Water, Acids and Bases Mr. Halfen Nov. 2016

Water Water is a unique and vital liquid. Humans are 60% water. Most living things require water to live, either living in it or drinking it. Our blood is mostly water. Water is made hydrogen and oxygen – H2O.

Structure of Water Many molecules are symmetric. One might expect a water molecule would be an oxygen between 2 hydrogens in a straight line. Instead, the molecules in water form a triangle. This results in water having charged parts. The oxygen corner is slightly negative and the hydrogen side is slightly positive. Thus water is called a “polar molecule.”

Water O H H

Properties of Water Water has a high surface tension due to hydrogen bonding. Many liquids form “films” on surfaces (salad oil or motor oil). Water tends to form drops. Both ionic and covalently bonded substances dissolve in water, hence the nickname “universal solvent.”

Water Molecules and Hydrogen Bonds

Acids Acids form hydronium ions in water. Hydronium ions: H3O+1 (H+ combined with H2O) Examples of acids are hydrochloric acid (HCl), acetic acid (vinegar) and lemon juice.

Hydronium O H H H

Properties of Acids pH < 7.0 Taste Sour React with metals to give off H2 (g) Turn blue litmus paper red Turn phenolphthalein colorless Universal Indicator is yellow to red Acids conduct electricity

Bases Bases form hydroxide ions in water. Or a base is a substance that reacts with an acid to form only a salt and water. Alkalis are soluble bases. Hydroxide is OH-1 (water missing a H+) Examples of bases include baking soda, ammonia, bleach and lye.

Hydroxide Ion O H

Properties of Bases pH > 7.0 Taste bitter and feel slippery Do not react with metals Conduct electricity Turn red litmus paper blue Turn phenolphthalein pink Universal Indicator is blue to purple

Measuring Acidity Whether a solution is acid or base is determined by its pH. The pH scale runs from 0 – 14. Pure water has a pH of 7.0 which we call neutral. Acids have a pH < 7. Bases have a pH > 7.

Oxides Reacting an oxide with water can produce an acid or a base. Non-metal oxides typically form acids. Metal oxides typically bases. Amphoteric oxides react to form a salt and water. ZnO and Al2O3 are the most common amphoteric oxides.

Weak and Strong Not all acids and bases are created equal. We consume soda and salad dressing without consequence, but both are acidic! Strong acids dissociate completely; weak acids do not; some of the original substance Is still present. The same applies to weak and strong alkalis.

Salts Salts are substances formed from the reaction of an acid and a base, e.g. NaOH + HCl  H2O + NaCl Salts can also be formed by reacting metals with acids. H2SO4 + Zn  ZnSO4 + H2 When salts are dissolved in water, the solution conducts electricity.

Work Worksheets Lab – Acids, Bases & pH Read Chap. 5, except 5.9 & 5.10 (opt.) p. 140- 1 & 3; p. 141 - 3 p. 143 - 1 & 3; p. 151 - 1 & 3 p. 153 - 1, 2, & 3 Exam-type 2, 5, 6, 10, & 11