How To Understand Chemical Chemistry

Transcription

1 CHEMISTRY: Identifying Acids and Bases with Red Cabbage Indicator By Darby Sloss and Marianne Smith Edited by Anne Starace Abstract Chemistry is an important part of our lives. Kitchen Chemistry uses some common household items to introduce a few basic chemical concepts. This module demonstrates a method of determining if a substance is acidic or basic. Keywords Chemical, molecule, compound, chemistry, chemical reaction, element, acids, bases, ph Funded by the National Science Foundation and the University of Nebraska

2 Content Standards K History & Process Standards K Skills Used/Developed: 2

3 Table of Contents I. OBJECTIVES 4 II. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS 4 III. SAFTEY 4 IV. LIST OF MATERIALS 4 V. INTRODUCTION 5 VI. PROCEDURE 7 VII. SAFETY 7 VIII. FREQUENTLY ASKED QUESTIONS 8 IX. TROUBLESHOOTING 8 X. HANDOUT MASTERS 8 XI. REFERENCES 8 Everything in the world is made of CHEMICALS! One of the most amazing things about chemicals is the way they can join together to make different new chemicals. That s why chemicals can make up all the zillions of different things in the world! WonderScience magazine 3

4 I Objectives Students will: - learn basic properties of acids and bases. - discover the approximate ph of some common items. II. Level, Time Required and Number of Participants Level: This activity is intended for grades Older participants can help make and use the indicator. Time Required: Preparation requires asking the facility manager about disposal of acids and bases, purchasing red cabbage and filling the pitcher with water. The activity requires 10 to 45 minutes. Number of Participants The activity is appropriate for groups of 5-25 people. III. List of Materials 2-3 leaves of red cabbage Pitcher of water Blender 6 beakers (250mL to 1000mL sizes will work fine) Hydrochloric acid (HCl) solution Sodium Hydroxide (NaOH) solution Vinegar The Works TM toilet bowl cleaner Drano TM Strainer Safety goggles Boric acid Baking soda IV. Safety Never allow anyone to taste any of the chemicals used in this module. Wear goggles. Don t let participants get too close unless they too are wearing goggles. 4

5 If a chemical comes into contact with skin, thoroughly wash the affected area with soap and water. If a chemical gets into eyes, thoroughly flush eyes with running water. See a doctor as soon as possible. If NaOH or any other strong base gets spilled on the floor, sprinkle it with boric acid, or some other weak, powdered acid, to neutralize it before wiping it up. (Boric acid is slightly toxic; do not ingest) If HCl or any other strong acid gets spilled on the floor, sprinkle it with baking soda, or some other weak, powdered base, to neutralize it before wiping it up. If a chemical is spilled on the floor, it should be wiped up using a dry towel and then washed with soap and water. Be careful not to get the chemical on yourself. HCl can make holes in clothes and paper. NaOH is very harmful to skin and burns on contact. HCl solution means that the HCl is mixed with water. Likewise for NaOH solution. Don t use a highly concentrated solution of HCl or NaOH. High concentrations are more dangerous than low concentrations. Do not get pure NaOH and HCl and mix it with water yourself. Instead, get solutions of NaOH and HCl. When acquiring HCl and NaOH solutions ask for 1 molar (molarity=1), it should be enough. Check with your facility manager about disposal of acids and bases. 5

6 V. Introduction The periodic table organizes and includes the all the known elements. For example, sodium (Na) and nitrogen (N) are elements. The smallest amount of an element is an atom. Only a few elements, called the noble gases, consist of individual atoms that move about independently of one another. In other words, the noble gases do not usually react with other elements. These elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Notice that all of these elements are in the same column. Other elements will usually react with each other, forming molecules. A chemical reaction is the breaking or forming of chemical bonds. A molecule is a group of atoms (2 or more) joined together by chemical bonds. Molecules move as a single unit. Molecules may consist of identical atoms, as in oxygen (O 2 ), or different atoms, as in water (H 2 O). The terms molecule, compound and chemical can be used interchangeably in this activity. A chemical bond is the force that holds two atoms together. For example, water is a molecule composed of one oxygen atom and two hydrogen atoms. The O hydrogen atoms are bound to the oxygen. Chemists use several different H H notations to represent a chemical bond. The most common is a line between two atoms. The molecular structure of water using lines to represent bonds is Figure 1 depicted in Figure 1. Chemical reactions: A chemical reaction requires the breaking and/or formation of chemical bonds. If bonds are not broken or formed, a chemical reaction has NOT occurred. For example, boiling water is water changing from the liquid phase to the gas phase (steam). Both liquid water and gaseous water (steam) are H 2 O, thus, boiling water is not a chemical reaction, it is a physical change. On the other hand, when baking soda (sodium bicarbonate) and vinegar (acetic acid) are mixed, a 6

