Chapter 20 Water and pH

Chemistry of water Water is a POLAR molecule. This means it has covalent bonds that do not share electrons equally. The oxygen is slightly negative, the hydrogens are slightly positive.

Chemistry of Water Water molecules form HYDROGEN BONDS with other water molecules. This creates a weak bond, and a ”sticky” molecule

7 really cool properties of water Because of hydrogen bonds, water has 7 unique properties that other forms of matter do not: 1. Cohesion 2. Adhesion 3. Universal Solvent 4. Ice is less dense 5. High specific heat 6. Evaporative Cooling 7. Forms ions

1. Properties of water: cohesion Hydrogen bonds between water molecules creates cohesion, where water sticks to other water molecules  Water is sticky  Surface tension on lakes  Water bugs  Drinking water through a straw  Water moving up a tree

2. Properties of water: adhesion Water molecules can stick to other things because of hydrogen bonds, this is called adhesion  Water climbing up a paper towel  A meniscus in a graduated cylinder

3. Properties of water: universal solvent Water is a good solvent due to its polarity  Polar molecules surround positive and negative ions  Solvents dissolve solutes creating aqueous solutions

Hydrophilic Substances that are hydrophilic have an affinity for water They are also polar and dissolve easily Examples: milk, salts, sugar, vinegar

Hydrophobic Substances that are hydrophobic do not have an affinity for water They are also non-polar and do not dissolve Examples include oil, wax, and fat

4. Properties of water: Ice floats Ice is less dense than liquid water, so ice floats  Hydrogen bonds form a crystal with a loose structure and space between the molecules  Ponds don’t freeze solid in winter

5. Properties of water: high specific heat Water has high specific heat, it takes a lot of energy to change the temperature of water because of the hydrogen bonds  This helps moderate the temperatures on Earth

6. Properties of water: evaporative cooling Organisms rely on heat of vaporization (energy required to turn liquid to gas) to remove heat which is called evaporative cooling  As bonds are broken, heat is released into the environment (sweating keeps us cool)

7. Properties of water: forms ions Liquid water splits apart to form the cation H + and the anion OH - H 2 O ----> H OH If concentration of 2 ions is equal, water is neutral If [H + ] > [ - OH], water is acidic If [ - OH] > [H + ], water is basic pH scale = how acidic or basic a solution is

pH The pH is the absolute value of the exponent of the H + concentration. [H + ] =  pH = 7 [H + ] =  pH = 4 [H + ] =  pH = 12

pH Or, the pH is the absolute value of 14 minus the absolute value of the [OH - ] concentration [OH - ] =  pH= 4 [OH - ] =  pH= 11 [OH - ] =  pH= 12

pH scale pH of a neutral solution = 7 Acidic solutions = more H + ions, pH less than 7 Basic solutions= more OH - ions, pH greater than 7 Most biological fluids have pH 6 – 8  pH values in human stomach can reach 2 Each pH unit represents a 10-fold difference in H + & OH - concentrations.  small change in pH actually indicates a substantial change in [H + ] & [OH - ]

pH Lab Use the garbage cans!! Do the front side BEFORE you start your lab. You will be testing 10 substances:  1. isopropyl alcohol  2. vinegar  3. baking soda solution  4. 7-up soda  5. hydrogen peroxide  6. alka seltzer solution  7. pineapple juice  8. bottled water  9. tap water (get some from the faucet)  10. your spit