Factors that Affect Solubility and Rate of Dissolving (8.2)
Solubility and Forces Between Particles
▪ formation of most solutions depends on the relative strength of three categories of forces:
1. forces that attract particles of the solute to each other
2. forces that attract particles of the solute to particles of the solvent
3. forces that attract particles of the solvent to each other
Solubility in Water
▪ water (polar) is a good solvent
▪ water molecules bond to each other with hydrogen bonds (bond between H on one molecule and a very
electronegative element on another molecule)
▪ fig 8.3
Solubility of Ionic Compounds in Water
▪ most ionic compounds are soluble in water
▪ water dipoles pull the compound apart
▪ water molecules surround the ions (hydration)
▪ fig 8.4
▪ strongly bound ionic compounds will not dissolve in water
Solubility of Polar Compounds in Water
▪ most polar compounds dissolve in water
▪ the bonds between the molecules are generally weaker than the bonds between water and the molecules
▪ hydrogen bonds form between the water and the polar compound
▪ non-polar molecules do not dissolve in water
Conductivity of Aqueous Solutions
▪ionic compounds will conduct electricity when dissolved in water (because the ions separate)
▪ molecular compounds will not conduct electricity when dissolved in water (because the molecules stay
intact)
Predicting Whether an Ionic Compound is Soluble in Water
▪Ion charge: the greater the charge on each ion, the less soluble the compound will be (ie. oxide compounds
are less soluble than fluoride compounds)
▪ Ion size: as ion size increase, so does solubility (usually) (ie. AgCl is insoluble, but AgNO3 is soluble because
the nitrate ion is large)
Solubility Guidelines
▪ page 363 (again!)
Predicting Whether a Molecular Compound is Soluble in Water
▪ smaller molecular compounds are soluble in water, because the polar part of the molecule is a bigger portion
of the molecule (ie. methanol and ethanol), while larger molecular compounds are insoluble
▪ Table 8.4
Like Dissolves Like
▪ non-polar substances can often dissolve non-polar substances
▪ Table 8.5
Solubility of Gases
▪ polar gases are soluble in water, while non-polar gases generally aren’t
▪ fig 8.9
Molecules That Have Both Polar and Non-Polar Components
▪ sucrose and alcohols have both types of bonds
▪ OH groups are often separated in chemical formulas because they are polar (ie. acetic acid is CH 3COOH and
NOT C2H4O2)
▪ some can dissolve in both polar and non-polar solvents
Temperature and Solubility
▪ solubility changes with temperature
▪ solubility of solid in liquids generally increases with temperature (ie. tea in cold water vs hot water)
▪ solubility curve: fig 8.11
▪ liquid in liquid solubility is generally not affected by temperature
▪ solubility of gases in liquids depends on temperature and pressure
▪ generally the solubility of a gas in a liquid decreases as temperature increases (because the gas will leave the
solution), this explains why pop goes flat faster at room temperature than when cold
Environmental Effects of Increased Temperatures
▪ gases in the atmosphere become less soluble in water as the temperature of the water increases
▪ commonly, heated water is returned to lakes from industrial processes (heat pollution)
▪ reduces the solubility of oxygen in the water, that’s a problem!
Pressure and Solubility
▪ little effect on the solubility of a liquid or solid
▪solubility of a gas is influenced by the pressure of that gas above the liquid (but not any other gases)
▪ example: CO2 pressure in a bottle of pop is much greater than atmosphere, CO2 dissolves in the pop under
pressure, but escapes from the pop when the pressure is released
Pressure, Solubility and Scuba Diving
▪ page 368 (application)
Factors That Affect the Rate of Dissolving
▪ rate = quickly (solubility = amount)
▪ cooks, bakers, lab techs, engineers, etc use a variety of techniques and equipment to increase the rate of
dissolving
▪ agitation/mixing – increases rate of dissolving (increases collision rate between solute and solvent particles)
▪ temperature – increases rate of dissolving (particles are moving faster)
▪ surface area – increasing surface area increases rate of dissolving (grinding)
Classwork:
▪ page 370 #3, 7, 8, 9, 10
▪ Write a procedure that would allow you to test the factors that affect rate of dissolving