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Water  Water is a polar molecule composed of two polar covalent O-H bonds in a bent or angular molecular geometry with two pairs of nonbonding electrons.

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Presentation on theme: "Water  Water is a polar molecule composed of two polar covalent O-H bonds in a bent or angular molecular geometry with two pairs of nonbonding electrons."— Presentation transcript:

1 Water  Water is a polar molecule composed of two polar covalent O-H bonds in a bent or angular molecular geometry with two pairs of nonbonding electrons.  water has 4 pairs of electrons arranged tetrahedral around the central oxygen atom  75% of the earth’s surface is covered with water;  about 97% of the total water available on Earth is salt water, about 2% is frozen as the polar ice caps, and the rest (1%) is fresh water. The Hydrologic cycle  The amount of water on this planet is fairly constant and cleans and replenishes itself via “The Hydrologic cycle”.  Water vapor in the atmosphere (clouds & such) returns to the earth via precipitation (rain, snow, etc.) where it flows into land pockets (oceans, lakes, or rivers), or it is absorbed into the ground, or it evaporates back up into the atmosphere (completing the cycle).

2 Water  Rainwater collects dust particles and gases as it travels from the atmosphere to the ground.  Gases like O 2, N 2, and CO 2 all dissolve to some degree in rainwater. An equilibrium is established between dissolved CO 2 in water with carbonic acid making rainwater (about pH 5) more acidic than pure water (pH 7). CO 2 + H 2 O  H 2 CO 3  As water flows beneath or atop the surface of the planet, it readily dissolves many substances from the soil and rocks.  Some common dissolved substances are Na +, K +, Ca 2+, Mg 2+, Fe 2+, Cl -, SO 4 2-, and HCO 3 -. Ca 2+, Mg 2+, and Fe 2+ salts are responsible for “Hard water” (these positive ions react with the negative ions in soap to form insoluble scum). Soft water contains soluble ions like sodium and potassium.

3 WATER QUALITY A relative measure of the water’s distinctive characteristics in reference to a particular use. Water quality ranges from: - drinking- washing- cooling - food processing- manufacturing- recreation - irrigation CONTAMINATION Pollutants degrade the water quality, preventing the use of water for some specific use. Not all constituents of water are contaminates. Natural water contains: - micro organisms: bacteria, viruses, fungus - gases: CH 4 H 2 S N 2 O 2 CO 2 - organic/Inorganic materials: salts, vegetation

4 Water Seawater contains about 35,000 ppm of dissolved salts thus making seawater unfit for agricultural and domestic use. Safe drinking water generally contains less than 1000 ppm of dissolved solids (salts).

5 ASPECTS A CHEMIST IS CONCERNED ABOUT: 1. Hardness - relates to the amount of Ca, Mg, and other salts dissolved in water. 2. Salinity - relates to the concentration of salts in water. Fresh water should contain 1000 ppm or less of dissolved solids. 3. pH - relates to the measure of acidity or alkalinity of water. pH 6 to 9 preferred 4. Turbidity - relates to the clarity of water, the amount of suspended matter in water. 5. Color, odor, taste- Organic matter and minerals may produce a distinct color. Minerals, bacteria and dissolved gases are responsible for taste and odor.

6 Water Water that contains dissolved calcium and magnesium salts is called hard water. Ordinary soap will not lather in hard water. Soap reacts with the calcium and magnesium ions to form an insoluble greasy scum. Synthetic soaps (i.e. detergents), have excellent cleaning qualities and will not form precipitates with hard water.

7 Water Hard water also causes “scale” to form on walls of water heaters, teakettles, coffee pots, etc. which reduces their efficiency.

8 Water Four techniques to soften hard water: 1. DISTILLATION: The water is boiled, and the steam formed is condensed into a liquid again, leaving the minerals behind in the distilling vessel. Commercial stills are able to produce hundreds of liters of distilled water per hour. 2. CALCIUM AND MAGNESIUM PRECIPITATION: Calcium and magnesium ions are precipitated from hard water by adding sodium carbonate and lime. Insoluble calcium carbonate and magnesium hydroxide are precipitated and are removed by filtration or sedimentation. 3. ION EXCHANGE: Hard water is softened as it is passed through a bed or tank of zeolite-a complex sodium aluminum silicate. Sodium ions replace objectionable calcium and magnesium ions, and the water is softened: Na 2 (zeolite)(s) + Ca 2+ (aq)  Ca(zeolite)(s) + 2Na + (aq) The zeolite is regenerated by back-flushing with concentrated sodium chloride solution, reversing the foregoing reaction. 4. DEMINERALIZATION: Both cations and anions are removed by a two-stage ion- exchange system. Special synthetic organic resins are used in the ion-exchange beds. In the first stage metal cations are replaced by hydrogen ions. Second stage anions are replaced by hydroxide ions. The hydrogen and hydroxide ions react and you get pure, mineral-free water.

