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.