Physical Properties

1. Physical Properties Topic 4.4

2. Physical Properties • The nature of the bonds within a substance can predict and explain some of its properties. This is particularly true for physical properties. • We’ll consider three of these properties: • Melting and boiling points • Solubility • Electrical conductivity

3. Melting and Boiling Points • Ionic compounds tend to have high melting and boiling points as the forces of electrostatic attraction between the ions in the lattice are strong and thus require high energy to break. • Ion-ion attractions are stronger than the intermolecular forces: H-bonding, dipole-dipole, and van der Waals’ forces.

4. Melting and Boiling Points • Ionic substances with multiply charged ions such as Al3+, S2-, etc. usually have higher mp/bp than singly charged ions such as Na+, Cl-, etc. When comparing ions with the same charge, the ion having the smallest radii will usually have higher mp/bp.

5. Melting and Boiling Points • Macromolecular or giant covalent structures also have high melting and boiling points as covalent bonds must be broken for these changes of state to occur. • They also exist as solids at room temperature.

6. Melting and Boiling Points • Covalent substances have lower melting and boiling point than ionic compounds as the forces needed to be overcome to separate the molecules are the relatively weak intermolecular forces. Consequently many covalent substances are liquids or gases at room temperature. • Intermolecular forces increase with: • Increasing molecular mass • The extent of polarity within the bonds

7. Solubility • Solubility refers to the ease with which a solid (the solute) becomes dispersed through a liquid (the solvent) and forms a solution. • There are several factors involved but, in general, solubility is determined by the degree to which the particles of the solute are attracted to the particles of the solvent.

8. Solubility • Solubility trends are based on the nature of the solute and solvent. The following rule of thumb is used: Like Dissolves Like. • Polar and ionic solutes tend to dissolve in polar solvents and non-polar solutes tend to dissolve in non-polar solvents. • So, if two substances have similar intermolecular forces, chances are that the solute will have a high solubility in the solvent.

9. Solubility • Consider an ionic compound being placed in water: • The partial charges of the water molecules are attracted to the ions of opposite charge in the lattice, which causes them to become dislodged from their position. • Ions are separated from the lattice in this way become surrounded by water molecules and become hydrated; when this happens, the solid is dissolved.

10. Solubility Dissolution of sodium chloride in water

11. Solubility • Similarly, many polar compounds are soluble in water because of the attraction between their dipoles and those of the water molecules. • Non-polar substances are not readily soluble in water (a polar solvent) because of the lack of attraction between the water molecules and the solute.

12. Solubility • In a non-polar solvent, ionic solutes will not dissolve because the ions are not attracted to the solvent. • Similarly, polar compounds have limited solubility in non-polar solvents. • Non-polar substances, however, are soluble in non-polar solvents due to their ability to interact with the non-polar solvent by van der Waals’ forces.

13. Electrical Conductivity • The ability of a compound to conduct electricity depends on whether it contains ions that are able to move and carry a charge. • Ionic compounds are not able to conduct electricity in the solid state as the ions are firmly held within the lattice and can’t move. • However, in the liquid state or dissolved in water, ionic compounds can conduct electricity because the ions are able to move.

14. Electrical Conductivity • Covalent compounds do not contain ions and so, are not able to conduct electricity in the solid or liquid states.