Water and Its Properties Honors Biology Ms. Kim

Water: Molecule That Supports All of Life Water is the biological medium here on Earth Provides an environment where metabolic rxns can happen H20 is often a product or reactant in biological rxns All living organisms require water more than any other substance (cells ~80%H20 )

¾ of the Earth’s surface is submerged in water The abundance of water is the main reason the Earth is habitable Figure 3.1

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

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

Polar and Nonpolar Molecules Polar Molecule – Unequal distribution of charges One side is more positive One side is more negative Dissolve 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…

Polar covalent bonds

Polar Structure

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”) Tendency of molecules of the SAME “kind” to stick together Partial - & partial + ends attract each other Examples: water “sticking” to itself, transpirational-pull, surface tension 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

Transpirational Pull a result of cohesion Helps pull water up through the microscopic vessels of plants w/out expending energy

Surface tension another type of cohesion a measure of how hard it is to 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

Surface Tension

The type of attraction that happens between two different molecules 2. Adhesion The type of attraction that happens between two different molecules Partial - & partial + ends of water attract to ends of other molecules Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton Adhesion forms stronger bonds than cohesion Examples: water “sticking” to stuff (i.e.-towels, clothes, paper, glass) More Examples: Meniscus forming on graduated cylinder! Transpiration - plants and trees remove water from the soil Paper towels soaking up water

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

Another Example of Cohesion and Adhesion…

3. 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 Water has a high specific heat, meaning it resists temperature change! Because of the hydrogen bonds between water molecules 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

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

Hydrogen bonds constantly break and re-form Hydrogen bonds are stable 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. The Solvent of Life 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

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

Why is water a good solvent? Water is called the universal solvent because more substances dissolve in water than in any other chemical. This has to do with the polarity of each water molecule. The hydrogen side of each water (H2O) molecule carries a slight positive electric charge, while the oxygen side carries a slight negative electric charge. This helps water dissociate ionic compounds into their positive and negative ions. The positive part of an ionic compound is attracted to the oxygen side of water while the negative portion of the compound is attracted to the hydrogen side of water. Despite its name as the "universal solvent" there are many compounds water won't dissolve or won't dissolve well. Such as nonpolar and hydrophobic 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?

Dissociation of water molecules leads to acidic and basic conditions that affect living organisms Dissociation = the separation of ions when an ionic compound dissolves H2O  H+ (hydrogen ion) + OH- (hydroxide ion) H+ + H2O  H3O (hydronium ion)

Acids Number of hydronium ions in solutions is greater than the number of hydroxide ions HCl  H+ + Cl-

Bases Number of hydroxide ions in solution is greater than the number of hydronium ions NaOH  Na+ + OH-

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

pH Scale Measures how acidic or basic a substance is: Scale: Determined by the concentration of H+ ions within a solution Uses a numerical scale of 1 to 14 scale Scale: 1 – 7: ACIDIC 7 – 8: NEUTRAL 8 – 14: BASIC Buffers – resist pH changes Can accept OR donate H+ Important in humans…why?

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

Buffers Buffers keep a neutral pH (pH 7) Buffers substances that minimize changes in [ ] of H+ and OH- ions in a solution Buffers keep a neutral pH (pH 7) The internal pH of most living cells must remain close to pH 7

The pH scale and pH values of various aqueous solutions Increasingly Acidic [H+] > [OH–] Increasingly Basic [H+] < [OH–] Neutral [H+] = [OH–] Oven cleaner 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH Scale Battery acid Digestive (stomach) juice, lemon juice Vinegar, beer, wine, cola Tomato juice Black coffee Rainwater Urine Pure water Human blood Seawater Milk of magnesia Household ammonia Household bleach Each increase or decrease is 10x

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)

Real Life Example…. The Threat of Acid Precipitation Refers to rain, snow, or fog with a pH lower than pH 5.6 Is caused primarily by the mixing of different pollutants with water in the air

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