Chapter 5 - The Water We Drink

Biomagnification:

The increase in concentration of certain persistent chemicals in successively higher levels of a food chain (pg. 229)

Water footprint

estimates of the volume of freshwater used to produce particular goods or to provide services (pg. 210)

How much freshwater is easiest for us to access?

lakes, rivers, and wetlands account for a mere .3% of the freshwater (pg. 209)

% concentration vs. ppm concentration

▪ % concentration is the ratio of parts (solute) per hundred (solution). [example] 7% salt solution means 7 g of salts in 100 g salt solution i.e., 7% salt solution = 7 g salt / 100 g solution ▪ ppm concentration is the ratio of parts (solute) per million (solution). [example] 7 ppm salt solution means 7 g of salts in 1000000 g salt solution. i.e., 7 ppm salt solution = 7 g / 1000000 g = 7000 mg / 1000 L (because 1g=1000mg & 1000g=1L) = 7 mg/L (because we divided by 1000)

solution

▪ A solution is a homogeneous mixture of uniform composition. ▪ Solutions are made up of solvents and solutes. ▪ solvents - Substances capable of dissolving other substances- usually present in the greater amount. ▪ solutes - Substances dissolved in a solvent- usually present in the lesser amount. ▪ When water is the solvent, you have an aqueous solution.

II. The role of hydrogen bonding

▪ Although not as strong as covalent bond, hydrogen bonds are quite strong compared with other types of intermolecular forces. ▪ In general, boiling points increase with molecular mass. Despite of its small mass, however, water has very high boiling point, because water molecules are tightly held together by hydrogen bonds. To boil water these bonds must be broken. Addition of more heat can only break them, resulting in high boiling point for water. ▪ Hydrogen bond is also important in stabilizing the shape of large biological molecules, such as proteins and nucleic acids (DNA and RNA). ▪ Hydrogen bond also explains why ice floats in water. Ice is a regular array of water molecule in which every H2O molecule is hydrogen-bonded to four others. (Dotted lines in red in the figure below illustrate hydrogen bonds.) ▪ The figure below shows the hydrogen-bonded lattice structure of ice, clearly illustrating the open channels. When ice melt, the regular array breaks down, individual H2O molecules enter the open channels, resulting in more compact distribution of the molecules than in ice. Thus, for the same volume liquid water has more mass than ice, making liquid water heavier than ice. So, ice float over water.

chlorination

▪ Disinfecting is to kill disease causing organisms and it is the most crucial step to make drinking water safe. It is most commonly done by chlorination. Hypochlorous acid (HClO), generated in chlorination, acts as the antibacterial agent. In some parts of the country, sodium fluoride (NaF) is added to the treated water to help protect against tooth decay. ▪ Drawbacks of chlorination: the taste and odor of residual chlorine may be objectionable to some people; the reaction of residual chlorine with other substances in the water may produce trihalomethanes (THMs) at potentially toxic levels. The most widely publicized THM is chloroform, CHCl3.

V. Protecting drinking water

▪ EPA sets legal limits for such contaminants (refer to Table 5.10) ▪ MCLG= maximum contaminant level goal. MCLG is the maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur. MCLG is not a legal limit; it is a goal established by the EPA. ▪ For known carcinogens, the EPA has set the MCLG at zero, under the assumption that any exposure to the substance could present a cancer risk. ▪ MCL= maximum contaminant level. MCL sets the legal limit for the concentration of a contaminant. The EPA sets legal limits for each impurity as close to the MCLG as possible. - The MCLG for any contaminant that is regulated as a carcinogen is set to zero - The MCL set by the EPA me be higher than the MCLG to account for the financial and technical barriers that may make achieving these goals difficult

Electrolytes and non-electrolytes

▪ Non-electrolyte is a solute that does not conduct electricity in aqueous solutions ▪ Electrolyte is a solute that conducts electricity in aqueous solutions - Sodium Chloride is an electrolyte

