Properties of Water

Water statistics

Covers 75% of Earth’s surface 97% oceans 3% freshwater

2% (of Total) in ice caps and glaciers 1% in lakes, underground, or in atmosphere

(usable by humans)

Makes up 70% of the human body 92% of blood plasma 80% of muscle tissues 60% of red blood cells

(why understanding it is important!)

Physical properties Water: Is clear, colorless, odorless, and tasteless

* Colors, tastes and odors are caused by substances dissolved in the water.

Boils at 100°C Freezes at 0°C Density = 1.0 g/mL (at 4°C)

CohesionAdhesionHigh Specific HeatHigh Heat of VaporizationLess Dense as a Solid

Properties of Water

Water Polar molecule

molecule with uneven distribution of charge

water is polar, O atom pulls electrons from H atoms causing an unequal sharing of electrons

Biochemistry

The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond.

Each water molecule can form hydrogen bonds with up to four neighbors.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.1

HYDROGEN BONDS The hydrogen

bonds joining water molecules are weak, about 1/20th as strong as covalent bonds.

They form, break, and reform with great frequency

Extraordinary Properties that are a result of hydrogen bonds.

Cohesive behavior Resists changes in

temperature High heat of vaporization Expands when it freezes Versatile solvent

Properties of Water

Cohesion is the tendency of molecules of the same kind to stick to one another.

Water has stronger cohesion than most liquids

Cohesion pulls molecules at the surface tightly together, forming a film-like boundary, creating water’s surface tension.

 Surface Tension The collective strength of hydrogen bonds

forms a film on the surface of water Ex: floating a paper clip, “water striders”, water

beading on waxed car

http://wine1.sb.fsu.edu/chm1045/notes/Forces/Liquids/Forces03.htm

AirWater

Adhesion-Attraction between unlike molecules Ex: water moving

up the roots of a tree this is called capillary action

Adhesion Water molecules attracted to sides

of container

Biochemistry

Capillary Action Capillary action = Water is able to

“climb” objects against the forces of gravity

Think of the (+) and (-) ends of a water molecule acting like suction cups, and a water molecule is using them to climb up a glass building

http://kingfish.coastal.edu/biology/sgilman/770lecwatersalt.htm

Water is Less Dense as a Solid Density of ice (at 0°C) is 0.917

g/mL, so ice floats in water. Most liquids become

more dense as you cool them. However, when water freezes, a large expansion occurs.

Water “organizing” with

H bonds as it freezes.

Forms hexagon shapes.

* Where have you see H2O in repeating

hexagonal patterns before?

So ice is able to float (just barely)

Universal Solvent Solvent – a substance that

dissolves another substance What dissolves in water? Salt, Sugar, Carbon Dioxide,

Oxygen, MANY more! For an object to dissolve in water it

must also be a polar molecule! This is why oil and water do not mix, oil

is NON-polar

High Specific Heat

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

•Water can absorb or release large amounts of heat energy with little change in actual temperature.

At the beach, why is the sand hotter than water at noon but cooler than water at midnight?

Water has a High Heat of Vaporization Evaporative Cooling -

the cooling of a surface occurs when the liquid evaporates

This is responsible for: Moderating earth’s

climate Preventing

organisms from overheating

Discussion Questions Which property of water causes the

cracks in the pavement in cold climates?

Why does a meniscus form on the surface of the water in a graduated cylinder?

What property of water best describes the ability of water to clean our clothes?

Acids, Bases, and pH

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pH scale

– Measurement system that indicates the concentration of hydrogen (H+) ions in solution A scale of 1-14 is used to describe

pH

AcidsAcids

have a pH range of 1-6

Produce lots of H+ ions

Bases

Bases have a pH range of 8 to 14

Contain lots of OH-ions and fewer H+ ions

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Acids and Bases Acid – any substance that forms hydrogen

ions (H+) in water Acidic solutions contain higher

concentrations of H+ ions and pH lower than 7.

Base – any substance that forms hydroxide ions (OH-) in water Basic, or alkaline, solutions contain lower

concentrations of H+ ions and pH higher than 7.

Buffers A substance that prevents sharp,

sudden changes in pH (neutralization).

Produced naturally by the body to maintain homeostasis

Weak Acid Weak Base