Chemistry and Properties of Water AP Biology Chemistry and Properties of Water

Matter consists of chemical elements in pure form and in combinations called compounds Organisms are composed of matter Matter is anything that takes up space and has mass Atoms are the basic units in chemicals Matter is made up of elements An element is a substance that cannot be broken down to other substances by chemical reactions A compound is a substance consisting of two or more elements in a fixed ratio A compound has characteristics different from those of its elements Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Table 2-1 Table 2-1

Electrons (– charge) form negative cloud and determine Fig. 2-UN3 Nucleus Protons (+ charge) determine element Electrons (– charge) form negative cloud and determine chemical behavior Neutrons (no charge) determine isotope Atom

Ionic bond Electron transfer forms ions Na Sodium atom Cl Fig. 2-UN6 Ionic bond Electron transfer forms ions Na Sodium atom Cl Chlorine atom Na+ Sodium ion (a cation) Cl– Chloride ion (an anion)

1H 2He 3Li 4Be 5B 6C 7N 8O 9F 10Ne 11Na 12Mg 13Al 14Si 15P 16S 17Cl Fig. 2-9 Hydrogen 1H 2 Atomic number Helium 2He He Atomic mass 4.00 First shell Element symbol Electron- distribution diagram Lithium 3Li Beryllium 4Be Boron 5B Carbon 6C Nitrogen 7N Oxygen 8O Fluorine 9F Neon 10Ne Second shell Sodium 11Na Magnesium 12Mg Aluminum 13Al Silicon 14Si Phosphorus 15P Sulfur 16S Chlorine 17Cl Argon 18Ar Figure 2.9 Electron-distribution diagrams for the first 18 elements in the periodic table Third shell

C Atomic Symbols 12 6 Carbon Each element represented by unique atomic symbol One or two letters First letter capitalized Superscripted number before: Represents mass number Count of protons plus count of neutrons Subscripted number before Represents to atomic number Number of protons in nucleus Mass Number 12 6 Carbon C Atomic Number Atomic Symbol

Isotopes C C C Carbon 12 Carbon 13 Carbon 14 Isotopes: Atoms of the same element with a differing numbers of neutrons Some isotopes spontaneously decay Radioactive Give off energy in the form of rays and subatomic particles Can be used as tracers Mutagenic – Can cause cancer 12 6 Carbon 12 C 13 6 Carbon 13 C 14 6 Carbon 14 C

NaCl

NaCl in Solution

Covalent Bonds formed when atoms share electrons sharing can be unequal

Neon, with two filled shells (10 electrons) (a) Electron-distribution Fig. 2-10-4 Neon, with two filled shells (10 electrons) (a) Electron-distribution diagram First shell Second shell (b) Separate electron orbitals x y z 1s orbital 2s orbital Three 2p orbitals Figure 2.10 Electron orbitals (c) Superimposed electron orbitals 1s, 2s, and 2p orbitals

Hydrogen Bonds A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom In living cells, the electronegative partners are usually oxygen or nitrogen atoms Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

  + Water (H2O) + Hydrogen bond   Ammonia (NH3) + + + Fig. 2-16   + Water (H2O) + Hydrogen bond   Ammonia (NH3) Figure 2.16 A hydrogen bond + + +

Overview: The Molecule That Supports All of Life Water is the biological medium on Earth All living organisms require water more than any other substance Most cells are surrounded by water, and cells themselves are about 70–95% water The abundance of water is the main reason the Earth is habitable Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The polarity of water molecules results in hydrogen bonding The water molecule is a polar molecule: The opposite ends have opposite charges Polarity allows water molecules to form hydrogen bonds with each other Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

H O H  –  +  –  +  –  +  –  + Hydrogen bond —— —— Fig. 3-2 Figure 3.2 Hydrogen bonds between water molecules  –  +

Four emergent properties of water contribute to Earth’s fitness for life Four of water’s properties that facilitate an environment for life are: Cohesive behavior Ability to moderate temperature Expansion upon freezing Versatility as a solvent Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cohesion Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion Cohesion helps the transport of water against gravity in plants Adhesion is an attraction between different substances, for example, between water and plant cell walls Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Adhesion Water-conducting cells Direction Cohesion of water movement Fig. 3-3 Adhesion Water-conducting cells Direction of water movement Cohesion Figure 3.3 Water transport in plants 150 µm

Surface tension is related to cohesion Surface tension is a measure of how hard it is to break the surface of a liquid Surface tension is related to cohesion Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 3-4 Figure 3.4 Walking on water