Other examples

▪ Other substances not listed in the table above (Table 5.10) in drinking water can pose health risks. For example, nitrate (NO3-) and nitrite (NO2-) limit the blood's activity to carry oxygen. ▪ Other examples are bacteria, viruses, and other microorganisms. The best known example of bacteria is coliforms which can cause symptoms such as diarrhea, cramps, nausea, and vomiting. ▪ Clean Water Act (CWA) establishes limits on the amounts of pollutants that industries can discharge into surface water, resulting in actions that have removed over a billion pounds of toxic pollution from US waters every year.

ozone

▪ Ozone is more effective than chlorine against water-borne viruses. But ozone is more expensive than chlorination. Major drawback of ozone is that it decomposes quickly and hence does not protect the water from contamination after the water leaves the treatment plant. ▪ UV disinfection is very fast and leaves no residual by-products, and is economical for small installations such as rural homes with unsafe well water). Major drawback of UV disinfection is that, like ozone, it does not protect the water from contamination after the water leaves the treatment site unless a low dose of chlorine is added.

Concentration Exercises

▪ Parter per hundred - suppose you have 10% of NaCl solution... how many grams of NaCl solute must be dissolved in 100 g of solution? - that question mark must be replaced by 10 ... so just replace whatever unknown solute amount by the percent number hiven - so 10% NaCl solution means that we really have 10 g of NaCl solute per 100 g of solution ▪ if you want to find out the net amount of water dissolving the amount of solute, you can find out the net amount of water - 100 g = 10 g + X g - the amount of solvent water is 90g - so if you dissolve 10 g of NaCl in 90 g of water, that becomes 10 % NaCl solution ▪ Now let's move onto ppm concentration ▪ suppose you have 1.3 ppm mercury concentration in the Scioto river ... how many grams of mercury are dissolved in one liter of water? - 1.3 milligrams... you replace that question mark by 1.3 ▪ Parts per billion concentration right beneath .. suppose we have 1.3 ppb concentration mercury ... how much is in one liter of water? - one microgram ! ▪ Let's take a look at the relationship between ppm and ppb ▪ 1 ppm = 1 mg/ 1 L - remember that 1 mg = 1000 micrograms - so 1000 microgram is obviously equal to 1 x 1000 microgram - 1 microgram per liter of water is 1 ppb ▪ As a result 1 ppm = 1000 ppb - so what does this mean? - this means that 1 ppm is 1000x more concentrated than 1 ppb - in other words 1 ppb is 1/1000 of 1 ppm , so ppb is a lot less concentrated than 1 ppm

hydrogen bonds vs. covalent bonds

▪ Polarized bonds allow hydrogen bonding to occur. ▪ A hydrogen bond is an electrostatic attraction between an atom bearing a partial positive charge in one molecule and an atom bearing a partial negative charge in a neighboring molecule. The H atom must be bonded to an O, N, or F atom. ▪ Hydrogen bonds typically are only about one-fifteenth as strong as the covalent bonds that connect atoms together within molecules. - Hydrogen bonds are intermolecular bonds. Covalent bonds are intramolecular bonds.

VI. Treatment of water to drink

▪ Screening → Flocculating → Settling → Filtering → → Disinfecting ▪ Screening excludes larger objects both natural and artificial (sticks, cans, etc). ▪ Flocculating collects suspended clay and dirt particles on the surface of the flocculating agents. The most common agent is a sticky floc, or gel, of aluminum hydroxide, Al(OH)3, which is formed when two chemicals, Al2(SO4)3 and Ca(OH)2, are added to the water to be treated. ▪ In the settling step, the Al(OH)3 gel settles, slowly carrying with it the suspended particles down into a settling tank. ▪ In the filtering step, any remaining particles are removed as the water is filtered through coal or gravel and then sand.

