15.1 Water and Its Properties
I. Liquid Water
A. Surface Tension
1. Surface Tension
a. A force that tends to pull adjacent parts of a liquid's surface together, thereby decreasing surface area to the smallest possible size
b. Hydrogen bonding in water creates stronger than normal surface tension
- accounts for DNA bonding, ice expanding, and surface tension
2. Capillary Action
a. The attraction of the surface of a liquid to the surface of a solid
- water wicks its way against gravity up into a paper towel or a piece of chromatography paper
3. Surfactant – a substance that breaks the surface tension of water by interfering with hydrogen bonding
a. soaps and detergents are surfactants (see more in the emulsion section)
B. Vapor Pressure
1. Water has a very low vapor pressure due to the strong hydrogen bonding on the surface (slow evaporation)
II. Water in the Solid State
A. Density
1. Water is one of only a few substances that is less dense as a solid than as a liquid
B. High melting point
1. No other substance with such small molar mass has so high freezing/melting point
III. Summary of Water’s Important Properties
Bond Type / Polar / Density of ice (0 °C) / 0.917 g/cm3Bond angle / 105° / Density of water (0 °C) / 0.999 g/cm3
Normal Boiling point / 100 °C / Point of maximum density / 3.98 °C
Normal Melting Point / 0 °C / Molar heat of fusion / 6.009 kJ/mole
Molar heat of vaporization / 40.79 kJ/mole
15.2 Homogeneous Aqueous Systems
I. Solutions
A. Soluble
1. Means capable of being dissolved
B. Solution
1. A homogeneous mixture of two or more substances in a single phase
2. An aqueous solution is a substance dissolved in water
C. 2 parts To a Solution
1. Solvent = The dissolving medium in a solution
2. Solute = The dissolved substance in a solution
D. Types of solutions
1. Gaseous mixtures 2. Solid solutions 3. Liquid solutions
a. Air is a solution a. Metal alloys a. Liquid dissolved in a liquid
b. Solid dissolved in a liquid
II. Solution Process (Solvation)
A. Water is Polar (positive hydrogen ends and negative oxygen ends)
B. Salts are Ionic (positive cation bonded to a negative anion)
C. Solvation of table salt– the process of the salt molecules become surrounded by the water molecules
1. The positive hydrogens on the water molecule are attracted to the negative chorines on the salt molecule
2. The negative oxygens are attracted to the positive sodium ions
D. Collision Theory = the collisions of the oppositely charged water poles against the salt poles account for the breaking apart of the salt molecules
1. Rates of Solvation
- stirring, heating, grinding (increasing surface area like powdered sugar, cubes, or crystals)
- they do not allow you to dissolve more, but allow you to dissolve faster
E. Some salts do not dissolve in water (Insoluble) (Think chalk = calcium carbonate)
1. Their ionic attractions inside of the salt molecules are stronger than their attraction to the water molecules
F. Oil doesn’t dissolve in water
1. Oil is nonpolar, therefore the water poles can’t pull it apart (Think oil spills in the ocean)
2. Gasoline will not dissolve in water, but it will dissolve in oil (Think solvents like turpentine)
3. “Like dissolves Like”
III Solutes: Electrolytes vs. Nonelectrolytes
A. Electrolyte
1. A substance that dissolves/disassociates in water to give a solution that conducts electric current
2. Solutions of acids, bases and salts are electrolytes
B. Nonelectrolyte
1. A substance that dissolves in water to give a solution that does not conduct an electric current]
- Sugar dissolves due to the charge on its hydroxyl groups, but doesn’t dissociate like salt
