Mechanism of dissolving process and factors affecting solubility
It is the process that occurs when the solute decomposes into negative and positive ions or into separated polar molecules , Each of them binds to the molecules of the solvent .
Mechanism of dissolving process
Although the water molecules seem static in the beaker , but they are really in a continuous motion , especially the surface molecules ( due to their kinetic energy ) .
Dissolving ionic compounds and polar compounds in a polar solvent
It is easy to dissolve ionic compound ( as sodium chloride ) & polar covalent compound ( as hydrogen chloride gas ) .
Dissolving of sodium chloride in the water
The polar water molecules collide with the crystals of NaCl by their kinetic energy.
The water molecules attract Na+ and Cl− ions by directing the suitable pole towards them to be separated from NaCl crystals .
The water molecules which surround the ions isolate positive ions from negative ions and prevent their binding again .
The speed of the dissolving process depends on the surface area of the solute , the stirring process and the temperature .
It is the ability of the solute to dissolve in a certain solvent or it is the ability of the solvent to dissolve a certain solute .
Degree of Solubility
It is the amount of solute in grams , which dissolves in 100 g of solvent to form a saturated solution at saturated temperature and pressure ( STP ) .
Factors affecting solubility
The solubility is affected by the temperature and the nature of solute & solvent .
The nature of solute & solvent
Like dissolve like is well known statement which controls the solubility process , this statement can be explained as follows :
Polar solvent such as the water dissolves ionic compounds ( sodium chloride NaCl , sodium hydroxide NaOH , Nickel nitrate Ni(NO3)2 ) and the water dissolves polar compounds ( Hydrogen chloride HCl , Ammonia NH3 ) .
Non-polar solvent ( organic solvent ) such as Benzene dissolves non-polar compounds ( methane , oil , fats ) and Methane dichloride dissolves non-polar material ( iodine ) .
Solubility of some substances in polar and non-polar solvents
Three tubes contain a heterogeneous mixture of the water and methane dichloride .
In the first tube , the methane dichloride does not dissolve in the water , because the water is a polar solvent , while the methane dichloride is a non-polar substance , in which the non-polar substances do not dissolve in polar solvents .
In the second tube , by adding the iodine solution to that heterogeneous mixture , it dissolves in methane dichloride , but does not dissolve in the water because the iodine solution is a non-polar substance and methane dichloride also is a non-polar solvent , but the water is a polar solvent , In which the non-polar substances do not dissolve in the polar solvents , but dissolve in the non-polar solvents .
In the third tube , By adding green nickel nitrate to that heterogeneous mixture , it dissolves in the water and does not dissolve in methane dichloride because nickel nitrate is a polar substance and the water is a polar solvent , while methane dichloride is a non-polar solvent , In which the ionic substances dissolve in the polar solvents , but do not dissolve in the non-polar solvents .
Oil is soluble in benzene because oil ( non-polar substance ) is dispersed between molecules of benzene ( non-polar solvent ) due to the weak bonds between the benzene molecules .
Effect of temperature on solubility
There are three cases , The solubility of most ionic substances increases greatly by increasing the temperature , such as NaNO3 , KNO3 , KCl , KClO3 , The solubility degree of potassium nitrate ( KNO3 ) is 20 g/100 g H2O ( l ) at 10 degree Celsius , On raising the temperature to 50 degree Celsius , the solubility increases and becomes 90 g / 100 g H2O ( l ) .
The solubility of some ionic substances increases slightly by increasing the temperature , such as NaCl .
The solubility of some ionic substances decreases by increasing the temperature , such as Ce2(SO4)3 .
Properties of solution
The particles of solution can not be distinguished by the naked eye or by the electron microscope .
The diameter of its particles ( ions or molecules ) is less than 1 nm .
The particles forming the solution are regularly distributed , So , the solution is homogeneous in its composition and properties .
The particles do not scatter the beam of light passing through the solution .
Concentration of solutions
The ratio of the amount of the solute to that of the solvent affects the concentration of solution to be diluted or concentrated .
Concentrated solution is the solution in which the amount of the solute is large ( not larger than the solvent ) .
Diluted solution is the solution in which the amount of the solute is small in proportion to the amount of the solvent .
There are several methods for expressing the concentration which are Percentage , Molarity and Molality .
This method is suitable to express concentration for food and medicines .
Mass percentage ( m/m )
Mass percentage is the percentage of the mass of solute in 100 g of solution .
Mass percentage = [ Solute mass ( g ) ÷ Solution mass ( g ) ] × 100 %
Solution mass = Solute mass + Solvent mass
When the mass percentage of a solution is 25 % , the mass of the solute in 100 g of the solution equals 25 g .
Volume percentage ( v/v )
Volume percentage is the percentage of the solute volume in 100 ml of solution .
Volume percentage = [ Solute volume ( ml ) ÷ Solution volume ( ml ) ] × 100 %
Solution volume = Solute volume + Solvent volume
Some solutions are prepared on a volume percentage because liquid volumes are so easily to be measured .
When the volume percentage of a solution is 20 % , The solute volume in 100 ml of the solution equals 20 ml .
The stickers placed on medicines and nutritional substances must show the units that express the solute percentage , due to the presence of many types of percentage .
Molarity ( M )
It is the number of moles of the solute dissolved in one litre of solution , The unit of molarity is ( mol / l ) or Molar ( M ) .
Molarity ( M ) = [ Number of moles of the solute ( mol ) ÷ Solution volume ( L ) ] × 100 %
Number of moles ( n ) = Mass of solute ( g ) ÷ Molar mass ( g/mol )
Molality ( m )
It is the number of moles of solute in one kilogram of solvent , The unit of molality is mol / kg or ( m ) .
Molality ( m ) = Number of solute moles ( mol ) ÷ Mass of solvent ( kg )