VIII. Drinking water on the globe

▪ Seawater is readily available resources but its high salt concentration makes it unfit for human consumption. ▪ Desalination is a general term describing any process that removes ions from salty water. There are several desalination methods: distillation and reverse osmosis. ▪ Osmosis is the natural tendency for a solvent to move through a membrane from a region of higher solvent concentration to a region of lower solvent concentration. ▪ This natural process of osmosis can be reversed. Reverse osmosis is using pressure to force the movement of a solvent through a semi-permeable membrane from a region of high solute concentration to a region of lower solute concentration. When using this process to purify water, pressure is applied to the saltwater side, forcing water through the membrane, leaving ions behind. ***********************************

polarity in water molecules

▪ Since a molecule of water is polar thus having partial negative and positive charges, molecules of water attract among themselves, resulting in strong intermolecular force. This strong intermolecular force is called hydrogen bond. ▪ The strong force is developed through the attraction between the positively charged hydrogen atoms in one water molecule and the negatively charged oxygen atom in the other neighboring water molecule. Since the hydrogen atoms are involved in this strong intermolecular attraction, these attractive forces are called hydrogen bonds (see the figure below).

Why is lead most commonly used?

▪ Since lead is most commonly used, Pb2+ is the most serious health risk. ▪ Why lead is commonly used? Because it is 50% more dense than iron or steel, abundant, soft and thus easily workable, and does not rust. Though now banned, lead had been used for water pipes until 1930. ▪ Solder used to join copper pipes contains 50-70% lead. The solder in contact with water releases Pb2+ to water. ▪ MCL for Pb2+ in drinking water is 15 ppb and MCLG is 0. (Table 5.10 above) ▪ The good news is that very little lead is present in most public water supplies. Most lead in drinking water comes from corrosion of plumbing systems, not from the source water itself.

I. Essential background knowledge

▪ Solvent is a substance capable of dissolving other substances. ▪ Solute is a substance that dissolves in a solvent. ▪ Solution is the resulting mixture of a solvent with a solute or solutes. ▪ In a solution the solvent takes the major component and the solutes the minor components.

specific heat

▪ Specific Heat: the heat required to raise the temperature of the unit mass of a given substance by a given amount (usually one degree). ▪ hydrogen bonds are NOT as strong as covalent bonds (pg. 206) - when drawing these structures, the hydrogen bonds are dashed, while the covalent bonds are solid lines - see the example also on pg. 206

IV. Source of drinking water

▪ The major sources of drinking water are ground water and surface water. ▪ Ground water, usually free of harmful contamination, is pumped from wells or aquifers. Aquifers are great pool of water trapped in sand and gravel 50-500 ft below the surface. The most famous and biggest aquifer is the Ogallala Aquifer which underlies parts of eight sates from South Dakota to Texas (map below). ▪ Surface water, water from lakes, rivers, and reservoirs, is treated before drinking.

III. Distribution of water on earth

▪ The primitive earth did not have much water as we see in today's earth. The water now in today's earth came from numerous volcanoes over the ages that spewed water vapors which were in turn condensed as rain. ▪ The vast majority of water in Earth (97%) is in the ocean as salt water. The remaining 3% is all the fresh water we have. ▪ Most of the 3% freshwater is locked in glaciers and polar ice caps. Only about 0.01% is conveniently located in lakes, rivers, and streams as fresh water. Consequently, the world's drinking water supplies are quite limited. ▪ In the US 80% of the 0.01% fresh water available is used to irrigate crops and to cool electrical power plants.

To express how much solute is present in a solution:

▪ To express how much a certain solute is present in a solution, we use the relation called concentration. ▪ The concentration is defined as the ratio of amount of solute to amount of solution. ▪ We introduce three kinds of concentration: percent, ppm and ppb ▪ Though different kinds, they all follow the same definition as the ratio of solute to solution. ********************************* - how much a certain solute is present in a solution, we use the relation called concentration

Concentrations

▪ To express how much a certain solute is present in a solution, we use the relation called concentration. ▪ The concentration is defined as the ratio of amount of solute to amount of solution. ▪ We introduce three kinds of concentration: percent, ppm, and ppb. ▪ Though different kinds, they all follow the same definition as the ratio of solute to solution.

