PALS - Task with Student Directions

8th Grade Performance Task
Contributed by: RAND
Designed by: University of California, Santa Barbara, Stanford University, and The Far West Laboratory

EQUIPMENT: You will need the following materials. Raise your hand if you are missing any of these materials:

Every solution is an acid, a base, or neutral. Acids and bases are chemical opposites of each other. Solutions that are neither acids or bases are neutral. Chemists use numbers to indicate the strengths of acids and bases. The numbers go from 1 to 14. Strong acids have low numbers and strong bases have high numbers. Neutral solutions are in the middle.

Chemists use a solution called Universal Indicator to identify acids and bases. Universal Indicator changes color when mixed with an acid or base. The Universal Indicator Color Guide shows that Universal Indicator turns red when it is added to a strong acid, it turns purple when it is added to a strong base, and it turns a yellowish-green when it is added to a neutral solution.

All acids in the range of 1 to 4 turn the indicator red. All bases in the range of 11 to 14 turn the indicator purple. Today you will learn how to test if one acid is stronger than another even if they both turn the indicator the same color.

1a. Which acid is stronger - one that turns Universal Indicator orange or one that turns Universal Indicator yellow?

1b. Which base is stronger - one that turns Universal Indicator blue or one that turns Universal Indicator purple?

Read the directions carefully and follow all the steps. Then answer the questions clearly and completely.

The color of the Universal Indicator can be used to show what happens when an acid and a base are mixed. If you mix the right amounts of an acid and a base you can obtain a neutral solution (yellowish-green color). Because the solution becomes neutral, chemists say acids and bases can neutralize one another.

Step 2: Add 7 drops of Base X to the solution in Cup 1. Gently swirl the cup to mix the solutions, observe the color, and record the color on the line for 0 drops in Table 1.

Step 3: Add 1 drop of Acid A to the solution in Cup 1. Gently swirl the cup to mix in the acid, observe the color, and record the color on the line for 1 drop in Table 1.

Step 4: Continue adding 1 drop of Acid A at a time to the solution until you have added 8 drops. After each drop, swirl the solution, observe the color, and record the color in Table 1.

DROPS OF ACID A ADDED	COLOR OF SOLUTION
0 (Base X + Indicator only)
1
2
3
4
5
6
7
8

2a. Look at the sequence of colors in Table 1. Compare it to the Universal Indicator Color Guide on your placemat. How did the solution in Cup 1 change as drops of Acid A were added?

2b. How many drops of Acid A did it take to turn the solution yellow? __________

2c. Suppose you conducted the same experiment with a stronger acid and recorded the colors in a table. How would the results in this new table be the same or different than those in Table 1? Explain your answer.

PART 3: TESTING ACID STRENGTH

In Part 1 you learned how to neutralize a base with an acid. Now you need to use this procedure to find out WHICH ACID IS STRONGER - ACID B OR ACID C.

Use Cup 2 and Acid B, as follows:

Step 1: Put 7 drops of Indicator into Cup 2
Step 2: Add 7 drops of Base X to the solution in Cup 2, observe the color, and write it in Table 2.
Step 3: Add 1 drop of Acid B, mix the solution, and write the color in Table 2
Step 4: Continue adding Acid B, drop by drop, until you have added 8 drops. After each drop mix the solution, observe the color, and write it in Table 2.

Use Cup 3 and Acid C, as follows:

Step 5: Put 7 drops of Indicator into Cup 3
Step 6: Add 7 drops of Base X to the solution in Cup 3, observe the color, and write it in Table 2.
Step 7: Add 1 drop of Acid C, mix the solution, and write the color in Table 2
Step 8: Continue adding Acid C, drop by drop, until you have added 8 drops. After each drop mix the solution, observe the color, and write it in Table 2

Table 2

DROPS OF ACID ADDED	COLOR OF SOLUTION - USING ACID B	COLOR OF SOLUTION - USING ACID C
0 (Base X + Indicator only)
1
2
3
4
5
6
7
8

PART 4: ANALYZING RESULTS

4a. Fill in the following blank spaces based on the results of your experiment.

______ drops of Base X + ______ drops of Acid B = neutral solution

______ drops of Base X + ______ drops of Acid C = neutral solution

4b. Which acid is stronger - Acid B or Acid C?

4c. How do you know this?

4d. Vinegar is a household acid that can be used in cooking. Most vinegars in the United States are diluted to make a 5% solution, but vinegars in Europe are made into a slightly stronger 6% solution. Chef Andrea uses both types of vinegars in her restaurant. There are three bottles of vinegar on her shelf. One is marked 5% and one is marked 6%. The label has fallen off the third bottle of vinegar. She wants to know if the third bottle is 5% or 6% vinegar. How could she use the equipment you used to figure out the strength of the vinegar in the third bottle?

4e. Sally has a bottle of Base Y and a bottle of Base Z. To find out which base is stronger:

she put 7 drops of Base Y, 7 drops of Indicator, and 10 drops of Acid B into a cup. The solution in the cup turns red.
Into a new cup she put 7 drops of Base Z, 7 drops of Indicator, and 10 drops of Acid B. The solution in this cup turns yellow.

Which base is stronger - Base Y or Base Z?

4f. How do you know this?