6/27/2015 Role of Water

6/27/2015 Water is the base of life Life on earth probably evolved in water Living cells are 70-95% H2O Water covers about ¾ of the earth In nature water exist in all three physical states of matter – solid, liquid and gas

6/27/2015 Water is the base of life It can be quite correctly argued that life exists on Earth because of the abundant liquid water. Other planets have water, but they either have it as a gas (Venus) or ice (Mars). Recent studies of Mars reveal the presence sometime in the past of running fluid, possibly water. Water can exist in all three states of matter on Earth, while only in one state on our two nearest neighboring planets.

6/27/2015 Polarity of water Water is polar molecule. Its polar bonds and assymetrical shape give water molecules opposite charges on opposite sides. Four valence orbitals of O point to corners of a tetrahedron. Two corners are orbitals with unshared pairs of electrons and weak negative charge.

6/27/2015 Polarity of water Two corners are occupied by H atoms which are in polar covalent bonds with O. Oxygen is so electronegative, that shared electrons spend more time around the O causing a weak positive charge near H’s.

6/27/2015 Polarity of water Hydrogen bounding orders water into a higher level of structural organization. The polar molecules of water are held together by hydrogen bonds. Positively charged H of one molecule is attracted to the negatively charged O of another water molecules. Each water molecule can form a maximum of four hydrogen bonds with neighboring water molecules.

6/27/2015 Polarity of water Water has extraordinary properties that emerge as a consequence of its polarity and hydrogen-bonding. Some of these properties are that water: Has cohesive behavior Resists changes in temperature Has a high heat of vaporization and cools surfaces as it evaporates Expands when it freezes Is a versatile solvent

6/27/2015 Polarity of water Water is polar molecule.polar Other molecules, such as Ethane, are nonpolar, having neither a positive nor a negative side.

6/27/2015 Hydrogen bonds Consequently, water has a great interconnectivity of individual molecules, which is caused by the individually weak hydrogen bonds that can be quite strong when taken by the billions.hydrogen bonds

6/27/2015 Cohesion of water molecules Collectively, the hydrogen bonds hold the substance together – a phenomenon called cohesion. Cohesion contributes to the transport of water against gravity in plants. Adhesion of water to the walls of the vessels helps counter the downward pull of gravity. Water has a great surface tension. At interface between water and air hydrogen is bounded to one another and to the water below.

6/27/2015 Water moderates temperatures on Earth Water absorbs heat from the warmer air and release the stored heat to the cooler air. Everything what moves has kinetic energy; the faster a molecule moves, the grater its kinetic energy. Heat is a measure of the total quantity of kinetic energy due to molecular motion in a body of matter

6/27/2015 Water moderates temperatures on Earth Temperature measures the intensity of heat due to the average kinetic energy of the molecules. When the two objects are brought together, molecules in the cooler object speed up at the expense of the kinetic energy of the warmer one. A calorie is the amount of heat energy required for raising the temperature of 1 g of water by 1  C. Conversely, one calorie is the amount of heat that one gram of water releases when it cools down by one degree Celsius.

6/27/2015 Water’s high specific heat Water has a high specific heat, which means that it resists temperature changes when it absorbs or releases heat. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1  C. The specific heat of water is 1 calorie per gram per degree Celsius, abbreviated as 1 cal/g/  C. It is unusually high when compared to the other substances (0.6 for alcohol).

6/27/2015 Water’s high specific heat As a result of hydrogen bonding among water molecules, it takes a relatively large heat loss or gain for each 1  C change in temperature. Hydrogen bonds must absorb heat to break, and they release heat when they form. Much of the absorbed heat energy is used to disrupt hydrogen bonds before water molecules can move faster (increase temperature)

6/27/2015 Water’s high specific heat A large body of water can act as a heat sink, absorbing heat from sunlight during the day and summer (while warming only few degrees) and releasing heat during the night and winter as the water gradually cools. As a result: Water, which covers three-fourths of the planet, keeps temperature fluctuations within a range suitable for life Coastal areas have milder climates than inland. The marine environment has a relatively stable temperature.

6/27/2015 Evaporative cooling The transformation from the liquid to gas is called vaporization. Molecules with enough kinetic energy to overcome the mutual attraction of molecules in a liquid, can escape into the air.

6/27/2015 Evaporative cooling Heat of vaporization is the quantity of heat a liquid must absorb for 1g of it to be converted from the liquid to the gaseous state. For water molecules to evaporate, hydrogen bonds must be broken which requires energy. Water has high heat of vaporization – for each gram of water 580 cal are needed (ammonia and alcohol two times less).

6/27/2015 Evaporative cooling With liquid evaporation the surface of the liquid that remains behind cools down. Evaporative cooling occurs because the “hottest” molecules (with greater energy) are the most likely to leave as gas. High concentration of water in the air inhibits evaporation.

