To carry out reactions we normally dissolve the solid (or liquid) reactants (known as a solutes) in a common liquid (known as a solvent) to give a solution. When the solute is a liquid we say it is miscible with the solvent, instead of referring to it as dissolving.

When the temperature involved is not specified it is understood to be at room temperature. In general compounds dissolve better at elevated temperature.

WATER AS A SOLVENT

Electron pair

sp3

Water is a common solvent for chemical reactions of ionic compounds. It is cheap and environment friendly. As the structure shows, it has two hydrogen atoms and two "lone-electron" pairs. Because of the difference in electronegativity of oxygen (more electronegative) than hydrogen, there exist a dipole moment (visualise it as a small magnet) for the oxygen-hydrogen bond. The hydrogen atom is slightly positively.

Let us consider the interaction between LiCl and water. When LiCl is stirred in water, the water molecules would swarm the lithium ion with the lone-electron "fighting" to get to the lithium cation. Similar the hydrogen will push its way to the chloride anion. It is just like fans meeting their idols in a rock concert. These actions separate the lithium and the chloride ions by offering them stability. This is how water dissolves LiCl.

Note: The other molecule notable for its lone electron pair is ammonia.

Why are some ionic compounds not soluble in water?

Of course not all ionic compounds are soluble in water. Otherwise all mountains will be dissolved by the rain water and flow into the sea.

Let us consider barium sulphate, BaSO₄. The barium ion is electrostatically bonded to the sulphate ion. However this is not the complete picture. In the solid we will have

The entire arrangement would form a crystal. The barium ion does not have a fixed sulphate partner, but is held by all the sulphate ions surrounding it (in 3 dimension). Similarly the sulphate ion would be electrostatically bonded to all the barium ions surrounding it.

The dissolution equation would be

In chemistry speak we say an ionic compound will dissolve in a solvent if the lattice (that is crystal) energy is less than the solvation energy. This again obey the universal law that says all systems tend to move to a lower energy state.

For barium sulphate the lattice energy is high so it would not be dissolved by water.

SOLUBILITY of SOME COMMON IONIC COMPOUNDS IN WATER

1. All Group 1A ionic compounds are soluble.

2. All nitrates, halides, and sulphates are soluble, except;

3. All oxides, hydroxides, carbonates, sulphites, phosphates are insoluble, except the Group 1A elements. The Group IIA oxides and hydroxides are slightly soluble.

In practice the word "insoluble" has no meaning. All compounds are soluble to a certain extend. When we say that a compound is insoluble it means that it dissolve in negligible quantity.

For example lead is not suppose to dissolve in water and so was used as piping in homes. Now it is replaced by plastic pipes with the understanding that this might reduce the risk of lead poisoning.

POLAR COVALENT COMPOUNDS

Ionic compounds are not the only molecules that can dissolve in water, covalent compounds having oxygen and nitrogen atoms (or those below them in the Periodic Table) can also do so. Beer and wine is a good example. Beer is a solution of about 5% w/v of ethanol in it. Ethanol is CH₃CH₂OH, with the hydrogen atom sticking out at the end of the molecule. This allows electrostatic attraction between the positively charged hydrogen of the ethanol with the "lone-electron" pair of the water molecule.

So it is possible to dissolve covalent molecules with polar groups in solvents like water.

NON-POLAR COVALENT COMPOUNDS

Most of us will know that oil and water do not mix (or in chemistry we say oil is not soluble in water). This is because oil, a molecule made up of hydrogen and carbon atoms, does not have any polar groups to interact with water. Carbon is not highly electronegative and so when it is covalent bonded to hydrogen there is very little dipole moment in the bond. For example hexane ;

The carbon will form the backbone with the hydrogen atoms sticking to it. The molecule is like a tube a core of carbon along the axis and a skin of hydrogen atoms. For all intent and purposes it is a "non-charge" entity.

Oil is just a much longer tube. So the water molecules prefer to interact with itself, with the "lone-electron" pair holding on to the hydrogen atom.

To dissolve non-polar covalent compounds we will have to look for non-polar covalent liquids. Hexane is a liquid and it can dissolve oil.

Tutorial 1

Why does the molecule of detergent have a long non-polar chain terminated by a polar group at one end.      Answer

SUMMARY

• Ionic and polar molecules are more likely to dissolve in polar solvents. They will not dissolve non-polar molecules.
• The number of solvent molecules surrounding an ion in a solution is known as the coordination number. In most cases the value is six. Two from the opposing ends of the x, y, z axis.
• Non-polar molecules are more likely to dissolve in non-polar solvents. They will not dissolve polar molecules.
• There is a limit to how much solute can dissolve in a solvent. If you keep adding a solute to the solvent there will be a point when the solvent had enough and will not dissolve any more solute (as seen be the presence of solid after a period of time). We say the solution is saturated.
  Like all chemistry processes saturation is a dynamic equilibrium. At saturation, the solid will keep on dissolving while some other molecules will come out of the solution to be solids, the amount of solute in the solution at any time remains the same. At equilibrium the amount of solute in the solution is referred to as the solubility of the compound.
• The concentration of a solution is generally measured as mol dm^-�. However in studies where the value is very sensitive to changes in temperature (since volume of solution expands and contracts with change in temperature) the unit used to measure solubility is mole of solute in one kilogram of solvent (not solution), termed molality.

Tutorial 2

Can you please explain what is a polar molecule and give an example. Samantha (High school sophomore, New York City).      Answer