KEY CONCEPT Water’s unique properties allow life to exist on Earth.

Life depends on hydrogen bonds in water. Water is a polar molecule. Polar molecules have slightly charged regions, or 2 “poles,” O H _ + These charged regions result in an unequal sharing of electrons leading to a positive end and a negative end, kind of like the ends of a magnet. This gives water a “Mickey Mouse” appearance

Hydrogen bonding between Water molecules Hydrogen bonds form between slightly positive hydrogen atoms and slightly negative atoms. The bonds between Oxygen and Hydrogen are “covalent” meaning the electrons are “shared” between the atoms.

PHASES AND PHASE CHANGES (of water)

PHASE CHANGES (cont’d) Liquid water becomes more dense at is cools to 4*C (almost freezing) But ice is less dense than liquid water! This is why ice floats Fish in certain habitats can survive in winter because ice floats—they continue to live and function in the water beneath the ice because the water below holds a certain amount of heat in (specific heat, we will discuss in a few minutes) See your textbook page 162 Figure 20 after presentation for more details

Hydrogen bonds are responsible for 4 important properties of water. high specific heat- water can hold large amounts of heat before the temperature begins to rise Cohesion – water is attracted to itself Adhesion - water is attracted to other substances Capillary Action – is the result of adhesion. adhesion cohesion

Some other ways Cohesion and Adhesion are important in Biology Capillary action is the result of adhesion. Water travels up the stem of a plan, and seeds swell and germinate by capillary action.

SPECIFIC HEAT CAPACITY Water has the highest specific heat capacity, which means the earth’s oceans and water systems can hold heat and maintain the planet’s temperatures within a moderate range capable of supporting life. Specific heat capacity is the amount of heat required to raise the temperature of a certain amount of substance.

MIXTURES WITH WATER: Solutions A solution is formed when one substance dissolves in another. Solvents dissolve other substances. Solutes dissolve in a solvent. Water is the universal solvent! A solution is a homogeneous mixture meaning they are mixtures of 2 or more substances that cannot be separated. solution

More mixtures: Heterogeneous mixtures: Suspensions Suspensions are mixtures with undissolved particles. Since they are not evenly mixed, they are called “heterogeneous” or “different phases”

Some solutions form acids or bases Some solutions form acids or bases. pH is a measure of Hydrogen ions in a solution An acid releases a hydrogen ion when it dissolves in water. high H+ concentration pH less than 7 A base removes hydrogen ions from a solution Low H+ concentration pH greater than 7 Pure Water is neutral at pH of 7

pH and Buffers The majority of biological processes in the cells in your body work best at a pH between 6.5 and 7.5. To maintain homeostasis, it is important to control H+ levels. Buffers can do this. For example, antacids help neutralize stomach acid when you have an upset stomach. The antacids have a pH higher (more basic) than the stomach acid so it “balances out” the pH Another example is the pH of blood which is around 7.4 The blood has buffers in it that help maintain that pH.

Review How do polar molecules form hydrogen bonds?

How do acids and bases differ?

Describe an example of cohesion and adhesion that you might observe during your daily life.

Give an example of a solution. Give an example of a suspension (heterogeneous mixture)