Water

Describe Water (46NBtop) How would you describe water to someone who had never seen it before? You might say that pure water has no color, no taste, and no odor. You might even say that water is a rather plain, ordinary substance. If you asked a chemist to describe water, the chemist would say that water is very unusual. Its properties differ from those of most other familiar substances.

Why is water so Important? All life on Earth depends on water Our fresh water resources are scarce Global warming is effecting the hydrosphere Different types of pollution are contaminating all of our water Etc, etc, etc…. Now…. Let’s look at what makes water so special in the first place….

Properties of Water

Polarity of Water In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. –A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.

Water has a variety of unusual properties because of attractions between these polar molecules. –Each water molecule can form hydrogen bonds with up to FOUR neighbors. –Gives water more structure than other liquids… and therefore causes lots of interesting things to happen……

Capillary Action Now, how does that work??

Cohesion & Adhesion Hydrogen bonds hold water together, a phenomenon called ….

Cohesion Co-hesion= 1.“Co-” = share 2.“-hesion”= bonded example -> (adhere) So… Cohesion refers to waters attraction to other water molecules.

–Adhesion refers to attraction to other substances. Water is adhesive to any substance with which it can form hydrogen bonds. Adhesion

Trees have specialized structures to transport water: - xylem and phloem “plumbing” * Water molecules are “dragged” from the roots to the top of the tree by capillary action and cohesion!

Capillary action water evaporates from leaves = transpiration adhesion, cohesion and capillary action All thanks to hydrogen bonding! water taken up by roots

Surface Tension Have you ever watched water striders (spiders!!!) skate across the surface of a pond without sinking? They are supported by the surface tension of the water….

Surface tension is the tightness across the surface of water that is caused by the polar molecules pulling on one another. (its related to cohesion) Some animals can stand, walk, or run on water without breaking the surface.

–Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. –Water behaves as if covered by an invisible film. Where else have you seen surface tension in action?

Specific Heat It is a steamy summer day. The air is hot, the sidewalk is hot, and the sandy beach is hot. But when you jump into the ocean, the water is surprisingly cool! If you go for an evening swim, however, the water is warmer than the cool air.

Because…. Water has a HIGH 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.

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 Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for: The Earth is over 75% water!

Universal Solvent

“Universal” Solvent What happens when you make a fruit drink from a powdered mix? homogeneous = solution. In an aqueous solution, water is the solvent. Shhh….Water is not really a universal solvent, but it is very versatile because of the polarity of water molecules.

Solvent for Life Can be either: –Solute (if its not main ingredient) –Solvent (= Aqueous solution ) Hydrophilic (H 2 0 loving) –Ionic compounds dissolve in water –Polar molecules (generally) are water soluble Hydrophobic (H 2 0 phobic) Nonpolar compounds

Cohesion and solubility allow…. Water to transport molecules dissolved in it: –Blood, a water-based solution, transports molecules of nutrients and wastes organisms –Nutrients dissolved in water get transported through plants –Unicellular organisms that live in water absorb needed dissolved substances

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.

Solid water (ice) is less dense than liquid same mass but a larger volume Ice is less dense than water: the molecules are spread out to their maximum distance Density = mass/volume

How does ice float? Ice is “Buoyant” –It floats on top of the more dense liquid water… –But what is “Buoyancy”?

 water expands as it solidifies  water reaches maximum density at 4-degrees C  water freezes from the top down  organisms can still live in the water underneath the ice during winter Oceans and lakes don’t freeze solid because ice floats

Water is Transparent The fact that water is clear allows light to pass through it –Aquatic plants can receive sunlight –Light can pass through the eyeball to receptor cells in the back

Wrap it up: What properties of water do you feel make it a Unique/Special Substance? Explain and justify your response.