How To Predict Boiling Points and Solubilities of Commpounds

Boiling Points

The boiling point of a substance is the temperature at which the vapour pressure of the liquid equals the pressure surrounding liquid and liquid changes into vapour. The boiling point of a liquid depends upon the bonding present in it, but it varies, as well as changed in surrounding environmental pressure. 

In general, compounds with ionic bonds have high normal boiling points, if they do not decompose before reaching such high temperature. Many metals have high boiling points, but not all. Very generally - with other factors being equal - in compounds with covalently bonded molecules, as the size of the molecule increases, the normal boiling point increases. Another factor that affects the normal boiling point of a compound is the polarity of its molecules. As the polarity of a compound's molecules increases, its normal boiling point increases, other factors being equal. In addition to these boiling points and vapour pressures of substance can be also affected by the presence of dissolved impurities ( solutes ) or other miscible compounds. The degree of effect depends on the concentration of impurities or other compounds. This effect is called boiling point elevation. For instance, salt water boils at a high temperature than pure water.

Solubility

Solubility is a chemical property of a substance called solute to dissolve in a solid, liquid, or gaseous solvent. Solubility, fundamentally depends on the physical and chemical properties of the solute or solvent as well as on temperature, pressure, and the pH of the solution. Solubility may also strongly depend on the presence of other species dissolved in the solvent, for example complex-forming anions ( ligands ) in liquids. All chemical substance are classified into miscible, immiscible, and partially miscible compounds, depending on their ability to get dissolve in a solvent molecule. The extent of solubility ranges widely, from infinitely soluble such as ethanol in water, to poorly soluble, such as silver chloride in water. The smaller particle is, the faster it dissolves in solvent. The solubility of a given solute in a given solvent may increase or decrease with increase in temperature depending on the nature of solute. For many solids dissolved in liquid water, the solubility increases with increase in temperature, while the solubility of ionic solutes tends to decrease due to the change of properties and structure of liquid water 

Predicting Trends in Boiling Points and Solubilities

Remember the following points when predicting trends in boiling points and water - solubilities of compounds:

• When considering boiling points of different types of organic compounds, first consider the type of intermolecular forces present in the molecule. For boiling points, the order is :

Ionic Compounds   >   Hydrogen Bonders   >   Dipole Dipole   >   Induced Dipole

Branching, etc, need not be considered. The non polar compound will have the lowest boiling point,

• In the absence of hydrogen bonding, boiling points increase with molecular weight. The higher the molecular weight, the higher is the boiling point.
• For compounds of similar molecular weights, boiling points increase with the degree of hydrogen bonding. The greater the degree of hydrogen bonding, the higher will be the boiling point.
• For alcohols of same molecular weights, branched-chain alcohols will have lower boiling points. This is because the decreased surface area leads to decreased Van der waals force.
• For alcohols of different molecular weight, the straight-chain alcohol containing the greater number of carbon atoms will have higher boiling point. This is because of increased van der waals force.
• Compounds capable of forming hydrogen bonds with water will be soluble in water.
• Degree of hydrogen bonding, chain-length ( molecular weight ), and branching will determine the extent of solubility.
• Compounds with shorter non-polar chain are most soluble in water.
• Among alcohols, the solubility in water depends mainly on the number of -OH groups. Greater the number of -OH groups, greater will be the solubility.