6/27/2015 Evaporative cooling Water’s high heat of vaporization: Moderates the Earth’s climate. Solar heat absorbed by tropical seas dissipates when surface water evaporates (evaporative cooling) As moist tropical air moves poleward, water vapor releases heat as it condenses into rain Stabilizes temperature in aquatic ecosystems (evaporative cooling). Helps organisms from overheating by evaporative cooling.

6/27/2015 The structure of ice. Each molecule is hydrogen-bonded to four neighbors in a three-dimensional crystal with open channels. Because the hydrogen bonds make the crystal spacious, ice contains fewer molecules than an equal volume of liquid water. In other words, ice is less dense than liquid water.

6/27/2015 The structure of ice. Water is densest at 4C. Water contracts as it cools to 4C As water cools from 4C to freezing (0C), it expands and becomes less dense than liquid water (ice floats)

6/27/2015 The structure of ice. When water begins to freeze, the molecules do not have enough kinetic energy to break hydrogen bonds. As the crystalline lattice forms, each water molecules forms a maximum of four hydrogen bonds, which keeps water molecules further apart than they would be in the liquid state.

6/27/2015 The structure of ice. Expansion of water contributes to the fitness of the environment for life: Prevents deep bodies of water from freezing solid from the bottom up. Since ice is less dense, it forms on the surface first. As water freezes it releases heat to the water below and insulates it. Makes the transitions between seasons less abrupt. As water freezes, hydrogen bonds form releasing heat. As ice melts, hydrogen bonds break absorbing heat.

6/27/2015 Universal solvent Water has been referred to as the universal solvent. Living things are composed of atoms and molecules within aqueous solutions (solutions that have materials dissolved in water).

6/27/2015 Universal solvent Solutions are uniform mixtures of the molecules of two or more substances. The solvent is usually the substance present in the greatest amount (and is usually also a liquid). The substances of lesser amounts are the solutes.

6/27/2015 Hydrophilic, hydrophobic Any substance (ionic or polar) that has an affinity for water is hydrophilic, even if the substance does not dissolve (cotton). Non-ionic and non-polar substances that repel water are termed hydrophobic.

6/27/2015 A water-soluble protein. Even a molecule as large as a protein can dissolve in water if it has enough ionic and polar regions on its surface. The mass of purple here represents a single such protein molecule, which water molecules are surrounding.

6/27/2015 How to prepare solutions Everything is done in moles. The advantage of measuring a quantity of chemicals in moles is that a mole of one substance has exactly the same number of molecules as a mole of any other substance. A mole (M) is equal to molecular weight – the sum of the weights of all the atoms in a molecule. The number of the molecules in a mole is called Avogadro’s number, 6.02x To get a litre (L) of 1 M of sucrose in water we need to weight 342 g of sucrose and add water up to 1L.

6/27/2015 Water dissociation Water tends to disassociate into H + and OH - ions. In this disassociation, the oxygen retains the electrons and only one of the hydrogens, becoming a negatively charged ion known as hydroxide. Pure water has the same number (or concentration) of H + as OH - ions.

6/27/2015 Water dissociation AcidicAcidic solutions have more H + ions than OH - ions. HCl  H + + Cl - BasicBasic solutions have the opposite. NaOH  Na + + OH - An acid causes an increase in the numbers of H + ions and a base causes an increase in the numbers of OH - ions.

6/27/2015 pH of some common items The pH scale is a logarithmic scale representing the concentration of H + ions in a solution. As the H + concentration increases the OH - concentration decreases and vice versa.

6/27/2015 If we have a solution with one in every ten molecules being H+, we refer to the concentration of H+ ions as 1/10. Remember from algebra that we can write a fraction as a negative exponent, thus 1/10 becomes Conversely 1/100 becomes 10 -2, 1/1000 becomes 10 -3, etc. Logarithms are exponents to which a number (usually 10) has been raised. For example log 10 (pronounced "the log of 10") = 1 (since 10 may be written as 10 1 ).

6/27/2015 The log 1/10 (or ) = -1. pH, a measure of the concentration of H+ ions, is the negative log of the H+ ion concentration. If the pH of water is 7, then the concentration of H+ ions is 10 -7, or 1/10,000,000. In the case of strong acids, such as hydrochloric acid (HCl), an acid secreted by the lining of your stomach, [H+] (the concentration of H+ ions, written in a chemical shorthand) is ; therefore the pH is 1.

6/27/2015 Buffers Biological fluids resist changes to their own pH when acids or bases are introduced because of the presence of buffers. Buffers are substances that minimize changes in the concentrations of H+ and OH- in a solution. The change of pH to less than 7 and more than 7.8 is lethal. The mode of action of a buffer: -donate the hydrogen ions when they have been depleted; -accept the hydrogen ions from the solution when they are in excess.

6/27/2015 Reading Ch. 3 (46-57)