Solubility of Substance and Crystallization

Solubility of a substance

The solubility of a substance (solute) at a given temperature is defined as the amount of substance (solute) dissolved in 100 gm. of solvent to make a saturated solution at that temperature.

Mathematically,

Solubility(s) =

Effect of heat on Solubility

The molecules of a solute in solid are compactly packed. But when we heat the solid, they start to vibrate about their mean position as they posses kinectic energy on them. When we increase the temperature their kinetic energy increases and they start to move with high speed. Similary, the molecules of solvent in liquid state are loosely packed as there is less intermolecular force of attraction between them. These molecules of liquid are constantly moving inside. When we heat the liquid, these molecules gain more and more kinectic energy and moves with very high speed. Also, b during shaking , the molecules moves with high speed as they gain kinectic energy during shaking too. As a result the molecules strike with eachother and are separated. This molecules then mixed with the molecules of solvent to make a solution.
More will be the surface area of solute (solid state), more it will mix with molecules of solvent at a faster rate. When we heat the molecules at high temperature, the intermolecular spaces between them increases as the intermolecular force become due to heat. It helps to adjust more molecules of solute in the increased space. So, solubility of the substances also increases with increase in temperature.

Solubility curve

A curve obtained by plotting the solubility of substances at different temperatures against these temperatures is known as solubility curve.

Information obtain from solubility curve

• The solubility of substances at a particular temperature can be known.
• The solubility curve helps us to compare the solubility of difference substances at same temperature.
• It gives the idea that solubility changes with the temperature.
• The solubility curve helps us to predict which substance will crystalize out first from hot solution containing two or more solutes.

Crystals

Crystals are the granular substances having sharp edges and smooth surface.

Characteristics of crystals

• They are solid particles with definite or regular shapes.
• They are pure substances with fixed melting point.
• They are arranged in a three dimensional patterns.

Crystallization

The process in which crystals are formed by cooling a hot saturated solution of solid materials is called crystallization.

To obtain crystals from copper sulphate solution

Take a solution of copper sulphate in a porcelain basin. Make the solution hot saturated either by dissolving more copper sulphate or by evaporating solvent as shown in the figure. Continue heating until the small crystals are seen. Now cool the apparatus. After a while, the crystals are formed. Now, separate the crystals from the mother liquor and place them on filter paper to dry.

Amorphous solids

Those solid substances, which do not have any definite geometrical forms or shapes, are called amorphous solids.

Crystalline solids

Those solid substances, which have definite geometrical forms or shapes, along with definite and rigid morphology, are called crystalline solids.