9 POLLUTANTS The change in water quality which makes water unsuitable for a particular use. NATURAL POLLUTANTS - high salinity- high acidity - high Arsenic concentrations- swamps HUMAN CONTRIBUTIONS - septic tanks & cesspools- oil fields - land fills- agriculture - industrial waste water- acid mine drain -underground storage- leaky pipes - highway de-icing- accidental spills - atmospheric pollutants - inadequate packaging of radioactive wastes

10 POLLUTANTS  There are different types of “Polluted” water:  Pathogenic (disease-causing) microorganisms like cholera, typhoid, hepatitis, and dysentery still effect over 70% of the world’s population.  Aerobic biodegradation (aerobic oxidation) happens when microorganisms break down organic material in the present of dissolved oxygen to produce, for example, CO 2, PO 4 3-, NO 3 - SO 4 2-, and HCO 3 -. The measure of oxygen needed to degrade organic material is referred to as the BOD (biochemical oxygen demand).  Anaerobic decay happens when the oxygen is depleted. The microorganisms reduce organic material (instead of oxidizing it) to produce nasty smelling substances like CH 4, NH 3, H 2 S, and amines. No life (excepts anaerobic microorganisms) can exist in such water.  Certain bacteria in water breaks down organic matter and, in the process, depletes the dissolved oxygen (which marine life is dependent on) while enriching the amount of plant nutrients (PO 4 3-, NO 3 - ) present. These nutrients promote algae growth. If the concentration of plant nutrients (from natural and human contributions) is left unchecked, it can lead to an excess of algae which, as it dies, increases the BOD eventually leading to anaerobic biodegradation. This process called eutrophication.  Industrial waste like VOC’s (volatile organic compounds like trichloroethylene), heavy metal ions/compounds (like Hg, Pb, & Cd), and a number of organic and inorganic materials from LUST (leaking underground storage tanks).  Acid rain produced from dissolved SOx and NOx compounds from air pollution and acid mine drainage from mining operations.

11 Water Treatment Natural fresh water is not pure, it may contain dissolved minerals, suspended matter, and/or harmful bacteria. Generally this water is unsafe to drink unless treated by some or all of the following processes: 1. Screening : removal of large objects (i.e. trash, fish, etc.) 2. Flocculation and sedimentation : Chemicals, usually lime, CaO and alum, Al 2 (SO 4 ) 3, are added to form a flocculent precipitate of aluminum hydroxide which traps most of the fine suspended matter and carries it to the bottom of the sedimentation basin. 3. Sand Filtration : Water is taken from the top of the sedimentation basin and passed through fine sand filters. Removing nearly all remaining suspended matter and bacteria. 4. Aeration : Water is drawn from the bottom of the sand filters and is aerated by spraying. This process is for removing objectionable odors and tastes. 5. Disinfection : In the final stage chlorine gas is injected into the water to kill harmful bacteria. Ozone is also used in some countries to disinfect water. In emergency situations, water can be disinfected by boiling it for a few minutes.

12 PURIFICATION OF DRINKING WATER 1. CHLORINATION: Cl 2 HOCl NaOCl  antibacterial Ca(OCl) 2 agent Negative aspects:- taste & odor - THM (tri halomethane’s) at toxic levels at toxic levels 2. OZONE:- more effective - more expensive Negative aspects:decomposes quickly 3. UV:- no by-products - economical Negative aspects: decomposes quickly

13 SUMMARY of CLASSIFICATION OF WATER POLLUTANTS Type of pollutant:Examples: Oxygen-demanding Decomposable organic wastes from domestic sewage and wastesindustrial wastes of plant and animal origin Infectious agentsBacteria, viruses, and other organisms from domestic sewage, animal wastes, and animal process wastes Plant nutrientsPrincipally compounds of nitrogen and phosphorus Organic chemicals Large numbers of chemicals synthesized by industry, pesticides, chlorinated organic compounds Other minerals andInorganic chemicals from industrial operations, chemicalsmining, oil field operations and agriculture Radioactive substancesWaste products from mining and processing radioactive materials, airborne radioactive fallout, increased use of radioactive materials in hospitals and research Heat from industryLarge quantities of heated water returned to water bodies from power plants and manufacturing facilities after use for cooling Sediment from land erosionSolid matter washed into streams and oceans by erosion, rain, and water runoff

