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.

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.

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

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

O2 and CO2

When it rains, some pollutant gases are VERY soluble and get dissolved in the rain water—SO2, Sulfur dioxide and NO2, Nitrogen dioxide and forming acidic solutions—”Acid rain”.

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.

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.

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)

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.

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.

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

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.

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.

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-CaCO3.—Or everything would dissolve and end up in the oceans!!

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.

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

NONPOLAR SOLVENTS