WATER’S LIFE-SUPPORTING PROPERTIES. Water’s polarity leads to hydrogen bonding and other unusual properties The charged regions on water molecules are

WATER’S LIFE-SUPPORTING PROPERTIES

Water’s polarity leads to hydrogen bonding and other unusual properties

• The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules– This attraction forms

weak bonds called hydrogen bonds

Like no other common substance, water exists in nature in all three physical states:

– as a solid– as a liquid– as a gas

2.11 Hydrogen bonds make liquid water cohesive

• Due to hydrogen bonding, water molecules can move from a plant’s roots to its leaves

• Insects can walk on water due to surface tension created by cohesive water molecules

2.12 Water’s hydrogen bonds moderate temperature

• It takes a lot of energy to disrupt hydrogen bonds– Therefore water is able to absorb a great deal of

heat energy without a large increase in temperature

– As water cools, a slight drop in temperature releases a large amount of heat

Water moderates temperatures

• A water molecule takes a large amount of energy with it when it evaporates– This leads to evaporative cooling

2.13 Ice is less dense than liquid water

• Molecules in ice are farther apart than those in liquid water – Ice is therefore less dense than liquid water, which causes

it to float

2.14 Water is a versatile solvent

• Solutes whose charges or polarity allow them to stick to water molecules dissolve in water

• They form aqueous solutions

Mixtures and Solutions• A mixture is a combination of substances in which the

individual components retain their own properties• Neither component of the mixture changes.

• A solution is a mixture in which one or more substances (solutes) are distributed evenly in another substance (solvent).

2.15 The chemistry of life is sensitive to acidic and basic conditions

• A few water molecules can break apart into ions• this is called Dissociation of Water

– Some are hydrogen ions (H+)– Some are hydroxide ions (OH–)

– Both are extremely reactive– A balance between the two is critical for chemical

processes to occur in a living organism

Copyright © 2009 Pearson Education, Inc.

Dissociation of Water

• At equilibrium the concentration of water molecules greatly exceeds that of H+ and OH-.

• Adding certain solutes, called acids and bases, disrupts the equilibrium and modifies the concentrations of hydrogen and hydroxide ions


• Chemicals other than water can contribute H+ to a solution– They are called acids– An example is hydrochloric acid (HCl)

– This is the acid in your stomach that aids in digestion

• An acidic solution has a higher concentration of H+ than OH–



• Some chemicals accept hydrogen ions and remove them from solution– These chemicals are called bases– For example, sodium hydroxide (NaOH) provides OH–

that combines with H+ to produce H2O (water)– This reduces the H+ concentration



• A pH scale (pH = potential of hydrogen) is used to describe whether a solution is acidic or basic– pH ranges from 0

(most acidic) to 14 (most basic)

– A solution that is neither acidic or basic is neutral (pH = 7)

pH scale• A compound that releases

H+ ions in solution is an acid, and one that accepts H+ ions in solution is a base thus increasing OH- ions

• The pH scale is used to describe how acidic or basic a solution is..– 0-7 is acidic – 8-14 is basic – Pure water and solutions that

are neither basic nor acidic are neutral, with a pH of 7

Neutralization

• Neutralization occurs when an acid is mixed with a base (correct amount) producing a neutral solution– H+ + OH- - H2OAnother product ,a salt is also formed

HCl + NaOH -> H2O + NaCl

Buffers

• pH can affect chemical reactions in an organism

• Cells are kept close to pH 7 by buffers• Buffers are substances that resist pH change

– They accept H+ ions when they are in excess and donate H+ ions when they are depleted

– Buffers are not foolproof

2.16 CONNECTION: Acid precipitation and ocean acidification threaten the environment

When we burn fossil fuels (gasoline and heating oil), air-polluting compounds and CO2 are released into the atmosphere

– Sulfur and nitrous oxides react with water in the air to form acids

– These fall to Earth as acid precipitation, which is rain, snow, or fog with a pH lower than 5.6

– Additional CO2 in the atmosphere contributes to the “greenhouse” effect and alters ocean chemistry


2.17 EVOLUTION CONNECTION: The search for extraterrestrial life centers on the search for water

An important question is, has life evolved elsewhere?

– Water is necessary for life as we know it

The National Aeronautics and Space Administration (NASA) has evidence that water was once abundant on Mars

– Scientists have proposed that reservoirs of water beneath the surface of Mars could harbor microbial life


2.18 Chemical reactions make and break bonds, changing the composition of matter

You learned that the structure of atoms and molecules determines the way they behave

– Remember that atoms combine to form molecules

– Hydrogen and oxygen can react to form water

2H2 + O2 2H2O


2.18 Chemical reactions make and break bonds, changing the composition of matter

The formation of water from hydrogen and oxygen is an example of a chemical reaction

The reactants (H2 and O2) are converted to H2O, the product

– Organisms do not make water, but they do carry out a large number of chemical reactions that rearrange matter

– Photosynthesis is an example where plants drive a sequence of chemical reactions that produce glucose


Documents

WATER’S LIFE-SUPPORTING PROPERTIES. Water’s polarity leads to hydrogen bonding and other unusual properties The charged regions on water molecules are