Properties of Water

Ch. 2

Pre-AP Biology

Ms. Haut

Properties of Water

• Cohesive nature of water

• Ability to moderate temperature

• Unusual phase changes

• Versatile solvent

• Water is a polar molecule, electrons are shared unequally

• Cohesion is the ability of water molecules to attach to other water molecules– Attach by hydrogen bonds

1. Cohesion of Water

THE PROPERTIES OF WATER

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

– This makes the oxygen end of the molecule slightly negatively charged

– The hydrogen end of the molecule is slightly positively charged

– Water is therefore a polar molecule

• In a water molecule, oxygen exerts a stronger pull on the shared electrons than hydrogen

Figure 2.9

(–)

O

(–)

(+)(+)

H H

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

• H+ atom is covalently bonded to the oxygen via a shared pair of electrons.

• Oxygen is an "electronegative" or electron "loving" atom compared with hydrogen

• "polar" molecule, meaning that there is an uneven distribution of electron density – This attraction forms weak

bonds called hydrogen bonds

Hydrogen bonds make water Cohesive• The oxygen end of water

has a negative charge and the hydrogen end has a positive charge.

• The hydrogens of one water molecule are attracted to the oxygen from other water molecule. This forms Hydrogen Bonds.

• Cohesion = the ability of water molecules to stick to other water molecules

Surface Tension

• Surface tension is the name we give to the cohesion of water molecules at the surface of a body of water

• 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

Figure 2.11

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

Water Strider

Adhesion

• Water can also be attracted to other materials.

Capillary Action

• Water molecules are attracted to the straw molecules.

• When one water molecule moves closer to the straw molecules the other water molecules (which are cohesively attracted to that water molecule) also move up into the straw.

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/FG13_04.JPG

Giant Redwoods—Muir Woods, CA

Plant Transpiration—aided by cohesion and adhesion

2. 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

Yankicha Island Boiling Water

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

Water has a high Specific Heat• Specific heat = heat

needed to raise temp. of 1g of substance by 1ºC

• Specific heat of water is very high

• Boiling Point High .... water stays liquid longer

• As such, water can act as heat reservoire, moderating Earth’s global temperature

http://bio.winona.msus.edu/berg/IMAGES/earth_from_space.jpg

– A water molecule takes a large amount of energy with it when it evaporates

– This leads to evaporative cooling

Figure 2.12

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

Figure 2.10B

3. Unusual Phase Change

• Like no other common substance, water exists in nature in all three physical states:– as a liquid– as a gas – as a solid

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

Gas

Liquid

Solid

Density of Water

• Ice is Less Dense than Liquid Water • Molecules in ice are farther apart than those in

liquid water • Water is densest at 4°C.

Figure 2.13

Hydrogen bond

ICEHydrogen bonds are

stable

LIQUID WATERHydrogen bonds

constantly break and re-form

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

• Frozen water floats (left) and frozen benzene sinks (right)

Figure 2.13x2

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

• Ice fishing

Figure 2.13x1

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

4. Water is a Versatile Solvent

• Also known as the “Universal solvent”. • Polar substances are enveloped by hydration

spheres. – “Like dissolves like!”

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

aqueous solutions

Figure 2.14

Ions in solution

Salt crystal

Cl–

Na+

Cl–

–

– –

–

–Na+

+

+

+

+

Modified from PowerPoint lectures http://occawlonline.pearsoned.com

Acknowledgements

• Unless otherwise noted, illustrations are credited to Pearson Education have been borrowed from BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.