1. Unit 2- The Chemistry of Life Part 3- Properties of Water

2. The Water Molecule  Remember the structure of a water molecule, H 2 O:  Like all molecules, a water molecule is neutral. The positive charges of its 10 protons balance out the negative charges of its 10 electrons. However, there is more to the story…

3.  Because the nucleus of the oxygen atom has more protons than the nuclei of the hydrogen atoms, the electrons that the atoms share are more attracted to the oxygen atom, and will most often be found nearer to it than to the hydrogen atoms.

4.  This means that the oxygen atom of the water molecule will have a SLIGHTLY negative charge, and the hydrogen atoms will have a SLIGHTLY positive charge  A molecule in which the charges are unevenly distributed is called a polar molecule (because it is like a magnet with poles). The slight negative and positive charges of the oxygen and hydrogen atoms in a water molecule make water a polar molecule!

5. Hydrogen Bonding  The polar nature of water molecules causes them to be slightly attracted to one another. The slightly positively charged end of one water molecule is attracted to the slightly negatively charged end of another water molecule.

6.  This attractive force is called a hydrogen bond. Hydrogen bonds exert attractive forces strong enough so that water clings to itself and some other substances.

7.  Hydrogen bonding accounts for many of the unique properties of water, which include: 1) cohesion 2) adhesion 3) high heat capacity 4) solvent

8. Cohesion and Adhesion  Cohesion is an attraction between molecules of the same substance. Because of hydrogen bonding, water is extremely cohesive.

9.  The cohesive property of water explains its surface tension, which we can observe many ways.

10.  Adhesion is an attraction between molecules of different substances, such as between water molecules and the molecules of a glass container.

11.  Cohesion and adhesion together contribute to the ability of water to rise in a narrow tube against the force of gravity. This is called capillarity. Adhesion between water and the tube causes the water to rise up, and cohesion between water molecules holds the water together as it rises.

12.  Capillarity is important in plants because it draws water out of the roots and up into the stem and leaves of the plant.

13. High Heat Capacity  Water has a high heat capacity, which means that water can absorb or release relatively large amounts of energy in the form of heat with only a slight change in temperature.

14.  During a hot summer day, water can absorb a large quantity of heat from the sun, cooling the air, without a large increase in the water’s temperature. At night, the gradually cooling water warms the air.

15.  This is how the Earth’s oceans stabilize global temperatures enough to allow life to exist.

16.  Water’s high heat capacity also allows organisms to keep cells at an even temperature when the temperature of their environment changes. (Because a large percentage of the cells of living things are water!)

17.  Evaporative cooling is another way this property of water helps living organisms. Evaporative cooling prevents organisms that live on land from overheating!

18.  Example: Perspiration: As sweat evaporates from a person’s skin, the surface of the skin that was under the sweat cools down. Perspiration releases body heat, preventing overheating on a hot day or during strenuous activity!

19. Solutions (Water as Solvent)  A solution is a mixture in which one or more substances are uniformly distributed throughout another substance. They can be mixtures of solids, liquids, or gases.  Example: Plasma, the liquid part of blood: many types of ions, large molecules, and gases dissolved in water.

20.  A solute is the substance dissolved in the solvent.  A solvent is the substance in which the solute is dissolved.  So, when sugar is dissolved in water, the sugar is the solute and the water is the solvent!

21.  The concentration of a solution is the amount of solute dissolved in a fixed amount of the solution. The more solute is dissolved, the greater the concentration.  A saturated solution is one in which no more solute can dissolve.

22.  Aqueous solutions are solutions in which water is the solvent. Aqueous solutions are important to living things for many reasons: 1) Marine organisms spend their lives immersed in the sea water which is an aqueous solution 2) Most nutrients that plants need are in aqueous solutions in moist soil.

23. 3)Body cells exist in aqueous solutions and are themselves filled with fluid 4)Most chemical reactions that occur in the body occur in aqueous solutions! Water is the greatest solvent on Earth!!!

24. Acids, Bases, and pH  As water molecules move around, sometimes they crash into each other- these crashes are sometimes strong enough to change the water molecule. Here’s what happens: 1) One molecule of water pulls apart another molecule: H 2 O H + + OH - The OH - ion is known as a hydroxide ion.

25. 2)The free H + ion reacts with another water molecule : H + + H 2 O H 3 O + The H 3 O + ion is known as a hydronium ion

26.  The pH scale is based on how many hydronium ions, or H 3 O +, are in a solution: IonsAcid or BasepH Scale H 3 O + equal to OH - H 3 O + greater than OH - H 3 O + less than OH -

27.  The pH of the fluids within most cells in the human body must generally be kept between 6.5 and 7.5. If the pH is lower or higher, it will affect the chemical reactions that take place within the cells.

28.  Therefore, controlling pH is an important part of maintaining homeostasis.  Buffers are chemical substances that neutralize strong acids or bases. Buffers are usually weak acids or bases that can react with strong acids or bases to prevent sudden, sharp changes in pH.