NOTES: 2.2 – Properties of Water

Water Overview… • Water is perhaps the most important compound in living organisms • Because so many substances can dissolve in water: molecules and ions are free to move and collide with one another in the reactions of life; water serves as a means of material transportation (i.e. blood, plant sap)

Water Overview… Living cells are 70%-95% water Water covers approx. ¾ of earth. In nature, water naturally exists in all 3 states of matter: solid, liquid and gas

Water is a POLAR molecule • the bonds that hold the hydrogen and oxygen atoms together are POLAR COVALENT BONDS…  this means that the electrons that make up the covalent bonds are shared UNEQUALLY …they spend more time near the oxygen atom than they do near the hydrogen atom so, a water molecule has a positive end and a negative end

Water is POLAR… • POLAR WATER MOLECULES attract one another, as well as ions and other polar molecules

Water Molecules… • The polarity of water molecules results in HYDROGEN BONDING • a HYDROGEN BOND is the attraction of opposite charges between hydrogen and oxygen • Each water molecule can form a maximum of 4 hydrogen bonds with neighboring water molecules

Properties of Water (as a result of polarity and H-bonding): ► COHESION: molecules are held together by H bonds • contributes to upward movement of water in plants from roots to leaves

Properties of Water ► ADHESION: water sticks to other surfaces (by polarity or H bonds); can counteract gravity in plant vessels

► SURFACE TENSION: measure of how difficult it is to stretch the surface of a liquid -water has greater surface tension than most liquids because at the air/water interface the surface water molecules are H-bonded to each other and to the water molecules below -causes water to “bead” -creates a “skin” on the surface

► HIGH SPECIFIC HEAT (resists changes in temp) • Specific Heat: amt. of heat that must be absorbed or lost for 1 gram of a substance to change its temp by 1°C • Water’s high specific heat means that it resists temp. changes when it absorbs or releases heat *Heat is absorbed to break H-bonds; and given off when they form

► HIGH HEAT OF VAPORIZATION • Heat of vaporization = amt. of heat a liquid must absorb for 1 g to be converted to gas state • for water molecules to evaporate, H bonds must be broken, which requires heat energy

► EVAPORATIVE COOLING: after high temp ► EVAPORATIVE COOLING: after high temp. molecules have evaporated, the remaining liquid is cooler (ex: sweating) -stabilizes temp. in aquatic ecosystems -protects organisms from overheating *(as 1 g of water evaporates from our skin, 539 g of body cools by 1°C)

The air in the shower stall is at the same temperature as the air outside, but there's less water vapor outside to condense on the skin.

HEAT OF VAPORIZATION (WHILE CHANGING STATE) EVAPORATIVE COOLING HEAT OF VAPORIZATION (WHILE CHANGING STATE) SPECIFIC HEAT (WHILE WATER IS HEATING)

► WATER EXPANDS WHEN IT FREEZES • because of H-bonding, water is less dense as a solid than it is as a liquid...ice floats! • since ice is less dense, it forms on the surface at first • as water freezes, it releases heat to the water below and insulates it! • Oceans and lakes don’t freeze solid!!

► WATER IS THE SOLVENT OF LIFE cell ► WATER IS THE SOLVENT OF LIFE • due to its polarity, water is a versatile solvent

Mixtures and Solutions: MIXTURE: a combination of 2 or more pure substances in which the individual components retain their own properties Examples: sand & sugar water & salt

SOLUTIONS: SOLUTION: a mixture in which the solute(s) are evenly distributed in the solvent Example: salt in water **Solutions are important to living things!!

SOLUTIONS: SOLUTE: the substance that is dissolved SOLVENT: the substance in which the solute dissolves (usually water)

Compounds that are HYDROPHILIC (“water loving”), and are soluble in water: ionic compounds: charged regions of polar water molecules have an electrical attraction to charged ions polar compounds: charged regions of polar water molecules are attracted to oppositely charged regions of other polar molecules

Compounds that are HYDROPHOBIC (“water fearing”) and are insoluble in water: Nonpolar compounds: symmetric distribution in charge, or composed of nonpolar bonds

► THE PROCESS OF DIFFUSION: • DIFFUSION is the net movement of particles from an area of higher concentration to an area of lower concentration  it results from the random motion of molecules / particles

Diffusion is affected by three key factors: 1) Concentration: the more concentrated a substance, the faster diffusion occurs 2) Temperature: an increase in temperature will increase rate of diffusion 3) Pressure: an increase in pressure will speed up molecular movement and, therefore, diffusion

**once substances are evenly distributed, no further change in concentration will occur…this is known as DYNAMIC EQUILIBRIUM.

• DIFFUSION is one of the methods by which cells move substances:  in and out of the cell (bring oxygen into cell; release CO2 to outside of cell);

 within the cell (ions dissolved in the cytoplasm remain evenly distributed within cell); and

outside of the cell (oxygen diffuses into the blood capillaries in the lungs because it is more concentrated in the air sacs).

ACIDS & BASES In living things, chemical reactions depend on the pH of the environment Water molecules (H2O) can split apart into H+ and OH- ions When H+ and OH- are present in equal amounts, the solution is NEUTRAL.

ACIDS & BASES ACID: a compound that forms H+ in solution (& adds H+ ions to the solution) BASE: a compound that forms OH- in solution (& adds OH- ions to the solution)

ACIDS & BASES pH = a measure of how acidic or basic a solution is

The pH scale: ACID: a solution with a pH below 7 BASE: a solution with a pH above 7

BUFFERS BUFFERS: dissolved compounds that prevent sharp, sudden changes in pH example: blood contains a number of buffers that help maintain a constant blood pH of 7.4