Water’s High Specific Heat The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºC The specific heat of water is 1 cal/g/ºC Water resists changing its temperature because of its high specific heat Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Insulation of Bodies of Water by Floating Ice Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense Water reaches its greatest density at 4°C If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Heat Capacity Water has a high heat capacity Temperature = rate of vibration of molecules Apply heat to liquid Molecules bounce faster Increases temperature But, when heat applied to water Hydrogen bonds restrain bouncing Temperature rises more slowly per unit heat Water at a given temp. has more heat than most liquids Thermal inertia – resistance to temperature change More heat required to raise water one degree than most other liquids (1 calorie per gram) Also, more heat is extracted/released when lowering water one degree than most other liquids

Properties of Water: Heat of Vaporization High heat of vaporization To raise water from 98 to 99 ºC; ~1 calorie To raise water from 99 to 100 ºC; ~1 calorie However, large numbers of hydrogen bonds must be broken to evaporate water To raise water from 100 to 101 ºC; ~540 calories! This is why sweating (and panting) cools Evaporative cooling is best when humidity is low because evaporation occurs rapidly Evaporative cooling works poorest when humidity is high because evaporation occurs slowly

Evaporative Cooling of Animals

Heat of Fusion Heat of fusion (melting) To raise ice from -2 to -1 ºC; ~1 calorie To raise water from -1 to 0 ºC; ~1 calorie To raise water from 0 to 1 ºC; ~80 calories! This is why ice at 0 ºC keeps stuff cold MUCH longer than water at 1 ºC This is why ice is used for cooling NOT because ice is cold But because it absorbs so much heat before it will warm by one degree

Heat Content of Water at Various Temperatures

Properties of Water: Uniqueness of Ice Frozen water less dense than liquid water Otherwise, oceans and deep lakes would fill with ice from the bottom up Ice acts as an insulator on top of a frozen body of water Melting ice draws heat from the environment

A Pond in Winter

Density of Water at Various Temperatures

Hydrogen bonds are stable Liquid water Fig. 3-6a Hydrogen bond Figure 3.6 Ice: crystalline structure and floating barrier Ice Hydrogen bonds are stable Liquid water Hydrogen bonds break and re-form

Hydrophilic and Hydrophobic Substances A hydrophilic substance is one that has an affinity for water A hydrophobic substance is one that does not have an affinity for water Oil molecules are hydrophobic because they have relatively nonpolar bonds A colloid is a stable suspension of fine particles in a liquid Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Properties of Water: Water as a Solvent Solutions consist of: A solvent (the most abundant part) and A solute (less abundant part) that is dissolved in the solvent Polar compounds readily dissolve; hydrophilic Nonpolar compounds dissolve only slightly; hydrophobic Ionic compounds dissociate in water Na+ Attracted to negative (O) end of H2O Each Na+ completely surrounded by H2O Cl- Attracted to positive (H2) end of H2O Each Cl- completely surrounded by H2O

pH of Water: Acids Acids Dissociate in water and release hydrogen ions (H+) Sour to taste Hydrochloric acid (stomach acid) is a gas with symbol HCl In water, it dissociates into H+ and Cl- Dissociation of HCl is almost total, therefore it is a strong acid

pH of Water: Bases Bases: Either take up hydrogen ions (H+) or release hydroxide ions (OH-) Bitter to taste Sodium hydroxide (drain cleaner) is a solid with symbol NaOH In water, it dissociates into Na+ and OH- Dissociation of NaOH is almost total, therefore it is a strong base

pH Scale pH scale used to indicate acidity and alkalinity of a solution. Values range from 0-14 0 to <7 = Acidic 7 = Neutral >7 to 14 = Basic (or alkaline) Logarithmic Scale Each unit change in pH represents a change of 10X pH of 4 is 10X as acidic as pH of 5 pH of 10 is 100X more basic than pH of 8

The pH Scale

Buffers and pH When H+ is added to pure water at pH 7, pH goes down and water becomes acidic When OH- is added to pure water at pH 7, pH goes up and water becomes alkaline Buffers are solutes in water that resist change in pH When H+ is added, buffer may absorb, or counter by adding OH- When OH- is added, buffer may absorb, or counter by adding H+

Biology, 9th ed,Sylvia Mader Chapter 02 Buffers in Biology Health of organisms requires maintaining pH of body fluids within narrow limits Human blood normally 7.4 (slightly alkaline) Many foods and metabolic processes add or subtract H+ or OH- ions Reducing blood pH to 7.0 results in acidosis Increasing blood pH to 7.8 results in alkalosis Both life threatening situations Bicarbonate ion (-HCO3) in blood buffers pH to 7.4 Basic Chemistry

Review Chemical Elements Atoms Isotopes Molecules and Compounds Chemical Bonding Ionic and Covalent Hydrogen Properties of Water Acids and Bases