Measuring pH

1. Measuring pH Vanderbilt Student Volunteers for Science Spring 2016

2. I. Introductory Demonstrations: Mystery Liquids • Tell the students you have some mystery liquids. • Pour the contents of the bottles labeled A, W, and B into clear glasses (first, second, third, respectively). • Pour all the cabbage juice from bottle #1 into the fourth glass. • Test each glass of clear liquid by dipping first the red and then the blue litmus paper into the glass. • Tell the students to record the changes. • Hold each glass of clear liquid up for the students to observe as you pour cabbage juice into each. • Pour until the color is vibrant. • Ask students to record the color changes. • The cabbage juice was the indicator used to cause this color change. • Indicators are substances that change colors when mixed with an acid or base. • The liquids turned different colors because one is an acid, one is a base, and one is a neutral. W = water A = vinegar B = window cleaner

3. II. Discussion of Acids and Bases • Ask students: Can you name some common acids? • Responses may include: references to battery acid, acid indigestion, stomach acid, acid rain, citrus acid, and chemicals in a lab • Ask students: Can you names some bases? • Common bases include soaps, detergents

4. II. Discussion ofAcids and Bases (cont.) Acids Bases Bases taste bitter and feel slippery Some bases are used to settle upset stomachs Detergents and many cleaning solutions are basic Strong bases can burn the skin • Natural acids in food give sour, sharp flavor • Strong acids can burn your skin • Many acids are corrosive • Some acids can be helpful (e.g. stomach acid) • Two acids (sulfuric acid and nitric acid) cause damage in acid rain

5. III.Explanation of pH Scale • Explain the pH scale (refer to manual for detailed explanations) and discuss the chart.

6. IV. Making a Color pH Scale Using Solutions of Known pH • Put the chart on the board • Organize the jars in pH 1-14 order • All liquids are clear, but have different strengths of acidity or basicity • Start with pH 1 bottle and add cabbage juice until color is strong enough to see. • Continue adding cabbage juice to the other jars, moving upward from pH 2-pH 14 Colors of Known pH Solutions

7. IV. Making a Color pH Scale Using Solutions of Known pH • Have them look at their chart to confirm the colors and record the colors on the table on the board. • Tell students that the pH 1 and 14 are the extremes of the pH scale. For every pH between 1 and 14, there is a unique color of red cabbage juice indicator. • The students now have a pH scale to use as a reference guide in determining the pH of their household solutions. • Ask students: • When red cabbage juice is used as the indicator, what color do the acid solutions turn? shades of pink or red. • When red cabbage juice is used as the indicator, what color do the basic solutions turn? shades of blue or green. pH 12 and 14 then turn yellow.

8. V. Testing household products • Tell students that they will test several household items to determine if they are acidic, basic, or neutral. • Put the Testing Household Items chart on the board (leave 2nd and 3rd column blank) • Student #1: add several squirts of cabbage juice to 6 of the wells in the 1st row and first well in the 2nd row. • Student #2: add a squirt of each household liquid to the well, following the order on the Instruction sheet. • Record the following: • Whether the product is an acid, base or neutral • Estimate the closest pH value using the colored pH standards at the top of the well plate.

9. VI. Analyzing Introductory Demonstrations • Get the three glasses of mystery liquids • Ask students whether the liquids are acids, bases, or neutral. Then ask students to guess what they are (A: vinegar-acid, W: tap water-neutral, B: window cleaner-base). W = water A = vinegar B = window cleaner

10. VII. Magic Foam • Place the cut-off bottle in the center of the clear plastic container. • Pour all of container of red cabbage juice into the tall bottle. • Have another VSVS volunteer carefully hold the plastic container with the bottle so all students can see the experiment. • NOTE: Be sure to hold the container with the bottle level since foam will come over the top of the bottle. • Add ALL of milk of magnesia (shake before adding) to cabbage juice. Slowly swirl the bottle. Ask students what color the cabbage juice is now (green)

11. VII. Magic Foam (cont.) • Add half of the sodium hydrogen sulfate solution and swirl. Be sure students note the color change (changes to red). • Add two spoonfuls of the solid sodium carbonate solid. The foam will form quickly and reach the top of the bottle and probably overflow. The solution will change colors, showing blue, purple, and pink at different places in the foam. • Add the remaining acidic (sodium hydrogen sulfate) solution and swirl. • NOTE: EYE OR SKIN CONTACT WITH THE CHEMICALS SHOULD BE AVOIDED. IF CONTACT OCCURS, IMMEDIATELY FLUSH WITH WATER. • CLEANUP: One of the VSVS volunteers should take the magic foam demonstration bottle to the nearest sink and pour the contents out. Wash down the drain with lots of water and rinse out the bottle. Be sure to return the bottle with the kit.

12. Review • What scale is used to determine how acidic or basic a dilute solution of an acid or base is?pH scale • How is this scale useful to scientists? It provides a comparison that is universally recognized, not only by scientists but by the informed public. • On the pH scale, what numbers represent acids? 0 – 6 • On the pH scale, what number represents a neutral? 7 • On the pH scale, what numbers represent bases? 8-14 • When using red cabbage juice as the indicator, what color do acidic solutions turn? shades of pink or red • When using red cabbage juice as the indicator, what color do base solutions turn? shades of blue and green, except pH 13 -14 which turns yellow • Is there an advantage to using red cabbage juice instead of litmus paper to test acids and bases? Yes, red cabbage juice has a range of color changes that indicate the range of acidity or basicity.