 # Solubility Curve

The solubility of a solid in a given solvent is defined as the number of grams of the solute required to saturate 100g of the solvent at a particular temperature. The solubility of a given substance at a given temperature can be determined from its solubility curve. Solubility curve can be used to determine the amount of substance deposited when the solution is cooled. Solubilities of different substances at a particular temperature can be determined.

When any solid is in contact with its solution for sufficient time, then the solid will dissolve if the solution is not saturated or solid will grow at the expense of the solution if the solution is supersaturated. In any case, the solubility curve represents equilibrium conditions.

## Solubility Curve Definition

The variation in the solubility of any given substance with the change of temperature is shown by the solubility curve. The curve line drawn on a graph showing the relationship between temperature and solubility of the substance at different temperatures is called a solubility curve.

A graphical relationship between the solubility and temperature is termed as the solubility curve. The solubility curve plots the changes of the solubility of a solid at different temperatures in a solvent. On a graph, the variations in temperature are plotted on the X-axis and the solubility is plotted on the Y-axis. Temperature plays an important role in solubility because the solubility of a substance is different at different temperatures.

## Importance of Solubility Curve

Solubility curve can be used to determine the amount of substance deposited when the solution is cooled. Solubilities of different substances at a particular temperature can be determined. The importance of solubility curve is listed below.

• The solubility of a substance at a particular temperature can be determined.
• The solubility process of a given substance at any temperature can be determined.
• The solubility curve helps us to predict which substance will crystallize out first from a solution containing two or more solutes.
• The solubility curve helps us to compare the solubilities of different substances at the same temperature.
• It brings the change in the solute composition substance.
• It gives a clear idea that solubility of substance changes with the temperature.

## Solubility Rules Chart

Solubility general rules are listed below.

• Chloride salts are soluble, Some exceptions are PbCl2, Hg2Cl2 and AgCl
• Salts of Na+, K+ and NH4+ are soluble.
• All sulfate salts are soluble, Some exceptions are BaSO4, CaSO4 and PbSO4.
• Almost nitrate (NO3) salts are soluble.
• Almost hydroxide compounds are sparingly soluble. The important exceptions are NaOH and KOH. Barium hydroxide and calcium hydroxide are moderately soluble.
• All sulfide, carbonate and phosphate salts are sparingly soluble.

## Solubility Curve Problems

Some of the solved problems based on the solubility curve are given below.

Question 1:

A solution in equilibrium with a precipitate of AgCl was found, on analysis, it contains 1.0 10-4 mol of Ag+/L and 1.7 10-6 mol of Cl/L. Calculate the solubility product of AgCl.

Solution:

The solubility product is by definition the product of the concentrations of the ions in equilibrium with a precipitate of a sparingly soluble substance.

For AgCl

Ksp = [Ag+][Cl]

= (1.0 10-4) (1.7 10-6)

= 1.7 10-10

Question 2:

A solution in equilibrium with a precipitate of Ag2S was found on analysis to contain 6.3 10-18mol of S2/L and 1.26 10-7mol of Ag+/L. Calculate the solubility product of Ag2S.

Solution:

Ksp (Ag2S) = [Ag+]2 [S2-]

= (1.26 10-17)2 (6.3 10-18)

= 1.0 10-51