Colloids
• Many colloids look similar to solutions because their particles cannot be seen. – Look homogeneous to the naked eye.
• Tyndall effect – Occurs when light is scattered by colloidal particles
dispersed in a transparent medium.
– Colloids frequently appear "murky" or "opaque". – Example: Milk
Colloids•The Tyndall effect can be used to distinguish between a solution and a colloid.
Mixture of waterand gelatin
Mixture of waterAnd NaCl
<--Beam of Light
Solutions
• Sugar and water – Sugar is described as “soluble in water.”
• Soluble – Capable of being dissolved.
• When sugar dissolves, all its molecules become uniformly distributed among the water molecules. – The solid sugar is no longer visible.
Solution
• Homogeneous mixture of two or more substances in a single phase.
• “Particle-by-particle mixture”
Components of Solutions
• Solute– Substance dissolved in a solution
• Solvent– Substance in which a solute dissolves
Saltwater
• What is the solute?
• What is the solvent?
solute + solvent = solution
Review
• What is an example of a solution, colloid and suspension?
• How can one distinguish a solution from a colloid if both appear homogeneous?
Electrolytes vs. Nonelectrolytes
• Electrolyte• A substance that dissolves in water to give a
solution that conducts electric current
• Examples:• NaCl (any soluble ionic compound)
• Dissociation• NaCl(s) Na+
(aq) + Cl-(aq)
• HCl (polar molecular compounds)• Ionization• HCl(g) + H2O(l) H3O+
(aq) + Cl-(aq)
Electrolytes vs. Nonelectrolytes
• Nonelectrolyte• A substance (solute) that dissolves in water
(solvent) to give a solution that does NOT conduct electric current.
• Compounds that do NOT ionize in water• Limited to covalent bonds
• Examples: • Sugar (C6H12O6)• Methane (CH4)
• Benzene (C6H6)
• Ethanol (C2H5OH)
Dissociation vs. Ionization
• Dissociation– Ions of the ionic compound split apart but do not
react with the water.• Ex: NaCl(s) Na+
(aq) + Cl-(aq)
• Ionization– The ionic compound reacts with the water to
form ions.• Ex: HCl(g) + H2O(l) H3O+
(aq) + Cl-(aq)
Strong and Weak Electrolytes
• Electrolytes • Substances that yield ions
• Conduct an electric current in solution.
• The strength with which substances conduct an electric current is related to their ability to form ions in solution.
• Strong and weak electrolytes differ in the degree of ionization or dissociation.
Strong Electrolytes
• Any compound whose dilute aqueous solutions conduct electricity well.
• Due to the presence of all or almost all of the dissolved compound in the form of ions.
• To whatever extent they dissolve in water, they yield only ions.
• Examples: HCl, HBr, HI, NaOH• All soluble ionic compounds
Weak Electrolytes
• Any compound whose dilute aqueous solutions conduct electricity poorly.• Due to the presence of a small amount of the dissolved
compound in the form of ions.
• Some molecular compounds form aqueous solutions that contain not only dissolved ions but also some dissolved molecules that are not ionized.
• Examples: CH3COOH CH3COOH ---------> CH3COO- + H+
before 100% 0 0 after 98.2% 1.8% 1.8%
What are the electrolytes in Gatorade?
• Key electrolytes – Sodium– Potassium – Chloride
Gatorade Website: http://www.gatorade.com/hydration/replenish/
Body Tour: http://www.gssiweb.com/interactive_tools.aspx
Drink Label: http://www.gssiweb.com/Article_Detail.aspx?articleid=572&level=2&topic=1
Why should people drink sports drinks?
• “To rapidly replace the fluids and electrolytes lost through sweat, and provide energy for active muscles." – Sweating is your body’s way of cooling down.
• Dehydration is the penalty we pay.
– Drinking sports drinks during physical activity can reduce dehydration to zero.
• Physical activity also increases our need for the carbohydrate energy that some sports drinks provide.
• Able to exercise longer and harder.
Reference -- Bob Murray, Ph.D., FACSM, Director, Gatorade Sports Science Institute
Solubility
• For every combination of solvent with a solid solute at a given temperature:– Limit to the amount of solid that can be dissolved.
• The point at which this limit is reached for any solute-solvent combination depends on the nature of the:– Solute– Solvent– Temperature
Solubility
• Amount of that substance required to form a saturated solution with a specific amount of solvent at a specified temperature.
– Example: The solubility of sugar is 204 g per 100 g of water at 20°C.
• Solubility's vary widely, and must be determined experimentally.
– Usually given as grams of solute per 100 g of solvent at a given temperature.
Solution Equilibrium
• When maximum solubility is reached:– Molecules are returning to the solid form at the
same rate at which they are going into solution.
• Physical state in which the opposing processes of dissolution and crystallization of a solute occur at the same rates.
Example of Solution Equilibrium
• Saturated solution in a closed system
– Solute is recrystallizing at the same rate that it is dissolving.
• Appears that there is no activity in system but the opposing processes are occurring at the same rate.
Saturated vs. Unsaturated
• Saturated Solution– Contains the maximum amount of dissolved
solute.• If more solute is added to a saturated solution, it falls
to the bottom of the container and does not dissolve.
– Result of equilibrium being established between ions leaving and entering the solid phase.
Soluble Substances in Water
• Any polar molecule
• C12H22O11
– Molecular compound with weak intermolecular forces
• Bonds broken and molecules released in solution
Soluble Substances in Water
• NaCl – Ionic solid salts
• Ions released into solution• Dissociation
• HCl– Reacts with water to form ions
• Ionization
Dissolving Ionic Compounds in Aqueous Solutions
• Polarity of water molecules • Important role in the formation of solutions of ionic
compounds in water.
• Slightly charged parts of water molecules attract the ions in the ionic compounds and surround them.
• Separating them from the crystal surface and drawing them into the solution.
• Hydration• Solution process with water as the solvent.
• The ions are said to be hydrated.
Liquid solutes and solvents
• Oil and water do not mix because oil is nonpolar whereas water is polar.
• Two polar substances, or two nonpolar substances, on the other hand, form solutions together easily because their intermolecular forces match.
• Immiscible• Liquids that are not soluble in each other
• Miscible• Liquids that dissolve freely in one another in any proportion
Effect of pressure on solubility
• Changes in pressure have very little effect on the solubilities of liquids or solids in liquid solvents.
• Increases in pressure increase gas solubilities in liquids.
• Equilibrium is established between a gas above a liquid solvent and the gas dissolved in a liquid.
• As long as this equilibrium is undisturbed, the solubility of the gas in the liquid is unchanged at a given pressure:
gas + solvent solution
Effects of Temperature on Solubility of Gases
• Increasing the temperature usually decreases gas solubility.
• As temperature increases, the average kinetic energy of molecules increases.
• A greater number of solute molecules are therefore able to escape from the attraction of solvent molecules and return to the gas phase.
• At higher temperatures, therefore, equilibrium is reached with fewer gas molecules in solution, and gases are generally less soluble.