I.V. Hydrates
A. Hydrate = a compound that contains water of hydration (not bonded)
B. Anhydrous = lacking any moisture
C. Efflorescent = gives moisture off into the atmosphere
D. Hygroscopic = removes moisture from the air to form a hydrate
1. Dessicant = a hygroscopic substance used as a drying agent
- rice and your cell phone when its wet, silica packets in electronics equipment
2. Deliquescent = substance capable of removing sufficient water from the air to dissolve themselves into solution
15.3 Heterogeneous Aqueous Systems
I. Suspensions = A mixture from which particles settle out upon standing (do not dissolve into the solvent)
A. relatively large particles compared to solutions and colloids (dirt in water)
II. Colloids (very similar to suspension, just smaller particles that can’t be seen by light)
A. Colloidal Dispersions (Colloids)
1. Tiny particles suspended in some medium
2. Particles range in size from 1 to 1000 nm.
B. Tyndall Effect (method to determine if substance is colloid, suspension or solution)
1. Scattering of light by particles
a. Light passes through a solution b. Light is scattered in a colloid
c. Light shows individual particles in a suspension
- laser in clean dark air, then through smoke, then through smoke and ask
Types of Colloids
Examples Dispersing Medium Dispersed Substance Colloid Type
Fog, aerosol sprays Gas Liquid Aerosol
Smoke, airborne bacteria Gas Solid Aerosol
Whipped cream, soap suds Liquid Gas Foam
Milk, mayonnaise Liquid Liquid Emulsion
Paint, clays, gelatin Liquid Solid Sol
Marshmallow, polystyrene foam Solid Gas Solid foam
Butter, cheese Solid Liquid Solid emulsion
Ruby glass Solid Solid Solid sol
A. Emulsion = a colloidal dispersion of a liquid in a liquid
a. Oil in water with a surfactant like soap
b. Soaps are “emulsifying agents”
- They have two “ends” to their molecules
- Hydrophobic = non polar end (no charge) water fleeing or fearing
- Hydrophillic = polar (slight charge) water attracting or “loving”
- They form a sphere (called a micelle) that surrounds the nonpolar “dirt or grease” with their
hydrophobic end and water then surrounds the other hydrophillic end.
- This allows the water molecules to surround grease and dirt molecules to sluice the dirt away
16.1 Properties of Solutions
I. Factors Affecting the Rate of Dissolution
A. Actions that make a solute dissolve faster = increasing the necessary collisions
1. Increasing the Surface Area of the Solute (Particle size or grinding it up like powdered sugar)
a. Finely divided substances dissolve more rapidly
2. Agitating a Solution
a. Stirring or shaking brings solvent into contact with more solute particles
b. Added energy temporarily increases solubility
3. Heating
1. Heating always increasing the rate of dissolution of solids in liquids
B. These Factors affect the RATE of dissolving but NOT the AMOUNT of dissolving
II. Solubility
A. Solution Equilibrium
1. The state that opposing processes of dissolution and crystallization of a solute occur at equal rates
B. Saturation Levels
1. Saturated solution
a. A solution that contains the maximum amount of dissolved solute at the given conditions
2. Unsaturated solutions
a. A solution that contains less solute than a saturated solution under the existing conditions
3. Supersaturated Solutions
a. A solution that contains more dissolved solute than a saturated solution contains under the
same conditions
C. Solubility Values (Measurements)
1. The solubility of a substance is the amount of that substance required to form a saturated solution
with a specific amount of solvent at a specified temperature
2. The rate at which a substance dissolves does not alter the substances solubility
III. Factors Affecting Solubility
A. "Like dissolves like"
1. Polar substances dissolve in polar solvents Nonpolar substances dissolve in nonpolar solvents
B. Dissolving Ionic Compounds in Aqueous Solutions
1. Electropositive hydrogen of the water molecule is attracted to negatively charged ions
2. Electronegative oxygen of the water molecule is attracted to positively charged ions
3. Hydration
a. The solution process with water as the solvent
4. Hydrates
a. Ionic substances that incorporate water molecules into their structure during recrystallization
CuSO4•5H2O
b. the ” •" means that the water is loosely attached
C. Nonpolar Solvents
1. Polar and ionic compounds are not soluble in nonpolar solvents
