2.2 Properties of Water

What makes water so What makes water so special?special?

2.2 Properties of Water

KEY CONCEPTWater’s unique properties allow life to exist on Earth.

2.2 Properties of Water

OHH

_

++

Polar Molecule and Hydrogen bonds

• Water is a polar molecule.– Polar molecules have slightly charged regions.

– Nonpolar molecules do not have charged regions.– Hydrogen bonds form between slightly positive hydrogen atoms and slightly negative atoms.–Because an oxygen nucleus has 8 protons and it pulls harder on the shared electrons than does Hydrogen with only 1 proton. Oxygen becomes slightly negative. Nonpolar share electrons more equally

2.2 Properties of Water

Properties of water

• Life depends on hydrogen bonds• Hydrogen bonds are responsible for three important

properties of water.– high specific heat– cohesion– adhesion

• Weak bond, but strong in numbers

2.2 Properties of Water

Cohesion

• Attraction between molecules of the same substance

• Caused by hydrogen bonding between water molecules

2.2 Properties of Water

Surface Tension

• Water molecules on the surface stick together

• Attraction is fairly strong

• Forms a “Film” at the surface

2.2 Properties of Water

Adhesion

• Attraction between molecules of different substances– Water sticking to web– Paint to walls

• Results in Capillary action which allows water to climb structures

2.2 Properties of Water

High Specific Heat

• Amount of heat needed to raise or lower 1g of a substance 1° C.

• Water resists temperature change, both for heating and cooling.

– Would you rather walk on sand or in water on a beach on a hot summer day?

2.2 Properties of Water

Water is Less Dense as a Solid

•Which is ice and which is water?

2.2 Properties of Water

Solutions and Suspensions• A solution is formed when one substance dissolves in

another.• Many compounds dissolve in water

– A solution is a homogeneous mixture.– Solvents dissolve other substances.– Solutes dissolve in a solvent.

solution

2.2 Properties of Water

Ex blood

• Liquid part plasma 95% water = Solvent• Solutes = sugars, proteins etc.

2.2 Properties of Water

• “Like dissolves like.”

– Polar solvents dissolve polar solutes.– Dissolve because attraction between water and

solute is greater than attraction between solute– Ionic compounds (NaCl) dissolve because ions

are attracted to water and water surrounds and rips apart molecule

– Nonpolar solvents dissolve nonpolar solutes.– Polar substances and nonpolar substances generally

remain separate. – Ex “Oil and water don’t mix”

2.2 Properties of Water

SuspensionsSuspensions

• Substances that don’t dissolve but separate into tiny pieces.

2.2 Properties of Water

Acids, Bases and pHAcids, Bases and pH

• One water molecule is made of two ions, (H+) and Hydroxide Ion (OH-)

Hydrogen Ion Hydroxide Ion

Acid Base

H2O H+ + OH-

2.2 Properties of Water

• Indicates the percent of H+ ions

• Ranges from 0 – 14• pH of 7 is neutral• Each pH unit represents

a factor of 10X change in concentration– pH 3 is 10 x 10 x 10

(1000x) stronger than a pH of 6

The pH ScaleThe pH Scale

2.2 Properties of Water

Acids, Bases and pH

• An acid releases a hydrogen ion when it dissolves in water.– high H+ concentration– pH less than 7

more acidic

stomach acid pH between 1 and 3

2.2 Properties of Water

• A base removes hydrogen ions from a solution.

– low H+ concentration– pH greater than 7

bile pH between 8 and 9

more basic

2.2 Properties of Water

• A neutral solution has a pH of 7.• Buffers are weak acids and bases produced by the body

to keep pH neutral– Maintain homeostasis– Can you think of a buffer to neutralize acid

indigestion?pure water pH 7