Solubility

Solubility of a substance is its maximum amount that can be dissolved in a specified amount of solvent at a specified temperature. It depends upon the nature of solute and solvent as well as temperature and pressure

Solutions of Solids in Liquids

Solutions of this type are most common. In solutions of solids in liquids, the liquid is invariably referred to as a solvent and the solid dissolved in it as the solute. If a solute is added in small amounts at a time to a given amount of a solvent at a constant temperature, with vigorous stirring of the solvent after each addition, a stage is reached when the added solute no more disappears, i.e. goes into solution but remains undissolved. The solution is then said to be saturated. A solution which remains in contact with undissolved solute is termed as saturated. It can also be defined as one which is in equilibrium with the excess of solid at a particular temperature.  

The amount of solute dissolved in 100g of a solvent to form a saturated solution at a given temperature is termed the solubility of the solute in the given solvent at that temperature. Each substance has a characteristic solubility in a given solvent at a definite temperature.  

When a solid is added to the solvent, the particles from the solid diffuse into it. The solute and solvent molecules move constantly in the solution phase. Some of the particles of the solute return to the solid state due to collisions. Thus, two opposite processes operate simultaneously.  

Dissolution: Particles of solute leaving the solid and dissolving in the solvent.  

Recrystallisation: Solute particles returning to the solid form.  

When these two processes move with same speed, an equilibrium stage is reached.  

Solute (solid)  \small \rightleftharpoons Solute (dissolved)  

Thus, a dynamic equilibrium exists in a saturated solution. When a saturated solution prepared at a higher temperature is cooled, it gives a solution which contains usually more of solute than required for the saturated solution at that temperature. Such a solution is referred to as a supersaturated solution. It is sually unstable and changes to saturated solution when excess of solute comes out in solid state.  

Refer to the following video for solubility

The following factors influence the solubility of a solid in a liquid:  

  • Nature of solute :   

The solutes (solids) can be classified as ionic and non-ionic solids. The ionic solids consist of positively and negatively charged ions. It is the force of attraction between the ions, i.e., lattic energy which opposes the tendency of a solute to dissolve. This force of attraction is different in different ionic solids depending on the charges present on the ions and distance between ions (ionic radii). The ionic solutes having high less lattice energy have more solubility. The ions are solvated by the solvent molecules and in this process energy (known as hydration energy) is released. When the hydration energy is high, the ionic solid is more soluble.  

Many non-ionic substances dissolve in polar solvents due to hydrogen bonding. Generally, if the solute and solvent have similar characteristics, i.e. both polar or both non-polar, the solubility is high and if both are dissimilar, the solubility is found low.    

  • Nature of solvent:  

Ionic solids dissolve to a larger extent in a solvent having a high dielectric constant as compared to solvents of low dielectric constants. Dielectric constant of water is 80 while that of methyl alcohol is 33.5 An ionic solid, therefore, dissolves more readily in water than in methyl alcohol. Benzene has a very low dielectric constant of 2.3 and, hence, ionic solids do not dissolve in benzene.  

For non-ionic solids, the guiding principle is ‘like dissolves like, i.e., if the solvent is polar, it will dissolve the polar solutes and if it is non-molar, it will dissolve the non-polar solutes in it.  

  • Temperature:  

The solubility of a solute in a given solvent varies appreciably with temperature. 

It is observed that the solubility of NaCl increases very slightly with an increase in temperature whereas those of KNO3, NaNO3, AgNO3, KI, etc., increase greatly. A sharp break in a solubility curve indicates the formation of a compound whose solubility is different from that of the substance from which it has been formed as in the case of Na2SO410H2O. It losses its water of crystallization at 32.3°C and is converted into anhydrous form. There are few substances like calcium acetate, cerium sulphate, calcium chromate etc., which show a decrease in solubility with rise in temperature.  

Generally, solubility depends on heat of solution. If a substance dissolves with absorption of heat, the solubility increases with rise of temperature. On the other hand, if a substance dissolves with evolution of heat, the solubility decreases with rise of temperature.