7 chemical reaction does occur. Chemical bonds are broken and formed. Many chemical reactions result in changes that can be easily visualized, as with the reaction of baking soda and vinegar. Acetic Acid Sodium Bicarbonate Carbon Dioxide Sodium Acetate water (CH 3 COOH) (NaHCO 3 ) (CO 2 ) (CH 3 COONa) (H 2 O) Note that the O-H, O-C, and O-Na bonds were broken and the O-H, O-Na, and C=O bonds were formed. One line between two elements (eg O-Na) represents a single bond (one electron being shared) and two lines between two elements (eg C=O) represents a double bond (two electrons being shared). Acids and bases The above reaction is actually an acid-base reaction. Acids are compounds that donate a positive hydrogen atom (H + ), like acetic acid. Bases are compounds that take positive hydrogen atoms (H+), like sodium bicarbonate. When acids and bases are put together, they exchange hydrogen atoms and make neutral compounds such as water. Acids form solutions that have a ph less than 7 and bases form solutions that have a ph greater than 7. Neutral compounds have a ph of exactly 7. ph is basically, no pun intended, a measure of the concentration of positive hydrogen atoms in a substance. ph= -log [ H+] [H+] is the concentration of positive hydrogen atoms in moles (of positive hydrogen) per liter. 1 mole = (6.02)(10^23) One mole of H+ is (6.02)(10^23) hydrogen atoms. A mole is just a number, as is a dozen. One dozen rolls is 12 rolls. This measure of concentration is called molarity. Molarity=moles/liters 7

8 VI. Procedure Red Cabbage Indicator The purpose of this activity is to make an acid/base indicator and use it to determine if various things are acids or bases. 1. Place one leaf (yes, 1 leaf) of the red cabbage in the blender. 2. Fill the blender three-fourths full with water. 3. Put the lid on the blender & blend. 4. Strain the blended solution through the strainer into the largest beaker you have. 5. Divide the strained solution into six other beakers. Now that you have your indicator, you will use it to determine the approximate ph (which will tell you if the substance is acidic or basic) of the solutions. You may want to ask the participants if they think the solution is an acid or a base before you pour it into the indicator. 6. Pour one of the liquids (hydrochloric acid (HCl) solution, sodium hydroxide (NaOH) solution, vinegar, The Works toilet bowl cleaner, Drano) into each beaker one at a time. 7. Watch the color change and determine which liquids are acids and which are bases. Like litmus paper, red cabbage is an acid/base indicator. It should turn red or pink in acids (ph less than 7), turn green or yellow in bases (ph greater than 7), and turn blue in neutral (ph exactly 7, neither acidic nor basic) solutions. HCl, vinegar, and The Works are acids, so they should turn pink or red. NaOH and Drano are bases, so they should turn yellow or green. Cleanup Extra cabbage indicator can go down the drain. Check with your facility manager about disposal of the acids and bases. Rinse all dirty glassware with water. Clean blender thoroughly. VII. Frequently Asked Questions Q Isn t this chemical in something else? A The chemicals in the module are used for many things. For example, people have hydrochloric acid (HCl) in their stomachs to digest food. However, drinking HCl is very harmful because it puts too much HCl in the stomach and causes tooth decay. Q A Can I touch or taste that? No one should intentionally touch or taste any of the liquids, solids or gases in this presentation. Q. Where do I obtain HCl and NaOH solutions? A. From a chemical supply store (try or you may be able to acquire some from a high school chemistry teacher. 8

9 Q. What other, easier to find, acids and bases can I use instead? A. Baking soda is a weak base. Vitamin C is an acid. For this activity, you will need to dissolve the baking soda in water, preferably distilled water. Crumble some vitamin C tablets and dissolve those in water. Dissolve as much as you can in the water to make the color change more noticeable. VII. Troubleshooting IX. Handout Masters X. References For general chemistry information: Chemistry by Steven S. Zumdahl For another description of the cabbage indicator activity: ml For a variation of this demonstrations see the Kitchen Chemistry Script 9