Acids and Bases pH Scale Acids are compounds that break into hydrogen (H+) ions and another compound when placed in an aqueous solution. Bases are compounds that break up into hydroxide (OH-) ions and another compound when placed in an aqueous solution. If you have an ionic compound and you put it in water, it will break apart into two ions. If one of those ions is H+, the solution is acidic. If one of the ions is OH-, the solution is basic. There are other ions that make acidic and basic solutions, but we will not learn about those now. That pH scale is actually a measure of the number of H+ ions in a solution. If there are a lot of H+ ions, the pH is very low. If there are a lot of OH- ions, that means the number of H+ ions is very low, so the pH is high. In this lab you will be using litmus paper to indicate the presence of acids or bases in two well-known substances (hydrochloric acid HCl, and sodium hydroxide NaOH) and in the following common household substances. Vinegar Ammonia Laundry detergent Lemon juice Clear soda Baking soda Distilled water 1. Record the properties/characteristics of acids and those of bases. 2. Note that all of these substances (not distilled water) are in solution. Based on your previous textbook readings, explain why these substances need to be in solution. 3. Research each of the substances and use that information to predict the pH of each. SAFETY PRECAUTIONS: WEAR GOGGLES, PULL HAIR BACK, NO TASTING OR SMELLING, RINSE SKIN IF IT COMES INTO CONTACT WITH HCl or NaOH. 4. Use your pipettes to put several drops of each of the substances into a separate well in the Chemplate. Be careful to NOT allow them to mix; this will contaminate your substances and invalidate your findings. Make sure that you keep record of which substance is in which well. 5. Use your pH paper to test each substance. Compare the color of your pH paper to the chart on the pH paper container. Compare these results to your predictions. 6. Which of the substances is the strongest acid? The strongest base? Neutral? Check the conductivity of each acid. Record your results. 7. Predict what would happen if you put zinc in HCl. Using the forceps (tweezers), take a single piece of the zinc (Zn) and place it in the HCl. Observe for 5 minutes. Note the results. Do the same for ONE of the other acidic solutions. 8. Obtain another sample of HCl in another well. Predict what would happen if Mg was placed in the HCl. Using the forceps, take a single piece of magnesium (Mg) and place it in the HCl. Observe for 5 minutes. Note your results. Repeat using ONE of the other acidic solutions. 9. Obtain another sample of HCl in another well. Predict what would happen if you put Fe in the HCl. Using the forceps, take a small amount of iron (Fe) and place it in the HCl. Observe for 5 minutes. Note your results. Repeat using ONE of the other acidic solutions. Neutralization Reactions: Research what happens when acids and bases react. http://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/Acid%2F%2FBase_Reactions/Neutralization Example: HCl + NaOH → H2O + NaCl Neutralization Reaction: 1. Make a small solution of baking soda and water. Test the pH of the solution using pH paper. Record your results. Test the pH of the vinegar using pH paper. Record your results. Add a small amount of the vinegar into the baking soda (stir to combine) Test the pH using litmus paper. If it does not test neutral, then add small amounts of vinegar and test with pH until the solution tests neutral. What were the indicators that a chemical reaction occurred? 2. Use your knowledge of the pH of each of the substances and predict which base would best neutralize which acid. 3. Research your predictions and note what you find. (We may try some of these in class) 4. What is Alka Seltzer and what is it used for? Predict whether it is an acid or a base. Place one tablet in a cup of water. Test the pH of the solution AFTER the fizzing stops. Record your findings. Were your findings what you expected? Read the ingredients on the package. Do you find anything unexpected? Let’s discuss this.