8.1 Formation of Solutions Chapter 8 8.1 Formation of Solutions

Solutions A homogeneous (different substances are not distinguishable) mixture of 2 or more substances.

Solution Made up of a solute and a solvent. The solute is dissolved in the solvent. Example: Salt is the solute in ocean water.

Questions What is the solute in a glass of tea? What is the solvent?

Answers Tea Water

Solutions Solutions may be solids, liquids, or gases. The solution will be whatever phase the solvent was before the solute was added. Example: Oxygen is a solute in your blood. Since the blood (the solvent) was a liquid before the oxygen was dissolved in it, the solution of blood and oxygen is liquid.

How are solutions formed? Substances can dissolve in water by: Dissociation Dispersion Ionization

Dissociation The process in which an ionic compound separates into ions as it dissolves. Solute and solvent particles must be attracted to each other. The attraction must be stronger than the attraction holding the solute together and the attraction holding the solvent together. This is a physical change.

Dissociation Example: Sodium chloride and water NaCl → Na+ + Cl- Sodium chloride separates into sodium ions and chlorine ions as it dissolves in water.

Dispersion The solute breaks into small pieces and spreads throughout the water. The polarity of the solute makes this possible. Remember that water is a polar molecule. This is a physical change. Example: Sugar dissolving in water (both sugar and water are polar)

Ionization The process in which neutral molecules gain or lose electrons. This is a chemical change because new compounds are being formed. Example: H2O + HCl →H3O+ + Cl-

Properties of Liquid Solutions Three physical properties of a solution that can differ from those of its solute and solvent: Conductivity Freezing point Boiling point

Conductivity Remember that metals are good conductors because of the free electrons that surround the atoms. Free electrons are similar to anions (negative ions). When ions dissolve in water, conductivity improves.

Freezing Point When ions are added to water it interferes with the way the water molecules arrange themselves in order to freeze. This decreases the freezing point. Example: Salting roads and sidewalks in the winter time lowers the freezing point of water to -150C.

Boiling Point For the same reason that the freezing point is lowered, the boiling point can be raised by a solute. Example: Ethylene glycol keeps the water in radiators in cars from boiling.

Energy During the formation of a solution, energy is either released or absorbed. So, solution process can be described as endothermic or exothermic.

Energy Energy is needed to break the particle attractions and allow solutions to form. Energy is released when new attractions form. The difference in the 2 amounts of energy is called the heat of solution.

Rate of Dissolving The rate of dissolving is affected by: Surface area (more surface area = faster dissolving rate) Stirring (stirring = faster dissolving rate) Temperature (warmer temperature = faster dissolving rate)