4.4 Water Properties

Properties of Water Result of H 2 O – being a polar molecule- Oxygen end has a slight negative charge; Hydrogen end has a slight positive charge. – Ability of water to form hydrogen bonds.

1. Cohesion Like molecules attach to like molecules. – Water forms bonds with other water molecules. Important in surface tension of water. Movement of water up a straw and up the roots of a plant to the stem and leaves.

2. Adhesion Unlike molecules are attracted to each other. Water forms bonds with other substances. Important in fighting the effects of gravity. Water forms bonds with cells in the transport tubes so water can move up the stems to the leaves. Why is this important to plants and us?

Cohesion and adhesion

3.Temperature moderation Because of hydrogen bonds water takes longer to heat up and longer to cool down. Heat is transferred from the warm to colder substances. This is why lakes and oceans moderate the temperatures of nearby land areas,so coastal areas generally have less extreme temperatures than inland areas. Also seen in sweating- When water molecules evaporate, the hydrogen bonds break releasing water into the air, cooling our bodies.

4. Low density of ice Water is unique- solid form-ice is less dense than the liquid form. As water cools the hydrogen bonds spread out.

5. Water is an excellent solvent Terms: A solution is a uniform mixture of two or more substances.solution A solvent (water) dissolves the other substance and is present in the greater amoun.solvent The solute is the substance that is dissolved and is present in a lesser amount (salt).solute An aqueous solution is when water is the solvent.aqueous solution

Water is the main solvent inside all cells, in blood, and in plant sap. The positive sodium ions of NaCl (salt) attract the oxygen ends of the water molecules. The negative chloride ions attract the hydrogen ends of the water molecules. The result is water molecules surround each ion, breaking the salt apart.

Acids, bases and pH In aqueous solutions the ions formed are: – positively charged hydrogen ions (H + ) – and negatively charged hydroxide ions (OH - ). – For the chemical processes of life to work a balance of H + ions and OH - ions is critical. Terms: An acid is a compound that donates H + ions to a solution.acid A base is a compound that removes H + ions from an aqueous solution.base

pH scale The pH scale describes how acidic or basic a solution is.pH scale The scale ranges from 0 (most acidic) to 14 (most basic) Each pH unit represents a tenfold change in the concentration of H + ions. For example, lemon juice at pH 2 has 10 times more H + ions than an equal amount of grapefruit juice at pH 3. Pure water has an equal amounts of H + and OH - ions It is neutral with a pH of 7. The pH of blood cells is 7.4

pH scale

Buffers Even a slight change in pH can be harmful to organisms. Many biological fluids contain buffers.buffers Buffers resist changes in pH by accepting H + ions when pH levels rise and donating H + ions when pH levels fall. Buffers

Ch. 4 Review Questions 4. A covalent bond forms when a. opposite ions attract. b. protons are shared between atoms. c. an electron is transferred from one atom to another. d. two atoms share a pair of electrons. 5. Substances formed by a chemical reaction are called a. reactants. b. products. c. chemical formulas. d. chemical equations. 7. A solution with a pH of 9 is a. acidic. b. basic. c. neutral. d. higher in H + ions than in OH - ions.

10. How are the number of electrons in an atom's highest energy level related to the reactivity of that atom? 11. How is a potassium atom (K) different from a potassium ion (K + )? 13. Explain what makes a water molecule polar. 15. Describe how the properties of water molecules contribute to the upward movement of water through a tall tree. 16. What is an aqueous solution? 17. How does a buffer keep a solution from becoming too acidic?

21. Analyzing Data Use the data table to answer. a. Which sample in the table is most acidic? b. Which sample listed has a pH that is closest to neutral? c. Which sample, normal rainfall or baking soda solution has a greater amount of hydrogen ions (H + )?