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Water and its PropertiesCP Biology: Chapter 2
The Water Molecule
needed by all living things– parts of cells, fluid around cells (humans 65% water)
– used in many chemical reactions– keeps temperatures stable– transports dissolved substances in blood or fluid
(nutrients, gases, chemical messages)
• covalent bonds molecules • electrons are NOT shared equally
in POLAR bonds • molecule has (+) and (-) poles
POLAR molecule has regions of partial charges
The Water Molecule
Polar Water Molecule
Oxygen end has slight negative charge (-) poleHydrogen ends have slight positive charge (+) poles• Molecule has bent shape
(+) (-) poles give water molecule many important properties
Water molecule is “bent”
NONPOLAR: A molecule with covalent bonds in which electrons
ARE shared equally
Polar and Nonpolar molecules
HYDROGEN BONDING
• + and – areas on polar molecules attract each other
• Hydrogen on one molecule oxygen on a nearby molecule (or sometimes nitrogen)
• Forms a weak bond, but usually many• Important in living things
Hydrogen Bonds in Water
Cohesion• Attraction between molecules of the same
substance• Hydrogen bonds make molecules stick
together• In water, bonds are fluid - constantly form,
break, form. . .
Cohesion in Water
Adhesion
• Attraction between molecules of different substances
Properties of Water Caused by Cohesion
1) High surface tension• Surface molecules bond• Acts like a membrane
Important Because…
• Help control movement into cells• Insects can “walk” on water
High Heat Capacity
• Water absorbs a lot of heat before its temperature rises
• Absorbed energy must first break hydrogen bonds before molecules can move faster, which makes temperature rise
• Also loses a lot of heat before its temperature drops
Important because…
• Slows temperature changes in the environment (living things can adjust to seasons)
• Keeps stable temperature in living things • Ocean temperatures stay stable all year• Evaporating water cools skin, plant leaves
Ice Floats
• When water freezes, hydrogen bonds stay formed
• Molecules are farther apart than in liquid water
• Ice is less dense than liquid water floats
Hydrogen bonds in ice create a stable, three-dimensional structure
Ice is less dense than liquid water
3 States of Water
Important because …• Surface of water freezes on ponds and lakes
– keeps liquid water below from freezing• Organisms in water can survive during cold
weather
Water Properties caused by AdhesionWater is the “Universal solvent”• More things dissolve in water than in any
other liquid
Important because…
• fluids carry important things into cells and throughout organisms – water dissolves most things cells need
• Water is main component in: cytoplasm, fluid between cells, blood, body fluids
• Plant sap, aquatic environments – substances that cells need are dissolved in the water
“Like Dissolves Like”
water dissolves other polar and ionic substances, but NOT nonpolar substances
NONPOLAR POLAR
Capillary Action
Water molecules climb up a narrow tube or spacea) Uses adhesion
- water sticks to walls of tube- hydrogen bonds: water-to-walls
b) uses cohesion, too- water molecules stick together- pulled up against gravity
Capillary Action
Important because…
water squeezes into small spaces–between cells–rises inside plant stems (vein = bundles
of tubes)–moves fluids like blood inside animals
Transpiration
Solutions and Suspensions
MIXTURE• two or more substances physically mixed• do NOT react chemically• most substances in living things are dissolved
in water
Solution
• liquid mixture, all parts are evenly dispersed
• because water is polar, it can dissolve both ionic and other polar substances
• water easily dissolves salts, sugar, minerals, gases
Solvent and Solute
• SOLVENT – the larger part of a mixture Substance that dissolves a solute SOLUTE – the smaller component, Substance that is dissolved ionic solute – dissociates into ions
Covalent – dissolves into molecules
Suspensions
• solute does not dissolve completely– larger particles
• blood is mostly water, with dissolved substances and suspended cells
Acids, bases, and pHThe pH scaleIn liquid, some water molecules break apart into ions H2O H+ + OH-
Water hydrogen ion + hydroxide ion (actually H3O+ )
Ions can also join to form a water molecule H+ + OH- H2O
pH in water
• In pure water, not many (1 X 10-7 moles H+/Liter) = pH 7• pH scale shows concentration of H+ ions
[H+]–below 7 is acid – have more H+ ions than
water–above 7 is basic - have fewer H+ ions than
water
The pH Scale
Each step on pH scale is a power of 10– ex. pH 4 has 10 times more H+ than pH 5, and 100 times more H+ than pH 6
[ H+ ]
pH #
Acids
ACID - any compound that forms H+ ions in water• Many foods are at least slightly acidic• Have a sour or tart taste Ex. lemon, vinegar,
apples• Strong acids have pH 1-3• Ex. HCl H+ + Cl- H2SO4 H+ + HSO4
-
Bases
Base (alkali) any compound that forms OH- ions in water• have bitter taste and slippery feel (ex. soap) Ex. KOH, NaOH
Buffers
• most cells have pH 7.0 – 7.4. • Outside this range, pH interferes with
chemical reactions in life processes• “buffers” are weak acids or bases that can
accept excess H+ or OH- ions • keep pH steady in cells
Buffers keep pH stable in fluids
• “neutralize” strong acids and bases homeostasis
• Blood has several buffers, such as bicarbonate ions and phosphate ions
keep blood pH stable
pH in the Human Body
• Most tissues and fluids in the body are at or near pH 7
• Stomach juices include acid (HCl) at pH 2• Juices from pancreas and liver are basic
- pH 8.5 - Neutralizes stomach acid
Neutralization reaction acid + base water + salt
pH in Digestive System
pH of food traveling through intestines gradually returns to pH 7.