Water Water A water molecule H 2 O

Water

Water • A water molecule (H 2 O), is made up of three atoms --- one oxygen and two hydrogen. H H O

Properties of Water • Polar molecule • Cohesion and adhesion • High specific heat • Density – greatest at 4 o. C • Universal solvent of life

Polarity of Water • In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids – Because oxygen is more electronegative, the region around oxygen has a partial negative charge. – The region near the two hydrogen atoms has a partial positive charge. • A water molecule is a polar molecule with opposite ends of the molecule with opposite charges. In each water molecule, the oxygen atom attracts more than its "fair share" of electrons The oxygen end “acts” negative The hydrogen end “acts” positive Causes the water to be POLAR However, Water is neutral (equal number of e- and p+) --- Zero Net Charge

HYDROGEN BONDS • Hold water molecules together • Each water molecule can form a maximum of 4 hydrogen 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

Organisms Depend on Cohesion Hydrogen bonds hold the substance together, a phenomenon called cohesion • Attraction between particles of the same substance ( why water is attracted to itself) • Results in Surface tension (a measure of the strength of water’s surface) • Produces a surface film on water that allows insects to walk on the surface of water

Adhesion • Attraction between two different substances. • Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton. • Capillary action-water molecules will “tow” each other along when in a thin glass tube. Which gives water the ability to “climb” structures Example: transpiration process which plants and trees remove water from the soil, and paper towels soak up water. Fig. 3. 3 Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Moderates Temperatures on Earth Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air. Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature. Celsius Scale at Sea Level 100 o. C Water boils 37 o. C 23 o. C Human body temperature Room temperature 0 o. C Water freezes • • What is kinetic energy? Heat? Temperature? What is specific heat?

Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1 o. C. Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for: 1. Prevention of temperature fluctuations that are outside the range suitable for life. 2. Coastal areas having a mild climate 3. A stable marine environment

Evaporative Cooling • The cooling of a surface occurs when the liquid evaporates (Under Armor) • This is responsible for: – Moderating earth’s climate – Stabilizes temperature in aquatic ecosystems – Preventing organisms from overheating

Density of Water • Most dense at 4 o. C • Contracts until 4 o. C • Expands from 4 o. C to 0 o. C The density of water: 1. Prevents water from freezing from the bottom up. 2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation. 3. Makes transition between season less abrupt.

– When water reaches 0 o. C, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners. – As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state. – Ice is about 10% less dense than water at 4 o. C. Fig. 3. 5 Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Water is Less Dense as a Solid Water Ice

Buoyancy • Buoyancy is the ability of a fluid to exert an upward force on an object floating in it. • Buoyant force (upward) must be greater than gravitational force (downward) in order for an object to float • Objects with a lower density than water will float in water. • Metal is more dense than water, so the shape of a boat helps it float.

Buoyancy and the Titanic

Solvent for Life • Solution – Solute – Solvent • Hydrophilic – Ionic compounds dissolve in water – Polar molecules (generally) are water soluble • Hydrophobic – Nonpolar compounds

Solutions & Suspensions • Water is usually part of a mixture. • There are two types of mixtures: – Solutions – Suspensions

Solution • • • Ionic compounds disperse as ions in water Evenly distributed SOLUTE – Substance that is being dissolved • SOLVENT – Substance into which the solute dissolves

Solution

Suspensions • Substances that don’t dissolve but separate into tiny pieces. • Water keeps the pieces suspended so they don’t settle out.

The p. H Scale • • • Indicates the concentration of H+ ions Ranges from 0 – 14 p. H of 7 is neutral p. H 0 up to 7 is acid … H+ p. H above 7 – 14 is basic… OHb Each p. H unit represents a factor of 10 X change in concentration • p. H 3 is 10 x 10 (1000) stronger than a p. H of 6

Acids and Bases • An acid is a substance that increases the hydrogen ion concentration in a solution. • Any substance that reduces the hydrogen ion concentration in a solution is a base. – Some bases reduce H+ directly by accepting hydrogen ions. • Strong acids and bases complete dissociate in water. • Weak acids and bases dissociate only partially and reversibly.

Acids • Strong Acids have a p. H of 1 -3 • Produce lots of H+ ions

Bases • Strong Bases have a p. H of 11 to 14 • Contain lots of OH-ions and fewer H+ ions

Buffers • Weak acids or bases that react with strong acids or bases to prevent sharp, sudden changes in p. H (neutralization). • Produced naturally by the body to maintain homeostasis Weak Acid Weak Base