Topic 9, Solutions

Topic 9, Solutions
(Key concepts)

A solution is a homogeneous mixture.  During this unit, we will discuss solutions made up of solids dissolved in water, such as salt water, NaCl(aq), and solutions made up of gases dissolved in water, such as seltzer water, CO2(aq).
Solutions have two parts, the solute and the solvent.  The solute is the part that is dissolved.  Salt is the solute in salt water.  The solvent is the part that does the dissolving.  Water is the solvent in salt water.

Solubility, or the ability to dissolve, varies among substances and can be influenced by temperature and, in the case of gases, pressure.

Solubility curves illustrate the relationship between solubility and temperature.

The three lines showing a decrease in solubility as temperature increases are for the gases SO2, NH3, and HCl.  As temperature increases, gases become less soluble.  Think of soda pop - cold soda is more carbonated (dissolved CO2) than warm soda.  Thermal pollution of rivers, lakes and streams is an issue because the oxygen gas dissolved in the water becomes less soluble as temperature increases.  Fish get their oxygen gas from the water and less dissolved oxygen gas is therefore a problem.

The other lines - each representing a different solid, show an increase in solubility as temperature increases.  In general, solids become more soluble as temperature increases.  Think of a cup of hot tea and a glass of iced tea - one can dissolve more sugar in hot tea than iced tea.

Gases are influenced by pressure changes because they can be expanded and compressed.  Increased pressure increases the solubility of gases.  This is why soda pop is packaged under pressure - to increase the solubility of the carbon dioxide gas.  And when the seal on a container of soda pop is broken, pressure is released causing the carbon dioxide to "come out of" solution.

Because solutions are homogeneous mixtures, their compositions can vary.  Salt water can be dilute, slightly salty, or concentrated, very salty.  Concentration can be expressed with molarity, percent composition by mass and parts per million.

Molarity (M) = moles of solute 
                      liters of solution

Percent mass = mass of part  x 100%
          mass of whole

Parts per million (ppm) = grams of solute  x 1,000,000 ppm
          grams of solution

Colligative properties are physical properties of a solution that depend on the number of dissolved particles rather than the type of particles present.  For example, the freezing and boiling points of water - both colligative properties, change when a solute is added.  When salt is dissolved in water, the freezing point decreases and the boiling point increases.  This is called freezing point depression and boiling point elevationThe more dissolved solute, the greater the effect.  Ionic compounds, like salt, dissociate when dissolved in water.  Adding one unit of NaCl is like adding two dissolved units; 1Na+ and 1Cl-.  Adding one unit of CaCl2 is like adding three dissolved units; 1Ca2+ and 2Cl-.  Covalent compounds, such as sugar, do not dissociate when dissolved in water.  Adding one unit of sugar adds one unit.  Therefore, adding ionic compounds has a greater effect on colligative properties than adding the same number of covalent molecules.  This is why we put salt on icy sidewalks and roads.  The salt lowers the freezing point of the ice causing it to melt.