14 EUTROPHICATION Fertilizers and other plant nutrients can be considered as a contaminate. Contains nitrates, Phosphates and potassium (salts) Pesticides are usually organic materials H  DDT Cl - O - C - O - Cl  di chloro di phenyl trichloro ethane Cl - C - Cl  Cl DDT is one example of chlorinated hydrocarbons which are persistent, reactive compounds mainly responsible for water pollution. - A body of water becomes enriched in nutrients - resulting in an excessive growth of algae “algae bloom” onto the surface of the water - in time, the algae and other vegetation dies - the organic materials decompose by aerobic bacterial action organic waste products are decomposed by bacteria which uses dissolved O 2 in the water in a combustion reaction: HC + O 2  CO 2 + H 2 O The bacteria obtains energy by “burning” organic molecules. - The greater the amount of organic wastes, the more O 2 needed for decomposition, thus depleting O 2 concentration in water. BOD - biochemical oxygen demand The amount of O 2 required during the breakdown of waste by bacteria..

15 - Once all of the dissolved O 2 is depleted by aerobic bacterial action: * fish and plants suffocate, producing more organic waste * aerobic bacteria dies - Now anaerobic bacteria grows, this bacteria does not require oxygen to decompose organic matter. This bacteria utilizes oxygen contained in matter. - This action results in H 2 S NH 3 CH 4 rotten egg smell rotten fish smell marsh gas - The body of water becomes a swamp (N x, P x O y and K x ) excess + algae  more algae (HC) HC + M.O. * + O 2  CO 2 + H 2 O M.O. * = aerobic micro organism HC + A.M.O.  H 2 S, or NH 3 or CH 4 A.M.O. = anaerobic micro organisms Pesticides and chlorinated hydrocarbons increase the amount of H.C. in water which is detrimental to M.O. * but increases the A.M.O. activities.

16 INDUSTRIAL WASTES Industry releases tons of heavy metals and other Inorganic compounds into rivers, lakes and oceans. Ba, As, Hg, Cd, Pb, Cu, CN -, etc Pb (Lead) galena PbS natural oxidation states Pb 2+, Pb 4+ uses: 1. Lead acid storage battery Pb(s) + PbO 2 (s) + 2H 2 SO 4  2 PbSO 4 (s) + 2H Antiknock agent 3. Soldering, glass and alloys 4. Radioactive storage and shielding 5. Paint pigment yellow PbCrO 4 white 2PbCO 3 Pb(om) 2 6. Ammunition white lead paint responsible for mental retardation and hyperactivity, irrational behavior, sleeplessness, starvation. Pb 2+ has similar ionic radius Ca 2+ it competes in the human body. Unlike Ca 2+, Pb 2+ accumulates in the bones, this build up of Pb 2+ causes the toxic problems. 1/2 life of Pb 2+ : 6 years in muscle and tissue, years in bones. Organic lead compounds are more toxic than the salts non-polar, lipid soluble

17 Hg MERCURY Quick silver - only metal which is a liquid at room temperature common oxidation states Hg 2+ & Hg 2 2+ Inorganic mercury Organic mercury - salts (toxic) - CH 3 HgCl methyl mercuric chloride - metal ores - (CH 3 ) 2 Hg dimethyl mercury (highly toxic) Organic mercury compounds damage all tissues with the methyl types especially damaging to the tissues of the central nervous systems in the brain. Neurotoxin: leads to quarrelsome behavior, headaches, depression, muscle tremors, numbness, fits and blindness B 12  L 5 Co-CH3 Hg + microbe  CH 3 HgCL L 5 Co-CH 3 + Hg 2+  L 5 Co + + CH 3 Hg + CH 3 Hg + + Cl - (or SCH 3 )  CH 3 HgCl or CH 3 HgSCH 3 PH7 or below 2CH 3 Hg +  (CH 3 ) 2 Hg PH7 or greater Treatment: 2, 3-dimercapto ethanol (BAI)

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