Solubility is the physical property of a substance. It is defined as the grams of solute dissolved per 100 ml of solvent. The solubility of a substance can not be changed. Where solubility is defined as the mass of solute dissolved at a fixed temperature and pressure.
However, the rate to achieve its solubility can be increased by physical or chemical means. The common factors that affect the rate of solubility are temperature, pressure, surface area, common ion effect, and, agitation (stirring).
Below is the conventional criterion to understand how much soluble a solute is in a given solvent:
- Less than 1g/100ml, Insoluble.
- 1-3 grams per 100 ml of solvent, sparingly soluble.
- Greater than 3 g/100ml, soluble.
Factors Affecting solubility
The factors affecting solubility have been explained in detail below:
- Effect of temperature
- Effect of pressure
- Common ion effect
- Surface area
1. Effect of Temperature
The rate of solubility is greatly affected by temperature. However, solid and gas solutes in liquid solvents experience different impacts on them with changes in temperature.
Solubility of solids solute in liquids solvents
In general, the rate of solubility of the solids, such as ionic salts, is increased at higher temperatures. However, the solubility is highly dependent upon the enthalpy of the solution. It works on Le Chatelier’s principle. If the enthalpy H of the solution is exothermic (heat is evolved), upon giving more heat, the reaction of dissolution goes backward. Hence, the solubility decreases. On the other hand, if the enthalpy of the solution is endothermic, heat is required to proceed with the dissolution. Then, upon heating, the solubility increases.
Solubility and solubility curves
The solubility of the solids (salts) can easily be understood by the solubility curves. The solubility curves show how the solubility of different compounds is changed with temperature.
Application in recrystallization
Purification of nonvolatile substances is based upon the effect of temperature on solubility. Firstly, the supersaturated solution of ionic salt is prepared. It is then heated up to the boing point of the solvent. This is done to dissolve maximum salt. Secondly, the solution is filtered to remove insoluble impurities. On cooling, the crystals of salt start appearing in the solution. This phenomenon is known as recrystallization.
Solubility of gases
Solubility of gases usually decreases upon increasing temperature. This happens due to two main reasons:
- The average kinetic energy of gas molecules is already very high.
- The gas dissolution process is generally an exothermic reaction.
An increased temperature results in high kinetic energy that leads to high solute and solvent particles energy. Eventually, the solute and solvent particles easily escape from the solution.
Also, the enthalpy of gas dissolution in liquids is exothermic. Hence, on heating, the reaction goes in such direction as to minimize the change, according to Le Chatelier’s principle. It is one of the primary reasons, why soft drinks need to be kept cold.
Gas solubility and BOD
Biological oxygen demand (BOD) is the minimum dissolved oxygen required by marine life to exist. Due to global warming, the temperature is increased which has decreased the amount of dissolved oxygen in the water and hence, adversely affected aquatic life.
2. Effect of pressure
The effect of pressure on the solubility of solids and liquids in liquid solvents is not so prominent. However, it has a remarkable impact on the solubility of gases.
The solubility of a gas in a particular solvent is increased by increasing the pressure.
This can be explained by Henry’s law of gases. It states that;
“The solubility of a gas in liquids is directly proportional to the applied pressure at a given temperature.”
C = k x Pgas
In Henry’s equation.
- C is the concentration of gas
- P is the external pressure
- K is the proportionality constant
Examples in real life
Scuba divers and depth limit
Scuba divers are advised not to go below the safe depth. This is so because, after a safe limit, the atmospheric pressure is high that increases the solubility of N2 gas in the blood. This may lead to abnormal bends or even death.
Soft drinks with dissolved CO2
Soft drinks bottles are subjected to high pressure to dissolve maximum CO2 in them. This carbon dioxide makes the drinks fizzy.
3. Common ion effect
Whenever a solution of an ionic substance comes in contact with another ionic compound with a common ion, the solubility of the ionic substance decreases significantly. This decreased solubility is explained by Le Chatelier’s principle.
For example, if we try to dissolve solid table salt (NaCl) in a solution where the chloride ion (Cl–) is already present. In order to accommodate more NaCl ions, the reaction shifts backward. The amount of NaCl that could dissolve in the solution to reach the saturation point would be lowered.
Application of common ion effect
The common ion effect takes a role in pH regulation in buffer solutions.
4. Surface area
The greater the surface area, the greater will be the rate of solubility. This can be understood with an example of a cube of a unit surface area placed in water.
It can be seen in the figure that the interaction of the cube with water is only at the surface in fig (a). If that cube is divided into 8 parts or more, the volume of the cube remains the same but the surface area is increased 4 times, fig (b). Thus, more interactions of solute and solvent would be possible. This means the saturation point will be achieved quickly.
5. Agitation (stirring)
Stirring is also among the factors that affect solubility. It is often employed by chemists to increase the rate of solubility in laboratories. On stirring, the solute particles are more likely to interact with the solvent. Thus, saturation occurs in lesser time.
Additional Resouces regarding solutions
How does viscosity affect solubility?
Viscosity is the friction between the layers of fluid. The viscous liquids cannot interact with solute particles, so, the viscosity decreases the rate of solubility.
What factors affect solubility directly?
There are two direct factors that affect solubility directly: Temperature and pressure. Temperature affects the solubility of both solids and gases, but pressure only affects the solubility of gases.
What is the nitrogen solubility index?
The solubility of nitrogen gas increases upon increasing pressure.
Is alum soluble in water?
Potash alum is an ionic salt and it is soluble in hot water.
Is mercury soluble in water?
Elemental mercury has strong cohesive forces, thus, insoluble in water.
Is acetaldehyde soluble in water?
Acetaldehyde has polar carbonyl bonds. The oxygen of carbonyl starts interacting with water’s proton. Hence, dissolves in water.
Are minerals fat-soluble or water-soluble?
All minerals, copper zinc iron are present in the ionized form and are soluble in water.
What are the three factors that affect the solubility of gases?
When gases are to be dissolved, the three important solubility factors are temperature, the nature of solvent and solute, and pressure.
- Fundamentals of Analytical Chemistry| (Second Edition) by Douglas A. Skoog (Stanford University) and Donald M. West (San Jose State University)