Water and its Properties CP 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  (-) pole Hydrogen 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 Adhesion Water 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 space a) 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 pH The pH scale In liquid, some water molecules break apart into ions H 2 O  H + + OH - Water  hydrogen ion + hydroxide ion (actually H 3 O + ) Ions can also join to form a water molecule H + + OH -  H 2 O

pH in water In pure water, not many (1 X 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 - H 2 SO 4  H+ + HSO 4 -

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 Neutralizes stomach acid Neutralization reaction acid + base  water + salt

pH in Digestive System pH of food traveling through intestines gradually returns to pH 7.