workpart, and presses, which apply a gradual pressure to the work.

There are two main types of forging — hot and cold. Hot forging requires the metal to be heated above its recrystallization temperature.

Forging is a manufacturing process involving the shaping of a metal through hammering, pressing, or rolling. These compressive forces are delivered with a hammer or die. Forging involves the shaping of metal through compressive forces such as hammering, pressing, or rolling.

The basic difference between forging and casting is that the forging process does not melt the work piece material to create a new shape. In casting, metal is melted and poured or forced into a mold made in the shape of the desired item.