Colloids share many properties with solutions. For example, the particles in both are invisible without a powerful microscope, do not settle on standing, and pass through most filters. However, the particles in a colloid scatter a beam of visible light, a phenomenon known as the Tyndall effect,The effect is named after its discoverer, John Tyndall, an English physicist (1820–1893), whereas the particles of a solution do not. The Tyndall effect is responsible for the way the beams from automobile headlights are clearly visible from the side on a foggy night but cannot be seen from the side on a clear night. It is also responsible for the colored rays of light seen in many sunsets, where the sun’s light is scattered by water droplets and dust particles high in the atmosphere. An example of the Tyndall effect is shown in Figure 13.10.1.

Emulsions are colloids formed by the dispersion of a hydrophobic liquid in water, thereby bringing two mutually insoluble liquids, such as oil and water, in close contact. Various agents have been developed to stabilize emulsions, the most successful being molecules that combine a relatively long hydrophobic “tail” with a hydrophilic “head”. Soaps are natural emulsifying agents and detergents are synthetic ones.  Figure 13.6.4 shows the similarity in structure between the soap sodium stearate [NaCH3(CH2)16CO2,] and detergent sodium dodecyl sulfate [NaCH3(CH2)11OSO3], both of which are salts with a charge on one end (the head) and a long tailed (nonpolar) hydrocarbon chain on the other.  The charged head is soluble in polar compounds like water while the long hydrophobic head is soluble in non polar compounds like fats and oils.

Volatile solute are solutes that have a vapor pressure. These are typically miscible liquids that form solutions of any proportion, and so it is not always useful to distinguish one as the solvent and the other as the solute. The vapor pressure above the solution is the sum of the vapor pressure of each component within the solution (Dalton's Law of Partial Pressure, section 10.4.3)., where the vapor pressure of each component is determined by Raoult's Law. For component A, PA=XAP0A, and for component B, PB=XBP0B .We will only look at two component systems, but the vapor pressure of a system with more than two volatile solutes is simply the sum of their individual Vapor Pressures.

Nonvolatile solutes do not have an appreciable vapor pressure of their own, and they decrease the vapor pressure of a solvent (over a solution) when added to a solvent. This can be understood by the dynamics depicted in figure 13.5.2. In part (a) you have a pure volatile substance (solvent) and the vapor pressure (Po) is the equilibrium pressure of the solvent when the rate of evaporation equals the rate of condensation (review 11.6, Vapor Pressure as Equilibrium Pressure). Note there are 5 red lines representing the evaporating molecules and 5 black lines representing the condensing molecules (so the rate of condensation equals evaporation and the number of vapor molecules is constant). A non-volatile solute is introduced (b), and when a solute molecule is near the surface it can't escape. This effectively reduces the surface area for evaporation, and so fewer molecules transfer to the vapor phase, but those condensing have no such reduction in surface area (a vaporized solvent molecule can lose energy and condense if it his a surface solute or solvent molecule). So in (b) there are 6 black arrows entering the liquid, but only 4 red arrows leaving. The system is no longer at equilibrium and more solute condense than evaporate, reducing the vapor pressure until the rate of evaporation equals condensation and a new equilibrium has been reached (c). The result is a reduction in the vapor pressure.

Henry's law states that the concentration of a gaseous solute in a liquid is proportional to the absolute pressure. This explains commonly observed phenomena like the degassing of a can of carbonated beverage upon opening. While sealed, there was no room for gas to expand and so the pressure would be high, resulting in a high dissolved concentration. This system was at equilibrium, but once it was opened, the gas pressure dropped and created a state of disequilibrium, with the gas leaving the fluid far mor rapidly than it was entering. Overtime, the system reached a new equilibrium, and the carbonated beverage became "flat".