2. Fats, oils and many petroleum products are soluble in nonpolar solvents
3. Nonpolar solvents include CCl4 and toluene (methyl benzene), C6H5CH3
D. Liquid Solutes and Solvents
1. Immiscible - Liquid solutes and solvents that are not soluble in each other
a. Oil and water
2. Miscible - Liquids that dissolve freely in one another in any proportion
a. Benzene and carbon tetrachloride (both nonpolar)
b. Water and ethanol (both polar)
E. Effects of Pressure on Solubility
1. Pressure has no real effect on the solubilities of liquids and solids in liquid solvents
2. Increasing pressure increases the solubility of gases in liquids
F. Effects of Temperature on Solubility
1. Solubility of solids (generally) increases with temperature
2. Solubility of gases decreases with temperature (GRAPH on page 474)
16.2 Concentration of Solutions
I. Concentration - A measurement of the amount of solute in a given amount of solvent or solution
(how many packets of sugar do you put in your glass of iced tea)
A. Methods of Concentration Measurement
1. Grams per liter = represent the mass of solute divided by the volume of solution, in liters. This measure of concentration is most often used when discussing the solubility of a solid in solution.
2. Molarity = describes the concentration of a solution in moles of solute divided by liters of solution.
a. this is the most widely used unit for concentration when preparing solutions in chemistry
b. the units of molarity are mol/L ( moles of solute divided by liters of solvent) and are represented by “M”
3. Parts per Million (ppm) = a ratio of parts of solute to one million parts of solution, and solutions.
a. It is often found in reports of concentration of water contaminants.
b. To calculate parts per million, divide the mass of the solute by the total mass of the solution. This number is then multiplied by 106 and expressed as parts per million (ppm).
4. Percent composition = the ratio of one part of solute to one hundred parts of solution, expressed as a percent.
a. Determine the mass of solute and solution and then divide the mass of the solute by the total mass
of the solution. This number is multiplied by 100 and given as a percent
16.3 Colligative Properties of Solutions
I. Colligative Properties: Properties that depend on the concentration of solute particles but not on their identity
A. Vapor Pressure Reduction
1. Volatility
a. Nonvolatile substances
- Substances that have little or no tendency to become a gas under existing conditions
b. Volatile substances
- Substances with a definite tendency to become gases under existing conditions
2. Effect of Solutes on Vapor-Pressure
a. Any nonvolatile solute will lower the vapor pressure of a solution, having two noticeable effects
- Raising the boiling point of the solution
- Lowering the freezing point of the solution
B. Freezing-Point Depression
1. The difference between the freezing points of pure solvent and a solution of a nonelectrolyte in that solvent
a. Molal Freezing-Point Constant for Water
- The Constant of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute
i) Kf = -1.86 °C/m
C. Boiling-Point Elevation
1. The difference between boiling points of the pure solvent and a solution of a nonelectrolyte in that solvent
a. Molal Boiling-Point Constant for Water
- The constant of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute
i) Kb = 0.51 °C/m
C. Osmotic Pressure
1. the pressure exerted across a semipermeable membrane between two solutions of different concentrations
a. Nature flows “high to low”
- high pressure cold fronts pushing into low pressure warm fronts accounts for wind
-high temperature soft drinks sending their heat into low temperature ice cubes “cools” off your drink
2. Two solutions of different concentrations on either side of a semipermeable membrane (i.e. your skin)
a. one solution has “high” concentration and wants to flow to the “low” concentration of the other
b. because the membrane is semi-permeable, some things can go through, others cannot
c. typically this means small water molecules can move though, but larger salt ions cannot
d. in order to balance out the concentrations, the solutions can’t exchange salt, so they exchange water
e. water flows across the membrane to lower the concentration of one, and raise the other until equal
f. This is why your “skin wrinkles” in water
g. Diagram