Water and Its Properties Honors Biology Ms. Kim

Water One side is more positive One side is more negative Water is composed of one atom of oxygen and two atoms of hydrogen held together by covalent bonds (H2O) Water is a Polar Molecule  unequal distribution of charges One side is more positive One side is more negative When water molecules are close, their opposite sides are attracted to each other because of polarity This attraction between water molecules is responsible for most of the properties of water.

Polar and Nonpolar Molecules Polar Molecule – Unequal distribution of charges One side is more positive One side is more negative Dissolves in water Ex: Water Nonpolar Molecule – no separation of charge, so no positive or negative poles are formed. Do not dissolve in water Ex: CO2, O2, lipids Think of the interaction of two magnets and how they either are attracted to each other or repel.

How do water molecules interact with each other? Polar water molecules can be attracted to each other The hydrogen atom with its’ partial positive charge (+) is attracted to the oxygen atom (partial negative charge) of a different water molecule! This is known as a hydrogen bond

Hydrogen Bonds Bonding between molecules Very weak, but very important for the various characteristics of of water…

Covalent bonds internally hold a water molecules together Different water molecules are held together by hydrogen bonds – very weak bonds Covalent Hydrogen

5 Characteristics of Water 1. Cohesion 2. Adhesion 3. High Specific Heat 4. Less Dense as a Solid 5. Water is a terrific solvent

Water Properties **H-bonding is responsible for these properties 1. Cohesion (“co-” means “together”) the attraction between molecules of the same substance (water) Tendency of molecules of the SAME “kind” to stick together 2. Surface Tension: Measure of how difficult it is to stretch or break the surface of a liquid/ resist an external force. H2O has high surface tension Due to H-bonds

Water “sticks” to itself

Surface tension another type of cohesion a measure of how difficult it is to stretch or break the surface of a liquid Water molecules form a “barrier” by H-bonding together Water does NOT pull apart very easily  acts like elastic

2. Adhesion Adhesion = The type of attraction that happens between two different molecules Glass, soil, plant tissue, cotton, etc. Forms stronger bonds than cohesion Example: Meniscus forming on graduated cylinder! Capillary action- seen in plants, trees and with straws too!

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

Another Example of Cohesion and Adhesion…

High Specific Heat The temperature of water does not increase or decrease easily Water has to absorb more heat energy to increase overall temperature compared to other compounds Ex: Lake Michigan is really cold until ~ August…it takes a long time to warm up!! Helps to regulate cell temperatures in organisms Due to the fact that the molecules hold each other together, the temperature of water does not rise or fall very easily

Water has a high specific heat so its temperature does NOT fluctuate very much  allows life to live in water moderate Earth's climate by buffering large fluctuations in temperature.

How come icebergs float?

4. Low Density in Solid Form Allows for insulation of bodies of water by floating ice Solid water (i.e.-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

Ice floats because ice is less dense than liquid water! This is because the H bonds hold the water molecules farther apart than in liquid water

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 stable

5. Water as a Solvent What is a solution? A solution is a mixture of substances looks the same throughout Made up of a solute and a solvent Water is a GREAT solvent (Water is the universal solvent) Solute – gets dissolved (Hot Cocoa mix) Solvent – does the dissolving (Water) Solution – uniform mixture of two or more substances

Hydrophilic and Hydrophobic Substances A hydrophilic substance Has an affinity for water Water “loving” Dissolves in water Example: salt and sugar A hydrophobic substance Does not have an affinity for water Water “fearing” Does NOT dissolve in water Example: Oil

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

Acids and Bases An acid compound that releases/donates a proton (H+) when dissolved in water Increases H+ concentration in solution Ex: HCl A base compound that accepts H+ and removes them from a solution Is any substance that reduces the hydrogen ion concentration of a solution The more basic the solution, the higher the OH- concentration Ex: NaOH pH Measure of acidity or alkalinity of an aqueous solution

What is the pH scale? Identifies acids and bases Indicates concentration of H+ ions in a solution pH 7 = Neutral = concentration of H+ and OH- are equal

How do you read the pH scale? pH of 7 Neutral Concentration of OH- and H+ are equal Ex – Water pH 1 – 6.5 Acid More H+ than OH- Ex – Lemon juice / HCl pH 7.5 – 14 Base More OH- than H+ Ex – Baking soda / ammonia

What is a buffer? Buffers prevent drastic changes in pH Buffers: weak acids or bases that can react with strong acids or bases to prevent sudden changes in pH Why are buffers important? Key in regulating our urinary, circulatory, and blood pH Necessary in maintaining homeostasis Buffers prevent our blood from becoming too acidic, our lungs/muscles from becoming over saturated in CO2

the lower the pH the stronger the acid the higher the pH the stronger the base pH 7.0 is neutral

Acid precipitation Can damage life in Earth’s ecosystems More acidic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 More acidic Acid rain Normal rain More basic

Thermal Energy Thermal energy is the energy that is generated and measured by heat.