WATER & pH

Fun Facts 50-95% of weight of living things Only common substance that exists in solid, liquid and gas forms If earth was smooth, depth of water would be 2.5m 98% of water is in liquid form –2% polar, glacial, soil, atmosphere, organisms –Most evaporation is from oceans

Special Properties Cohesion/ Adhesion High surface tension High specific heat –required to heat 1g of water 1 degree C High boiling point High heat of vaporization –Heat required to convert 1 g of water to water vapor – 580 cal Ice floats Versatile Solvent

Hydrogen bonds between water molecules

Water transport in plants COHESION & ADHESION

Walking on water HIGH SURFACE TENSION: RESULT OF H-BONDS

Evaporative cooling

The structure of ice

ICE FLOATS BECAUSE IT IS LESS DENSE

Ice, water, and steam WATER IS MOST DENSE AT 4 DEGREES C WaterIceSteam

Ice floats and frozen benzene sinks

Floating ice and the fitness of the environment

A crystal of table salt dissolving in water WATER DISSOLVES POLAR AND CHARGED MOLECULES

A water-soluble protein

Dissociation Dissociation of water: –H 2 O H + + OH - Dissociation of other things: –Ex: HCl H + + Cl - Acids yield H + ions Bases yield OH - ions

Chemical reaction: hydrogen bond shift

pH Aqueous 25 o C –[H + ][OH - ] = –[ ] = molar concentration pH = -log [H + ] pOH = -log [OH - ] Ex: neutral solution, [H + ] = –pH = -log [10 -7 ] = -(-7) = 7

pH Each step in pH scale is an order of magnitude (x10)

The effects of acid precipitation on a forest

Pulp mill

Acid rain damage to statuary, 1908 & 1968

Buffers Minimize changes in pH –Accepts H+ in excess –donates H+ when depleted Weak acid and corresponding base Example: H 2 CO 3 HCO H + Rise in pH Drop in pH

Water Movement Water potential – –Potential energy of water molecules gravity, pressure, conc. of solutes, etc. –Movement from a region of greater potential to a region of lower water potential Osmotic potential – –Movement across a selectively permeable membrane –Determined by measuring the pressure required to stop the osmotic movement of water in the solution

Water Movement Water moves across a selectively permeable membrane From a region of:To a region of: Higher water potentialLower water potential Lower solute conc.Higher solute conc. Hypotonic soln. (less solute) Hypertonic soln. (more solute) Lower osmotic potentialHigher osmotic potential

Question 1 Many mammals control their body temperature by sweating. Which property of water is most directly responsible for the ability of sweat to lower body temperature? A.Water’s change in density when it condenses B.Water’s ability to dissolve molecules in the air C.The release of heat by the formation of hydrogen bonds D.The absorption of heat by the breaking of hydrogen bonds E.Water’s high surface tension

Question 2 The bonds that are broken when water vaporizes are A.Ionic bonds B.Bonds between water molecules C.Bonds between atoms within individual water molecules D.Polar covalent bonds E.Nonpolar covalent bonds

Question 3 Which of the following is an example of a hydrophobic material? A.Paper B.Table salt C.Wax D.Sugar E.Pasta

Question 4 Acid precipitation has lowered the pH of a particular lake to 4.0. What is the hydrogen ion concentration of the lake? A.4.0 M B M C M D.10 4 M E.4%

Question 5 What is the hydroxide ion concentration of the lake described above? A M B M C M D M E.10 M

Summary Polarity of water results in H-bonding –Water’s special properties Organisms depend on cohesion of water –Pull water up plant vessels –Surface tension Water moderates temperatures on earth –High specific heat –Evaporative cooling/ high heat of vaporization Ice floats –Lakes and oceans don’t freeze, life goes on under ice Water is the solvent of life –Dissolves charged/ polar substances