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Water & Solutions. Chapter 11. Household Water. How is water used in the typical household? 2% for cooking & drinking 6% for cleaning dishes 11% for laundry 23% for bathing 29% for toilets 29% for lawns and gardens. Pollutant Classes. Lead : causes nerve damage
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Water & Solutions Chapter 11
Household Water • How is water used in the typical household? • 2% for cooking & drinking • 6% for cleaning dishes • 11% for laundry • 23% for bathing • 29% for toilets • 29% for lawns and gardens
Pollutant Classes • Lead: causes nerve damage • Chlorinated Solvents: cause cancer • Trihalomethanes: cause liver damage and kidney damage • PCBs: cause liver damage and possible cancer http://cancerweb.ncl.ac.uk/cgi-bin/omd?pcb's • Bacteria and viruses: caused gastrointestinal problems and serious disease • Water treatments must remove these pollutants in order for us to use water for most household purposes
Properties of Water • Some common properties of water include: • Universal solvent: ability to dissolve and transport many solids • Expansion upon freezing: ice has a lower density than liquid water, thus it floats • High specific heat: much energy is required to change the temperature of liquid water, so it acts to moderate temperature change on the Earth • High latent heat: much energy is required to evaporate water, so the evaporation of moisture from our skin cools us efficiently • Why does water have these particular properties?
Structure of Water • Water forms a polar covalent bond • The oxygen and hydrogen share the bonding pair of electrons, but not equally. The electronegativity of oxygen is greater, and the pair spend more time near the oxygen than near the hydrogen • The end of the water molecule with oxygen is slightly negative, the end with hydrogens is slightly positive • Water has a permanent dipole moment, and we call it a polar molecule • This polar structure allows for hydrogen bonding, in which a bond is formed between the positive hydrogen end of the molecule and the negative end of another molecule • The hydrogen bond is not as strong as the covalent bond, but is strong enough to account for many important properties of the water molecule
Dissolving Process • Dissolving is the process of making a solution • A solution is a homogeneous mixture of ions or molecules of 2 or more substances • The solvent is the component present in the larger amount • The solute is the component present in the smaller amount • In air, nitrogen is more abundant than oxygen. Is nitrogen a solvent or a solute? • An aqueous solution is a solution that is formed in water (water is the solvent).
Solutions • A solution is formed when the molecules or ions of two or more substances become homogeneously mixed. Click here for video But: • Solutions can become saturated (solubility has its limits) • Some substances are insoluble (no appreciable dissolving) • General rule: Like dissolves like • A polar solid will dissolve in a polar liquid, and a non-polar solid will dissolve in a non-polar liquid • Soaps: grease tends to be non-polar, and water is polar. So, how can you get grease out of clothing? • Soap has a polar end and a non-polar end. The non-polar end is soluble in the grease, and the polar end is soluble in water. http://science.csustan.edu/nhuy/chem1002/soapexp.htm
Concentrations • The concentration of a solution is an expression of the relative amounts of solute and solvent • Words: “dilute” and “concentrated” • Numerical: Parts per million (ppm) and parts per billion (ppb) can express very small ratios • Percent by volume: ratio of volume of solute to volume of solvent as a percentage • Percent by weight: ratio of weight of solute to weight of solvent as a percentage • Molarity: ratio of moles of solute to liters of solution M = (moles of solute)/(liters of solution)
Solubility • The solubility of a solute is defined as the concentration that is reached in a saturated solution at a particular temperature • Solubility varies with temperature • For solids, solubility tends to increase with temperature • For gases, solubility in water decreases with temperature • Solubility also varies with pressure (especially for a gas dissolved in a liquid) • Think of a bottle of soda pop left open to the air. The fizz, which is dissolved CO2, soon leaves after the pressure is reduced
Boiling point of water • Boiling point of water is affected by pressure • Boiling occurs when the vapor pressure of water equals the atmospheric pressure • At high altitudes, the atmospheric pressure is less, so that water boils at a lower temperature • A pressure cooker is designed to increase the pressure, meaning that the water will not boil until it reaches a higher temperature, and thus foods cook faster • In your automobile, the pressure cap on your radiator is designed to make the pressure in the cooling system higher, which allows a higher temperature before the water becomes steam and boils away • Solutions of water also boil at higher temperature. • One mole of solute per 1,000 grams of water will elevate the boiling point by 0.521°C.
Freezing Point • Freezing occurs when the kinetic energy of molecules has been reduced sufficiently so that the molecules can come together forming the crystal structure of the solid. • The normal freezing point of water is 0°C • The presence of solute particles lowers the freezing point, and thus solutions freeze at a lower temperature than pure solvent. • Salt dissolved in water reduces the temperature at which the water will freeze (or melt), so salt is used for making ice cream and for clearing ice and snow from road surfaces
Acids and Bases • Acids and bases are defined by the amount of hydronium ion present in an aqueous solution • Example: HCl + H2O H3O+ + Cl- • The H3O+ ion is called hydronium, and is a water molecule with an attached proton • Pure water has a H3O+ concentration of 1 x 10-7 moles / liter • Solutions with a larger concentration are acidic, solutions with a smaller concentration are basic • Click here for video
Acids • The following are the properties of acids dissolved in water: • Acids have a sour taste such as the taste of citrus fruits • Acids change the color of certain substances • Acids react with active metals, such as magnesium or zinc, releasing hydrogen gas • Acids neutralize bases, forming water and salts from the reaction • Common acids include: Acetic acid (vinegar), boric acid (eyedrops), carbonic acid (carbonated beverages), formic acid (bee stings), lactic acid (sour milk), and HCl, HNO3, and H2SO4.
Bases • The properties of bases dissolved in water are: • Bases have a bitter taste, for example, the taste of caffeine • Bases reverse the color changes caused by acids • Basic solutions feel slippery on the skin • Bases neutralize acids, forming water and salts from the reaction • Common bases include: NaOH, KOH, NH3, Ca(OH)2, and Mg(OH)2
The pH Scale • The strength of an acid or a base is usually expressed in terms of its pH. The pH scale is based upon the concentration of the hydronium ion in moles per liter • Neutral solutions have a H3O+concentration of 1 x 10-7 moles per liter • Acidic solutions have higher concentrations, basic solutions have lower concentrations • It is convenient to use a logarithmic scale because the range of concentrations is so large • Therefore, pH = - log [H3O+ ] • Of greatest significance to us, a pH of 1-6 indicates an acidic solution, with pH of 6 being a weak acid, and pH of 1 being very strong • Similarly, a pH between 8 and 14 indicates a basic solution, with pH of 8 being a weak base, and pH of 14 being very strong • Think of it along a number line….
