8th Grade Performance Task
Contributed by: RAND
Designed by: University of California, Santa Barbara, Stanford
University, and The Far West Laboratory
Acids and Bases - Form T
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.
Strong Acid
|
Weak Acid
|
Neutral
|
Weak Base
|
Strong Base
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
11
|
12
|
13
|
14
|
Red...... |
............ |
Red
|
Orange
|
Yellow
|
Yellowish
Green
|
Green |
Blue |
|
Purple |
........ |
..................................Purple |
|
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.
PART 1: READING THE SCALE
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?
PART 2: NEUTRALIZING ACIDS AND BASES
An eighth grade student named Tony conducted an
experiment using the following equipment:
- 1 bottle labeled INDICATOR
- 1 bottle labeled BASE X
- 3 plastic cups
- 1 bottle labeled ACID A
- 1 bottle labeled ACID B
- 1 bottle labeled ACID C
Read Tony's experiment carefully. 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.
To see how this is done, Tony followed these steps:
Step 1: Tony put 7 drops of Indicator into Cup 1. The color was
yellowish-green.
Step 2: Tony added 7 drops of Base X to the solution in Cup 1.
The color turned purple. He wrote the color in Table 1.
Step 3: Tony added 1 drop of Acid A, mixed the solution, observed
the color, and wrote it in Table 1. He continued adding Acid A,
drop by drop, until he had added 8 drops. After each drop he mixed
the solution, observed the color, and wrote it in the table. Here
is the color he saw after each drop:

Here is the information Tony wrote in the table:
Table 1
DROPS OF ACID A ADDED
|
COLOR OF SOLUTION
|
0 (Base X + Indicator only)
|
Purple |
1
|
Blue |
2
|
Greenish-blue |
3
|
Green |
4
|
Yellowish-green |
5
|
Yellow |
6
|
Orange |
7
|
Red |
8
|
Red |
2a. Look at the sequence of colors in Table 1. Compare
it to the Universal Indicator Color Guide on the first page. 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 AND STRENGTH
In Part 1 Tony learned a technique for neutralizing
a base with an acid. Next he did an experiment to find out WHICH
ACID IS STRONGER - ACID B OR ACID C.
First, he used Cup 2 and Acid B, as follows:
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 |
 |
Here are the results of Tony's experiment:
Table 2
DROPS OF ACID ADDED
|
COLOR OF SOLUTION - USING ACID B
|
COLOR OF SOLUTION - USING ACID C
|
0 (Base X + Indicator only)
|
Purple |
Purple |
1
|
Blue |
Bluish-purple |
2
|
Yellowish-green |
Blue |
3
|
Orange |
Bluish-green |
4
|
Orange-red |
Green |
5
|
Red |
Green |
6
|
Red |
Yellowish-green |
7
|
Red |
Yellow |
8
|
Red |
Orange |
3a. Which acid, B or C, took the most drops to turn
the solution orange?
______________________________
3b. About how many drops of Acid C would
it take to turn the solution red?
3c. Explain why you chose this number.
PART 4: ANALYZING RESULTS
4a. Fill in the following blank spaces based on
the results of Tony's 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 Tony 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 puts 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 puts 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?
|