Solutions of Gases in Liquids

All gases are soluble in water as well as in other liquids to a greater or lesser extent. The solubility of a gas in liquids depends upon the following factors:  

  • Nature of the gas  

  • Nature of the solvent  

  • Temperature and  

  • Pressure  

Generally, the gases which can be easily liquefied are more soluble in common solvents. For example, CO2 is more soluble than hydrogen or oxygen in water. the gases which are capable of forming ions to aqueous solutions are much more soluble in water than in other solvents. Gases like hydrogen chloride (HCl) and ammonia (NH3) are highly soluble in water but not in organic solvents in which they do not ionize.  

The solubility of most gases in liquids decreases with increase of temperature. When a solution of a gas is heated, the gas is usually expelled. However, some gases are more soluble at higher temperature than at lower.  

The most important factor which influences the solubility of a gas in liquid is the pressure. the quantitative connection between the solubility and pressure is given by Henry’s law. According to this law,

“The mass of a gas dissolved by a given volume of a liquid, at constant temperature, is proportional to the pressure of the gas”.  

or

“The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution”.

or

“Mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution”.

The most commonly used form of Henry’s law states that

The partial pressure of the gas in vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution” 

Mathematically,

p = KHX

Here KH is the Henry’s law constant.

It has been observed that most gases obey Henrys law provided,  

  • The pressure is not too high.  

  • The temperature is not too low.  

Applications of Henrys law:

  • Scuba divers must cope with high concentrations of dissolved gases while breathing air at high pressure underwater. Increased pressure increases the solubility of atmospheric gases in blood. When the divers come towards surface, the pressure gradually decreases. This releases the dissolved gases and leads to the formation of bubbles of nitrogen in the blood. This blocks capillaries and creates a medical condition known as bends, which are painful and dangerous to life. To avoid bends, as well as, the toxic effects of high concentrations of nitrogen in the blood, the tanks used by scuba divers are filled with air diluted with helium (11.7% helium, 56.2% nitrogen and 32.1% oxygen).

  • To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under high pressure.

  • At high altitudes the partial pressure of oxygen is less than that at the ground level. This leads to low concentrations of oxygen in the blood and tissues of people living at high altitudes or climbers. Low blood oxygen causes climbers to become weak and unable to think clearly, symptoms of a condition known as anoxia.

Solutions of Liquids in Liquids 

When one liquid dissolves in another, the molecules of the solvent are caused to move apart so as to accommodate the solute molecules. Similarly, the solute molecules must also be separated so that they can take their places in the mixture. In both these processes energy is required. Finally, as the solute and solvent molecules are brought together, energy is released because of the attractive forces between them. When solute and solvent molecules are strongly attracted to each other, more energy is released in the final step. Three cases may arise under these circumstances. The overall dissolution process results either in evolution of heat or absorption of heat, or energy released in the final step is the same as the absorbed in the first two, i.e., net change is zero.  

Examples:  

  • Benzene and carbon tetrachloride : No evolution or absorption of Heat.  

  • Acetone and water: Evolution of heat.  

  • Ethyl alcohol and water : Absorption of heat.  

A liquid may or may not be soluble in another liquid. Depending upon the relative solubility of a liquid in another, the following three cases are possible.    

  • Liquids that are completely miscible.   Examples: Benzene and toluene; Ethyl alcohol and water; carbon tetrachloride and benzene.       

  • Liquids that are partially miscible. Examples: Ether and water; Phenol and water; Nicotine and water.    

  • Liquids that are practically immiscible. Examples: Benzene and water; carbon tetrachloride and water; Benzene and alcohol                                                              

Question 1: Which of the following factors does not affact the solubility of solids in liquid to large extent?

a. Nature of solvent

b. Temperature

c. Nature of solute

d. Pressure

Question 2: A solution which remains in contact with undissolved solute is termed as 

a. ideal solution

b. non-ideal solution

c. saturated solution

d. unsaturated solution

Question 3: Which of the following liquid pairs is completely miscible in each other?

a. Benzene and water

b. Carbon tetrachloride and water

c. Benzene and alcohol 

d. Ethyl alcohol and water

Question 4: To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under ..

a. high pressure and low temperature

b.high pressure and high temperature

c. low pressure and high temperature

d. low pressure and low temperature

Q.1

Q.2

Q.3

Q.4

d

c

d

a

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