Solutions: Processes Solutions: Types and Properties Chapter 15

What is a solution? Solute – the substance being dissolved Solution - a mixture that has the same composition, color, density & taste throughout. (can be a solid, liquid or a gas) Solute – the substance being dissolved Solvent – substance doing the dissolving Ex: In salt water, salt is the solute and the water is the solvent

How does dissolving happen? (sugar water) The sugar molecules are 1st pulled away from each other by the H2O molecules. B/c the dissolved sugar molecules are surrounded by H2O molecules, they are not attracted together (the H2O molecules block the attractive force). The sugar molecules are then distributed throughout the H2O.

Dissolving Demos http://www.mhhe.com/physsci/chemistry/es sentialchemistry/flash/molvie1.swf

Can dissolving be reversed? The H2O can be removed by boiling or allowing it to evaporate. When the H2O boils or evaporates away, the sugar molecules will once again be attracted, one to another. Sugar crystals will reform.

What affects the rate at which substances dissolve? 1. Temperature The higher (↑)the temperature, the faster the rate of dissolving. At higher temps the H2O molecules move faster & collide w/ the surface of the solute more often. At higher temps the sugar molecules are vibrating faster allowing them to overcome their attraction easier.

What affects the rate at which substances dissolve? 2. Particle size The smaller the size of the particles, the faster they dissolve. The smaller the particle size, the more surface area the particle will have in contact w/ the H2O molecules.

What affects the rate at which substances dissolve? 3. Agitation (stirring) The more the solution is agitated, the faster the rate of dissolving. When a solution is stirred, the H2O particles collide w/ the surface of the solute more frequently & dissolving occurs faster. However, if a substance is soluble in H2O, it will eventually dissolve even if the particle size is large, the temperature is low and there is no agitation.

Colligative Properties Depend on quantity of solute particles, not the type Vapor-Pressure Lowering: addition of solute particles decrease free solvent particles, so the resulting solution has a lower vapor-pressure than the pure solvent

Freezing-Point Depression Addition of solute particles disrupts the orderly formation of the solid and increases the kinetic energy of the solution. Higher kinetic energy means it takes more energy for liquids to freeze, so the freezing point is lowered (takes longer to freeze)

Boiling-Point Elevation Boiling occurs when a solution’s vapor pressure is equal to the atmospheric pressure Adding solute particles decreases the vapor pressure of the solution (vapor- pressure lowering) More kinetic energy is needed to raise vapor-pressure to atmospheric pressure, thus increasing the temperature at which the liquid boils

What is solubility? Solubility – the maximum amount of a solute that can be dissolved in a given amount of solvent at a given temp

What are saturated solutions? Unsaturated Solution – solution that can dissolve more solute at a given temp Saturated Solution - solution that contains all the solute it can hold at a given temp Super Saturated Solution – solution that contains more solute than a saturated one at the same temp