Water and Its Properties AP Biology Ms. Day

Water is a polar molecule The polarity of water molecules Allows it form many hydrogen bonds Contributes to many properties of water Oxygen has a greater “pull” on e-’s than hydrogen forms partial – and partial + end of molecule (dipole nature) + H  –

Bonds in H2O Where is the Polar Covalent Bond in water? What is the difference btw a “POLAR COVALENT BOND” and a “NONPOLAR COVALENT BOND?

What is “polarity”? A molecule that has different charges on each end or “pole” Asymmetrical ends = polar Water polarity allows for H-bonding (attraction)

Polar covalent bonds

Water Properties **H-bonding is responsible for these properties 1. Cohesion (“co-” means “together”) a high % of H20’s bond to neighboring H20’s Partial - & partial + ends attract each other Examples: water “sticking” to itself, transpirational-pull, surface tension

Water “sticks” to itself

Transpirational Pull a result of cohesion Helps pull water up through microscopic vessels of plants w/out using energy (ATP)

Surface tension another type of cohension a measure of how hard it is to break the surface of a liquid H20’s form a “barrier” by H-bonding together Water does NOT pull apart very easily  acts like elastic At the surface of a liquid, the molecules are only puller downward and towards the sides.

The surface tension of water is GREATER than the pull of gravity on the needle or spider’s tiny feet!

Surface Tension At the surface of a liquid, the molecules are only puller downward and towards the sides.

2. Water demonstrates Adhesion a high % of H20’s bond to other different molecules Partial - & partial + ends of water attract to ends of other molecules Examples: water “sticking” to stuff (i.e.-towels, clothes, paper, glass)

Cohesion & Adhesion Acting Together Helps pull water up through the microscopic vessels of plants Called capillary action

Another Example of Cohesion and Adhesion…

3. Water has a HIGH heat capacity Water changes temp slowly Absorbs large amounts of energy before it begins to get hot  needs to break A LOT of H-bonds Releases heat slowly too Allows large bodies of water to remain stable even when temperature of air changes

Water has a High Specific Heat specific heat = amount of heat that must be absorbed or lost for 1 gram of substance to change its temp. by 1ºC WHY IS THIS PROPERTY IMPORTANT TO BIOLOGY? Contents of cells unlikely to freeze  stable internal temperature despite fluctuating external temp’s Helps moderates Earth's climate by buffering large fluctuations in temperature.

Water has a High heat of Vaporization Amount of energy to convert 1g of a substance from a liquid to a gas In order for water to evaporate, hydrogen bonds must be broken. As water evaporates, it removes A LOT of heat with it (cooling effect)

Water on Earth Water is the only substance that exists under NORMAL conditions on Earth as solid, liquid and gas

4. Low Density in Solid Form Allows for insulation of bodies of water by floating ice Solid water (ice) Is less dense than liquid water Floats in liquid water Since ice floats in water  Life can exist under frozen surfaces of lakes/polar seas

Polar Structure Separated by 105o when in the gaseous or liquid phase 109.5o when ice.

The hydrogen bonds in ice Are more “ordered” than in liquid water, making ice less dense Liquid water Hydrogen bonds constantly break and re-form Ice Hydrogen bonds are more stable

5. Water is an Excellent Solvent Water is a versatile solvent (“dissolver”)due to its polarity It can form aqueous (“watery”) solutions very easily! MOST biochemical reactions occur in water!!!

Ex: salt water solution Solution = a UNIFORM mixture of 2+ substances LOOK the same (uniform) throughout Solvent = substance that is doing the dissolving The “dissolver” Solute = substance(s) getting dissolved Ex: salt water solution

The different ends of the polar water molecule can interact with ionic compounds (the solutes) and dissolve them + Cl – – Na+ Cl–

Hydrophilic and Hydrophobic Substances A hydrophilic substance Has affinity for water Water “loving” A hydrophobic substance Does not have affinity for water Water “fearing” Dissociation = the separation of ions when an ionic compound dissolves

Changes in [ ] of these ions have great affect on living organisms Water can dissociate Into hydronium ions and hydroxide ions H2O + H2O  H3O+ + OH- Changes in [ ] of these ions have great affect on living organisms Hydronium ion (H3O+) Hydroxide ion (OH–) +

Acids vs. Bases Hydrogen Ion Hydroxide Ion Acid Base H2O  H+ + OH-

Acids and Bases An acid any substance that increases hydrogen ion (H+) concentration of a solution A base any substance that reduces hydrogen ion concentration of a solution pH Measure of acidity or alkalinity of an aqueous solution

The pH Scale The pH of a solution high in an acid (Higher [H+] ) determined by relative [ ] of H+’s high in an acid (Higher [H+] ) low in a base (Lower [H+] thus higher [OH-] )

Acids = low pH #’s HIGH [ ] of H+ Bases = High pH #’s HIGH [ ] of OH- Remember…. A…..B…. In the alphabet and 1….14 A= 1st letter (H+ has 1 letter) B= 2nd letter (OH- has 2 letters)

Buffers NEUTRALIZATION!!! Buffers substances that minimize changes in [ ] of H+ and OH- ions in a solution Consist of an acid-base pair that can REVERSE changes NEUTRALIZATION!!! The internal pH of most living cells must remain close to pH 7 (Ex: H2CO3) Need biological buffers to regulate systems (absorbs excess H+ or OH-) Most important buffer in humans = bicarbonate ion

Let’s Practice in Groups… pH 2 has more _____ ions and is an acid or base? Weak or strong? pH 12 has more _____ ions and is an acid or base? Weak or strong? pH 9 has more _____ ions and is an acid or base? Weak or strong? pH 7 has more _____ ions and is an acid or base? Weak or strong?

More Practice in Groups… Why is water a versatile (universal) solvent? In general, what kinds of materials will not dissolve in water? Why?

More Practice…Which Property? During winter, air temp’s in northern United States can remain below 0∘C for months; however, fish and other animal living in the lakes survive. When you pour water into a 25ml graduated cylinder, a meniscus forms at the top of the water column. Sweating and evaporation of sweat from the body surface help reduce a human's body temperature. Water drops fall on a surface of a leaf & form beads. If you touch the edge of a paper towel to a drop of colored water, the water will move into the towel helps Earth have a “global blanket” helps maintain homeostasis in cells