VII. Why lead poisoning is still a major concern?

▪ Unlike many other toxic substances, lead is a cumulative poison and is not transformed into a nontoxic substance. Once it enters the body, it accumulates in bones and the brain. Pb2+ can be incorporated rapidly into bone along with Ca2+. ▪ When ingested, lead causes severe and permanent neurological problems. This is particularly tragic for children. In children, who have less bone mass than adults, the Pb2+ remains in the blood longer, where it can damage cells, especially in the brain. They may suffer mental retardation and hyperactivity as a result of lead exposure, even at relatively low concentrations. ▪ Lead and most of the metals close to it on the periodic table like mercury and cadmium are toxic (Pb=lead; Hg=mercury; Cd=cadmium). Their cations, Pb2+, Hg2+ and Cd2+, in water are toxic and deadly.

Biological Oxygen Demand (BOD):

▪ a measure of the amount of dissolved oxygen that microorganisms use up as they decompose organic wastes found in water (pg. 236) - Untreated sewage water would have the highest BOD?

Concentration types

▪ here we have different types of concentrations ▪ Parts per hundred (percent) concentration, ppm concentration, and ppb concentration ▪ even though we have these different types of concentrations, they all follow the same definition, which is the ratio of the amount of solute to the amount of solution ... always in that ratio - amount of solute top, amount of solvent bottom ▪ 20% NaCl solution means there are 20 g of NaCl solute per every 100 g solution (water) ▪ from this ratio, we can also find out the net amount of solvent water - how much water actually dissolved this 20 g of solute ▪ look at the right in pink ... since solution is equal to solute + solvent... to find the amount of solvent you would fo 100 = 20 + x ... so the amount of solvent would be 80 g ▪ ppm ... suppose we have 1 ppm concentration ▪ that would mean that we have 1 g of solute for every 1,000,000 g of solution - we can make that modification ▪ 1g = 1000 mg solute ▪ on the bottom you have 1000 x 1000 ... the 1000s cancel out .... so that would be 1 mg per 1,000 g of solution ... 1000 g = 1 Liter .... so you have 1 mg solute to 1 Liter of water - so 1ppm is equal to 1 mg solute/1 liter of water - we change solution to water, this is because the amount of solute is so low that the solution amount is almost equivalent to the solvent amount ... so we can change 1 L solution to 1 L water .... in that solution it's because the amount of solute is so small ▪ ppm = mg solute/1 L water ▪ using that relation we can say that 2.5 ppm Lead concentration , means that we have 2.5 mg of Lead that dissolves in 1 L of water ▪ now lets get down to ppb, one ppb becomes 1 microgram solute per liter ▪ remember 1 ppb = 1 microgram solute per 1 liter of water - this is even less concentrated because 1 microgram is smaller than one milligram - 1 ppb concentration is much lower than 1 ppm concentration by thousands , 1/1000 ▪ using that relation above, we can understand that 1.7 ppb concentration of mercury means that we have 1.7 micrograms of mercury in 1 Liter of water - this is a very small concentration, but even that can be toxic

What are some consequences of hydrogen bonding between water molecules?

▪ ice floats on water ▪ water has a high boiling point ▪ water has a high specific heat

Solution

▪ solution is homogeneous mixture of uniform composition ▪ it consists of two components: solutes and solvents ▪ Solutes: the substances dissolved in a solvent - usually present in the lesser amount ▪ Solvent: the substance capable of dissolving other substances - usually present in the greater amount ▪ Example: - Salt solution: there are obviously two components, salt and water ... salt becomes the solute and water is the solvent - salt is present in the lesser amount ... water is available in the majority ▪ When water is the solvent, we call this an Aqueous solution ... coming from the word aqua