Chapter 13 Section 4 - Physical Properties of Solutions

2. Chapter 13 Section 4 - Physical Properties of Solutions Electrical Conductivity in Solutions Some substances conduct electricity and some cannot. The conductivity of a substance depends on whether it contains charged particles, and these particles must be able to move. Electrons move freely within a metal, thus allowing it to conduct electricity Electrolytes Provide Ions in Solution An electrolyte is a substance that dissolves in a liquid solvent and provides ions that conduct electricity. Strong electrolytes completely dissociate into ions and conduct electricity well. Weak electrolytes provide few ions in solution. Therefore, even in high concentrations, solutions of weak electrolytes conduct electricity weakly. Ionic compounds are usually strong electrolytes.

3. Remember: To have a liquid conduct electricity it must have charged particle that are able to move.

4. Tap Water Conducts Electricity Have you wondered why you are warned not to use electrical appliances when you are near water? Have you wondered why you are also warned not to go swimming when a thunderstorm is near? The reason is because of the electrolytes in the water. If you collect rainwater in a relatively unpolluted area, you will discover that the rainwater is essentially a nonconductor of electricity. A small concentration of carbonic acid from the carbon dioxide in the air added to the rainwater causes the rain water to be a weak conductor. Pure rainwater conducts almost as poorly as distilled water. However, most of the water we use comes from wells, lakes, or rivers. This water has been in contact with soil and rocks, which contain ionic compounds that dissolve in the water. Consequently, tap water conducts electricity.

5. Colligative Properties Salt can be added to icy sidewalks to melt the ice. The salt actually lowers the freezing point of water. Therefore, ice is able to melt at a lower temperature than it normally would. This change is called freezing-point depression. Nonvolatile solutes such as salt also increase the boiling point of a solvent. This change is called boiling-point elevation. For example, glycol in a car’s radiator increases the boiling point of water in the radiator, which prevents overheating. It also lowers the freezing point, preventing freezing in cold weather.

6. Only the Concentration of Dissolved Particles Is Important Any physical effect of the solute on the solvent is a colligative property The lowering of the freezing point and the raising of the boiling point are examples of colligative properties. Only nonvolatile solutes have predictable effects on boiling point, but besides that requirement, the identity of the solute is relatively unimportant. The greater the number of particle of solute in the solution the greater the freezing point will lower and the boiling point increase. Look carefully at the equations below. C12H22O11(s) → C12H22O11(aq) (1 dissolved particle) NaCl(s) → Na+(aq) + Cl−(aq) (2 dissolved particles) CaCl2(s) → Ca2+(aq) + 2Cl−(aq) (3 dissolved particles) Which will change the freezing and boiling points the greatest?

7. C12H22O11(s) → C12H22O11(aq) (1 dissolved particle) NaCl(s) → Na+(aq) + Cl−(aq) (2 dissolved particles) CaCl2(s) → Ca2+(aq) + 2Cl−(aq) (3 dissolved particles)