Chemistry 9th 2017-2018 ACIDS, BASES, AND NEUTRALIZATION Instructions: Copy the table for question 1 before you start reading, and fill it in as you read. Have you ever listened to advertisements for medicines for "acid stomach," or telling you to buy pH balanced shampoos? These commercials are referring to a property of substances that classifies them as acids, bases (also called alkalis) or neutral substances. At first chemists based their classification of substances on properties they could test with their senses, including taste and touch. Acids taste sour, while bases taste bitter. Taste some orange, lemon or lime juice. These taste sour because they contain citric acid; the more acid, the more sour. Which one do you think has the most acid in it? Try some vinegar in water. Is it sour? Vinegar is acetic acid. Take a pinch of baking soda and put it on your tongue. Is it an acid or a base? . Do you dare put your tongue on a wet bar of soap? Yuck! Old-time soap makers used to taste the soap mix to decide when the amount of alkali or base was just right. How much would they have to pay you to be an old-fashioned style soap maker? Bases tend to have a slippery feeling on your skin (like soap). Rub some baking soda in water between your fingers and you can feel it. Acids tend to just feel watery (unless they are very strong, in which case you don't want to feel them, as they will burn your skin off). Later chemists based their classification on the chemical reactions and structure of acids and bases. They used some chemical substances called indicators, which have different colors when they are in contact with acids than when they are in contact with bases. Indicators include chemicals such as litmus, phenolphthalein, bromthymol blue, and methyl orange. Chemists classified chemicals that turned blue litmus to red as acids, those that turned red litmus to blue as bases (alkalis), and those that did neither as neutral substances. When chemists started looking at the chemical formulas of substances that were acids or bases, they found that these other properties were based on the structure of the molecules. They found that most acids contain one or more hydrogen atoms in their molecules. These hydrogen atoms can split away from the molecule, leaving their electron behind to form "hydrogen ions" (remember ions are atoms or groups of atoms that have gained or lost electrons). The symbol for a hydrogen ion is H+. Substances such as HCl (hydrochloric acid) and H2SO4 (sulfuric acid) form hydrogen ions, so are acids. Many bases contain an oxygen atom with a hydrogen atom connected to it. These two can split away together, taking an extra electron with them, to form an "hydroxide Chemistry 9th 2017-2018 ion". The symbol for an hydroxide ion is OH-. Substances such as NaOH (sodium hydroxide), and Ca(OH)2 (calcium hydroxide) form hydroxide ions, so are bases. Neutral substances either do not form hydrogen or hydroxide ions, or form equal numbers of both. Water is a neutral substance. Its formula is H2O. It can split into one hydrogen ion and one hydroxide ion: H+ and OH- . When acids and bases get together the hydrogen ions from the acid combine with the hydroxide ions from the base to make water. Since water is neutral, this is called neutralization. The leftover parts of the acid and base molecules can also combine to form a substance called a salt. For instance, if equal amounts of hydrochloric acid and sodium hydroxide are mixed, they form water and the salt sodium chloride (table salt). If the amounts of acid and base are not equal then there will be some acid or some base left over also, and it will not be a complete neutralization. Some acid-base neutralization reactions are used in everyday life. Remember those "acid stomach" commercials? Antacids contain bases such as sodium bicarbonate (baking soda) which can help neutralize stomach acid (hydrochloric acid). Baked goods such as cakes, biscuits, and breads which use baking powder in their recipes use a neutralization reaction. Baking powder contains the base sodium bicarbonate and a dry acid. When mixed with water in a batter and heated, they neutralize each other and form carbon dioxide bubbles at the same time. These bubbles form inside the batter, making it light and fluffy. The strength of an acid or base is measured by the pH scale. This scale tells how many hydrogen or hydroxide ions the acid or base produces. The middle of the pH scale is 7. Seven is the neutral number, with equal amounts of hydrogen and hydroxide ions. Pure water has a pH of seven. pH numbers below 7 are acid, and above 7 are basic (or alkaline). The lower the pH of acids, the stronger the acid. An acid with a pH of 3 is ten times stronger than an acid with a pH of 4. The higher the pH of a base, the stronger the base. A base with a pH of 9 is ten times stronger than a base with a pH of 8. 0....1...2....3....4....5....6....7....8....9....10....11...12...13....14 acid neutral base You can use a commercial "pH paper" to determine the pH of a substance by dipping it in the substance, then matching the color with a set of colors for each pH. Many natural plant pigments can be used as indicators as well, especially those called anthocyanins, which give plants and flowers red and blue colors. Red cabbage juice Chemistry 9th 2017-2018 is one. It shows a range of colors depending on the pH of the substance it is in contact with. Can you find other natural plant pigments that would make good indicators? Study questions: Answer these in complete sentences. 1. Fill in this table: Properties of Acids and Bases Property Acids Bases Neutral Substances Taste Sour Bitter Neutral Feel Often watery unless they are very strong, in which case, they will burn your skin off. Slippery Watery Reaction with red litmus Stay red Turn blue Stay red Reaction with blue litmus Turn red Stay blue Stay blue Kind of ion Hydrogen ions Hydroxide ions Either they do not form hydrogen or hydroxide ions, or form equal numbers of both. pH numbers pH numbers below pH numbers above 7 7 7 2. Describe what happens in neutralization. When acids and bases get together the hydrogen ions from the acid combine with the hydroxide ions from from the base to make water. Since water is neutral, this is called neutralization. 3. What is a salt? Chemistry 9th 2017-2018 The leftover parts of the acid and base molecules can also combine to make a substance called salt. 4. What is an indicator? Chemical substances that change color when they are in contact with acids and with bases. The color determines which one of them it is. 5. neutral? How can the pH scale tell you if a substance is an acid, a base, or The strength of an acid or base in measured by the pH scale. This scale tells how many hydrogen or hydroxide ions the acid or base produces. Seven is the neutral number with equal amounts of hydrogen and hydroxide ions. pH numbers below 7 are acid and above 7 are basic. Challenge Question for an extra point: 1. Hair is naturally about pH 7. It is damaged if the pH goes below 3 or above 10. What do you think "pH balanced" shampoos do? They use the neutralization process to balance the pH. If the hair is acid, it will contain hydrogen ions and the shampoo will contain the same amount of hydroxide ions to repair the damage and vice versa.
