Solubility Definition and Solubility Crave Chart

The solubility of a solute in a given solvent is the concentration of the solute in a saturated solution at a given temperature. The concentration of a solute in a saturated solution is called the solubility of the solute in the solvent. 

The solubility of a solute in a given solvent depends on the nature of the solute, the nature of the solvent and the prevailing temperature (solubility mainly depends on temperature). Solubility data are generally given as parts of the weight of anhydrous solute material per 100 parts by weight of the solvent, e.g., the solubility of MgSO4 in water at 293K (20°C) is 35.5 kg MgSO4 per 100 kg water. Solubility data are plotted as solubility curves wherein solubilities are plotted against temperature. 

 {tocify} $title={Table of Contents}

The solubility of the solute in a given solvent is different at different temperatures and it forms the basis of crystallisation by cooling. 

Saturation Definition 

Consider a process of dissolving copper sulphate in water (solvent) at a given temperature. Initially, when some amount of copper sulphate is added, all of it goes into the solution. An additional amount can be dissolved further till a stage comes when no more copper sulphate can be dissolved in a given amount of the solvent. At this stage, the solution is called a saturated solution. A saturated solution is defined as one which is in equilibrium with an excess of solid solute at a given temperature.

If the temperature of the solution is increased more solute [CuSO4] can be dissolved. Therefore a saturated solution will contain different amounts of solute dissolved in it at different temperatures. When a saturated solution at a higher temperature (say as T1) is cooled to say T2 (T1>T2)  then theoretically the amount of the solute corresponding to the difference in solubilities at these two temperatures will come out of the solution in the form of crystals. 

Solubility Curves

A graphical relationship between the solubility and temperature is termed the solubility curve. 

The concentration is necessary for crystal formation and chemical species that separate can be determined from solubility curves. Such curves are obtained by plotting the solubility of a solute as a function of temperature (solubilities against temperature). 

It shows the effect of temperature on the solubility of the solute substance. The solubility of solutes in a given solvent may increase, decrease, or more or less remain constant with temperature. Solubility curves have no general shape or slope. 

Solubility curves example

Solubility curves of potassium chlorate and sodium chloride are continuous solubility curves as they show no sharp break anywhere. Sometimes, the solubility curve exhibits sudden changes of direction and these curves are therefore referred to as discontinuous solubility curves e.g. FeSO4, Na2SO4 etc. 

For some substances, their solubility decreases with an increase in temperature and in such cases, their solubility curves are called inverted solubility curves (e.g. that of MnSO4•H2O in H2O) 

Usually, an increase in the temperature of the solution increases the solubility of the solute when no true compounds are formed between the solute and solvent, KClO3, KNO3 in H2O. In the case of hydrated salt, the solubility increases with an increase in temperature over a certain temperature range and then decreases. 

The image shows solubility curves for several salts in water. In the case of potassium chlorate, the solubility increases with temperature so it can be readily crystallised by cooling a saturated solution. 

The solubility of NaCl in water is almost independent of temperature (a slight increase in the solubility results from a large increase in temperature), so for crystallisation to occur, some of the solvents should be evaporated. 

Testing Methods Of Casein Protein Powder | Solubility of Casein protein powder in water

Type of Solution based on Solubility