Water is a Polar Molecule! What is the molecular formula of water? Draw water and label it. Place a negative sign by oxygen and a positive sign by the hydrogens. The oxygen atom has a slight negative charge and the hydrogens have a slight positive charge Molecules with this unequal distribution of charges are called polar molecules.

Polar Covalent Bonds Water is made up of atoms with different electronegativities. Oxygen is very electronegative. Oxygen attracts the shared electrons more than the hydrogens. So the electrons spend more time with the oxygen molecule. This unequal sharing of electrons form bonds called polar covalent bonds.

(–) O HH (+)

Hydrogen Bonds Hydrogen bonds are weaker than covalent bonds Hydrogen will share attractions with other electronegative atoms (like nitrogen) Each water molecule can form up to 4 H bonds with neighbors Because the positively charged region is always a hydrogen atom, the bond is called a hydrogen bond

Do Now Find where you drew your last water molecule. Label the Polar Covalent Bonds Add in 4 more water molecules around your initial molecule. Label the hydrogen bonds between them.

Hydrogen bond Polar Covalent Bond

Water’s Life Supporting Properties Water’s unique properties are due to hydrogen bonding! Cohesion and adhesion Moderation of temperature Ice is less dense than water Water is the solvent of life

Cohesion and Adhesion Cohesion - Hydrogen bonding causes water molecules to stick together Cohesion is much stronger for water than other liquids This is useful in plants that depend upon cohesion to help transport water and nutrients up the plant

Adhesion – clinging of one substance to another Example – water clinging to glass Adhesion helps water to stick to the cell walls of plants.

Water-conducting cells Adhesion Cohesion 150 µm Direction of water movement

Cohesion is related to surface tension Surface tension -a measure of how difficult it is to break the surface of a liquid – Hydrogen bonds are responsible for surface tension

Water Moderates Temperature on Earth Water has a greater ability to resist temperature change than other liquids because of the hydrogen bonds This is because water has a high specific heat This allows for life in water. Heat - the energy associated with movement of atoms and molecules in matter Temperature - measures the intensity of heat Heat must be absorbed to break hydrogen bonds; heat is released when hydrogen bonds form

Water also stabilizes air temp. Water absorbs heat from air that is warmer and releases stored heat to air that is cooler. Water can also moderate temperature by evaporative cooling. Evaporative cooling – when a substance evaporates, the surface of the liquid remaining behind cools down as the molecules with the greatest energy (hottest ones) leaves.

Ice is less dense than water Water can exist as a gas, liquid, and solid Water is less dense as a solid due to hydrogen bonding When water freezes, each molecule forms a stable hydrogen bond with four neighbors This forms a three-dimensional crystal There is space between the water molecules so there are less molecules in ice than in the equal volume of liquid water. Ice is less dense than water, so it floats Why is this important in the winter?

Liquid water Hydrogen bonds constantly break and re-form Ice Hydrogen bonds are stable Hydrogen bond

Water is the solvent of life Solution - a liquid consisting of a uniform mixture of two or more substances The dissolving agent = solvent The substance that is dissolved = solute Aqueous solution – water is the solvent When the substance dissolves in a liquid, the substance separates into its individual molecules. Water is a versatile solvent that is fundamental to life processes Its versatility results from its polarity

Ion in solution Salt crystal –Table salt is an example of a solute that will go into solution in water –Sodium and chloride ions and water are attracted to each other because of their charges

Chemistry of Life and pH A few water molecules can break apart into ions – Some are hydrogen ions (H + ) – Some are hydroxide ions (OH – ) – Both are extremely reactive – A balance between the two is critical for chemical processes to occur in a living organism

Acids - chemicals other than water that can contribute H + to a solution – An example is hydrochloric acid (HCl) An acidic solution has a higher concentration of H + than OH –

Base - chemicals that accept hydrogen ions and remove them from solution – For example, sodium hydroxide (NaOH) provides OH – that combines with H + to produce H 2 O (water) – This reduces the H + concentration

pH scale (pH = potential of hydrogen) - is used to describe whether a solution is acidic or basic – pH ranges from 0 (most acidic) to 14 (most basic) – A solution that is neither acidic or basic is neutral (pH = 7) – Blood – 7.4 to 7.3 Buffers- minimize changes in pH by accepting or donating H+ ions Biological fluids have buffers.

Acidic solution pH scale Battery acid Lemon juice, gastric juice Grapefruit juice, soft drink, vinegar, beer Tomato juice Rain water Human urine Saliva Pure water 6 7 Human blood, tears Seawater Milk of magnesia Household ammonia Household bleach Oven cleaner Neutral solution Basic solution NEUTRAL [H + ]=OH – ] Increasingly ACIDIC (Higher concentration of H + ) 14 Increasingly BASIC (Lower concentration of H + )

Acidic solution Neutral solution Basic solution

Acid Precipitation When we burn fossil fuels (gasoline and heating oil), air-polluting compounds and CO 2 are released into the atmosphere Sulfur and nitrous oxides react with water in the air to form acids These fall to Earth as acid precipitation, which is rain, snow, or fog with a pH lower than 5.6 How can this affect living things?