Water(H2O)

Objective

• The importance of the water cycle.• Method of water purification.• Important of dissolved oxygen in water to

aquatic life• Water pollution• Impact of water pollutants on the aquatic

environment.

The water cycle

• The water cycle also know as the hydrologic cycle is a natural process where energy from the sun changes the state of water to a solid, liquid or gas on earth.

• Water on the earth’s surface such as lakes, rivers and oceans enter the atmosphere by evaporation then condenses to form clouds and fall back by precipitation to the earth.

The important the water cycle

• The water cycle is a natural self purifying system• Without this continuous cycle of water to replace

water on earth organism could not exist.• Water cycle provides fresh drinking , agriculture,

industry and domestic usage• The water cycle is affects climate on earth because

it regulates temperature by absorbing heat from the equator and circulation around the earth.

The water cycle

Structure of water

• Water is made up of 2 hydrogen atoms and 1 oxygen which is held by single covalent bonds (dipole)

• Water has a neutral charge but differs because oxygen has a slight +ve and hydrogen –ve. The unequal distribution of change in the molecule is dipole.

• Water molecules are attracted by hydrogen bonding

• Hydrogen bonding is due to polarity of the molecules in which one charge is attracted by opposite charge poles

Properties of water

Surface Tension, Heat of Vaporization, and Vapor Pressure

• Water has the highest surface tension for all liquids. Water's high surface tension is due to the hydrogen bonding in water molecules.

• Water also has an exceptionally high heat of vaporization. Vaporization occurs when a liquid changes to a gas, which makes it an endothermic reaction. Water's heat of vaporization is 41 kJ/mol.

• Vapor pressure is inversely related to intermolecular forces, so those with stronger intermolecular forces have a lower vapor pressure. Water has very strong intermolecular forces, hence the low vapor pressure, but it's even lower compared to larger molecules with low vapor pressures.

Viscosity and Cohesion• Viscosity is the property of fluid having high resistance to flow. We

normally think of liquids like honey or motor oil being viscous, but when compared to other substances with like structures, water is viscous. Liquids with stronger intermolecular interactions are usually more viscous than liquids with weak intermolecular interactions.

• Cohesion is intermolecular forces between like molecules; this is why water molecules are able to hold themselves together in a drop. Water molecules are very cohesive because of the molecule's polarity. This is why you can fill a glass of water just barely above the rim without it spilling.

Solid State• When water is cooled, it becomes more dense and forms ice becuase water

continues to become more dense until it reaches 4°C. After it reaches 4°C, it becomes LESS dense. When freezing, molecules within water begin to move around more slowly, making it easier for them to form hydrogen bonds and eventually arrange themselves into an open crystalline, hexagonal structure.

Liquid State• is liquid at room temperature moves aroWater und quicker than it is as solid,

enabling the molecules to form fewer hydrogen bonds resulting in the molecules being packed more closely together. Each water molecule links to four others creating a tetrahedral arrangement, however they are able to move freely and slide past each other, while ice forms a solid, larger hexagonal structure.

Gas State• As water boils, its hydrogen bonds are broken. Steam particles move very far apart

and fast, so barely any hydrogen bonds have the time to form. So, less and less hydrogen bonds are present as the particles reach the critical point above steam.

Water purification methodsDistillation• Distilled water is water that has many of its impurities removed

throughdistillation. Distillation involves boiling the water and then condensing the steam into a clean container.

Advantages• Removes a broad range of contaminants• ReusableDisadvanges• Some contaminants can be carried into the condensate• Requires careful maintenance to ensure purity• Consumes large amounts of energy

Ion change• Softening is used primarily as a pretreatment method to reduce water hardness

prior to reverse osmosis (RO) processing. The softeners contain beads that exchange two sodium ions for every calcium or magnesium ion removed from the "softened" water.

• Deionization (DI) beads exchange either hydrogen ions for cations or hydroxyl ions for anions. The cation exchange resins, made of styrene and divinylbenzene containing sulfonic acid groups, will exchange a hydrogen ion for any cations they encounter (e.g., Na+, Ca++, Al+++). Similarly, the anion exchange resins, made of styrene and containing quaternary ammonium groups, will exchange a hydroxyl ion for any anions (e.g., Cl-). The hydrogen ion from the cation exchanger unites with the hydroxyl ion of the anion exchanger to form pure water.

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Filtration • Screening – large object such as fish, logs and leaves screen as it is

drawn into plant.• Sedimentation-the chemical Al2SO44 is add to the raw water in a

large tank this chemical clings to impurities in the water(co adulation ) forming large heavier particle flux in a process called flocculation these large particle then sink to the bottom of the tank and collect to form sludge which is then removed.CaCO3 and other chemical add to improve the quality of the water

• Filtration –Small particles are removed from the water my passing it through sand and gravel and this should have removed impurity like microorganisms.

• Chlorination-is the process by which filtered water is disinfected to kill disease causing bacteria after this process water is portable

UV radiation

Desalination• Removal of salt (sodium chloride) and other minerals from the sea water to make

it suitable for human consumption and/or industrial use. The most common desalination methods employ reverse-osmosis in which salt water is forced through a membrane that allows water molecules to pass but blocks the molecules of salt and other minerals.

The importants of Dissolved O2 in water

• Dissolved Oxygen (DO): Dissolved oxygen refers to the amount of oxygen gas (O2) that has dissolved into the water. This is an important test for water quality because oxygen levels reveal a great deal about the amount of pollutants in the water. In general, higher dissolved oxygen levels indicate higher water quality. Dissolved oxygen is required for respiration in aquatic life.

• Factors that affect O2 levels in water is temperature and excess nutrients which can cause eutrophication .Increased temperature decreased the amount of O2 water And excess nutrients in water can cause stimulation of algae growth on the surface of water .