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HOW AND WHY DO SUBSTANCES DISSOLVE IN WATER? Learning Targets: *I can explain aqueous solutions and the role of water as the universal solvent. *I can.

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Presentation on theme: "HOW AND WHY DO SUBSTANCES DISSOLVE IN WATER? Learning Targets: *I can explain aqueous solutions and the role of water as the universal solvent. *I can."— Presentation transcript:

1 HOW AND WHY DO SUBSTANCES DISSOLVE IN WATER? Learning Targets: *I can explain aqueous solutions and the role of water as the universal solvent. *I can investigate factors that affect the rate at which water functions as the universal solvent. *I can describe the role of surfactants as solubility agents. *I can explain the saying “like dissolves like” and relate this to biomagnification of water contaminants in the food chain. *I can describe an example of human impact (biomagnification of pesticides) on ecological systems.

2 AQUEOUS SOLUTIONS  To understand water quality, you need to know  WHAT dissolves in water,  WHY it dissolves, and  HOW to specify the concentration of the resulting solution.

3 BASIC VOCABULARY  SOLVENT- A substance (usually a liquid) capable of dissolving one or more pure substances.  SOLUTE- Solid, liquid or gas that is dissolved in a solvent.  SOLUTION- A homogeneous (looks the same throughout) mixture of a solvent and one or more solutes.  AQUEOUS SOLUTION- Solution in which water is the solvent.  SOLUBLE- will dissolve  INSOLUBLE- will NOT dissolve

4 BECAUSE WATER IS SUCH A GOOD SOLVENT, IT...  is practically NEVER 100% pure.  contains impurities.  dissolves tiny amounts of substances when it flows over rocks and minerals.  can dissolve minerals containing ions that are toxic like arsenic and fluoride.  can dissolve tiny amounts of gases in the air like O 2 and CO 2  When it rains, some pollutant gases are VERY soluble and get dissolved in the rain water—SO 2, Sulfur dioxide and NO 2, Nitrogen dioxide and forming acidic solutions—”Acid rain”.

5 HUMANS CONTRIBUTE TO SUBSTANCES DISSOLVED IN WATER SOURCES  When we wash clothes, we add detergent and particles that washed off dirty clothes.  When we flush toilets, we add liquid and solid wastes.  Our urban streets add solutes to rainwater/storm run-off.  Our agricultural practices add fertilizers and other soluble compounds.

6 WHAT DOES WATER BEING A GOOD SOLVENT MEAN FOR OUR DRINKING WATER?  In order to decide whether drinking water is “good” or “bad”—potable or non-potable, you have to know what substances are in it and HOW MUCH.  Amounts of dissolved substances are expressed as CONCENTRATION.  CONCENTRATION - the ratio of the amount of solute to the amount of solution.

7 SOLUTE CONCENTRATIONS IN AQUEOUS SOLUTIONS  Example: Sweet Tea –If one teaspoon of sugar dissolved in 1 cup of tea--or 3 teaspoons of sugar dissolved in 3 cups of tea, the concentration is the same for both --1tsp sugar per cup of tea  4 Ways to Express Concentration:  Percent (%)  Parts per million (ppm)  Parts per billion (ppb)  Molarity (M)

8 PERCENT (%)  Parts per hundred  Medical “Normal saline” solution given intravenously is 0.9% solution by mass. 0.9 grams of sodium chloride (NaCl) in 100 grams of solution. ( 99.1 grams of distilled water )  Rubbing alcohol/Antiseptic isopropyl alcohol is 70% aqueous solution by volume. 70 ml of isopropyl alcohol in every 100 ml of solution.

9 PARTS PER MILLION (ppm)  Used for low concentrations, typical in drinking water  Water that contains 1 ppm of calcium ions contains the equivalent of 1 gram of calcium (in the form of the calcium ion) dissolved in 1 million grams of water.  Drinking water contains naturally occurring substances present in the parts per million range.  US standards set acceptable limits. For example: for nitrate ion, found in some well water in agricultural areas is 10 ppm; the limit for the fluoride ion is 4 ppm.  Measuring 1 million grams of water is not convenient so converted to Liters.  1 ppm of any substance = 1 mg solute dissolved in 1 L water  Mg/L used by water utilities to report the minerals and other substances dissolved in tap water.

10 PARTS PER BILLION (ppb)  Contaminants are dangerous at much lower concentrations than parts per million and are reported in parts per billion (ppb).  ppb represents extremely small amounts—Example 1 ppb is like a few centimeters on the circumference of the Earth!!  Mercury is a dangerous contaminant that is monitored in water. The acceptable limit for mercury in drinking water is 2 ppb.  1 ppb is measured in more convenient terms as 1 microgram of solute in 1 liter of water.  1 microgram = 1 x 10 -6 g or 0.000001g

11 MOLARITY (M)  Molarity (M) – a unit of concentration represented by the number of moles (mol) of solute present in 1 liter of solution.  M = moles of solute liter of solution  Molar mass of NaCl is 58.5 g (Na-23 + Cl-35.5), therefore, 1 mol of NaCl has a mass of 58.5 g.  By dissolving 58.5 g of NaCl in some water and then adding enough water to make exactly 1.00 L of solution, we would have prepared a 1.00 M NaCl aqueous (aq) solution.

12 THE OCEAN—AN AQUEOUS SOLUTION WITH MANY IONS  97% of all water on our planet is found in the oceans.  This source contains much more that simple table salt (NaCl)  When salt crystals dissolve in water, the polar H2O molecules are attracted to the sodium ion (Na+ ) and the Chlorine ion (Cl-). Over time, the ions are separated and then surrounded by water molecules.  NaCl (s)  Na+(aq) + Cl- (aq)  Some ionic compounds dissolve easily others do NOT.

13 SOLUBILITY  The sizes and charges of the ions determine how strongly the ions are attracted to water molecules.  Landmasses on Earth are largely composed of minerals—ionic compounds.  Most have extremely low solubility in water— like limestone-CaCO 3.—Or everything would dissolve and end up in the oceans!!

14 SOLUBILITY RULE: “LIKE DISSOLVES LIKE”  Polar covalent compounds—like sugar-also dissolve in water.  When sucrose (sugar) dissolves in water, the molecules disperse evenly among the H2O molecules. The molecules do not separate into ions.  Solubility happens when an attraction exists between the solvent molecules and the solute molecules.  POLAR molecules are soluble in other POLAR molecules!  Hydrocarbon molecules in oil are NONPOLAR and are insoluble in water.  We cannot use water to wash off grease and oil.

15 SURFACTANTS  Compounds that help polar and nonpolar compounds to mix.  Contain both polar and nonpolar groups.  Polar groups dissolve in the water while the nonpolar ones are able to dissolve in the grease.  Examples: soap, detergent


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