Chemistry 2015
Chemistry 2015
Data: pH Lab Name __________
Introduction The measure of the concentration of hydrogen ions in a solution is the pH. Solutions with a pH below 7 are acidic. Solutions with a pH above 7 are basic. A pH of 7 represents a neutral solution, one that is neither an acid nor a base. Pure water has a pH of 7. The pH scale is from 0 to 14. Solutions can be identified as being acidic or basic by testing them with certain indicators. Some indicators (pH test paper) can be used to determine the specific pH of a solution as its color changes within a specific range. Red and blue litmus papers indicate whether a solution is an acidic or a basic solution. Red litmus paper turns blue in a basic solution. Blue litmus paper turns red in an acidic solution. Test Chemical Universal pH paper
Color Red Litmus
Blue Litmus
pH H2 O
HCl
NaOH
Vinegar
NaCl(aq)
Purpose: To use various indicators to determine the pH of common substances. Procedure: Materials: Blue Litmus, Red Litmus, Universal Indicator, Reaction Surface, Cabbage, Goggles, Chemicals 1. Put on safety goggles.
2. Cut small pieces (2 cm) of blue litmus paper, red litmus paper, and pH paper and place them in separate petri dishes.
3. Put 2 drops of a solution in each of the wells of the first row in the well tray. Write the name of
the solution in the 1st column of the data table. 4. Use other solutions to fill the second third and fourth rows. Write the name of the solutions in the 1st column of the data table (next page).
5. Insert a piece of blue litmus paper into the first solution in the first column. Immediately observe
the color of the paper. Record the color by shading the squares with colored pencils.
6. Repeat step 5 for the other solutions.
7. Repeat step 5 using red litmus paper in the second column of the well tray.
8. Repeat step 5 using pH paper in the third column of the well tray. Use the color chart to
…….determine the pH number of each solution and put this number in the appropriate spot on the …….data table. 9. Repeat step 5 using the cabbage juice. See directions on the back.
NaHCO3(aq) Dish Soap
Alcohol
Ammonia
pH scale ___________________________________________________________ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Post‐Lab Questions Cabbage Juice Indicator Chemistry 2015
1. Why must you use clean glassware and other lab equipment in experiments involving
pH?
2. What does pH measure?
3. What is a neutral solution? What is the pH of a neutral solution?
Chemistry 2015
Red Cabbage pH Indicator Name _________________ Red cabbage turns different colors depending on the pH of the soil in which it grows. That is because it contains a pigment molecule called anthocyanin which turns different colors depending on the pH. This water‐soluble pigment is also found in apple skin, plums, poppies, cornflowers, and grapes. Very acidic solutions will turn anthocyanin a red color. Neutral solutions result in a purplish color. Basic solutions appear in greenish‐yellow. By extracting the anthocyanins from cabbage, or other fruits or vegetables, it is possible to determine the pH of a solution based on the color it turns. Pupose: To make a pH indicator using red cabbage. 4. What substances did you find to be neutral?
Procedure: Materials: red cabbage, scissors, beakers, hot plate 5. Is the pH of hydrochloric acid greater than or less than distilled water?
1.
2.
3.
6. Which solution was the most acidic?
4.
5.
6.
7. Which solution was the most basic?
7.
8. If you chose one indicator to use in a series of experiments which one would you use?
Why is this indicator (previous question) the best choice?
9. Is the pH of a strong base higher or lower than the pH of a strong acid?
Cut the cabbage into tiny pieces until you have about a golf ball ‐ size clump of finely
chopped cabbage.
Place the cabbage in a beaker and add about 20 mL of water to cover the cabbage.
Heat the cabbage until the liquid is a dark purple color. This should take about 10
minutes. Don’t let it boil dry! This will crack the beaker. This liquid is at about pH 7. (The exact color you get depends on the pH of the water.)
Pour your indicator into a small beaker or vial. Using the reaction plates, place a few drops of each test chemical in a well and add a
few drops of indicator.
Using colored pencils, shade in the color for each pH value below. You may have to
infer the pH of untested values. For example: Let’s say you don’t test an item with a pH
of 4. If pH 2 is red and 6 is purple‐red, make 4 intermediate between those two colors.
Extra Credit: ‐‐Due May 7th (Same day as pH Lab) Make your own pH paper strips using
red cabbage. Take filter paper (or coffee filter) and soak it in a concentrated red cabbage juice solution. After a few hours, remove the paper and allow it to dry (hang it by a clothespin or string). Cut the filter into strips and use them to test the pH of foods
at home. You must attach your test strips to the paper for credit. Test ONLY food and
household substances that are safe to touch. (10 Lab points.)
pH Cabbage Indicator Key
10. Is the pH of a stronger acid higher or lower than the pH of a weaker acid?
2 4 6 8 10 12