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 D
EQUIPMENT: You will need the following materials.
Raise your hand if you are missing any of these materials:
-
1 bottle labeled INDICATOR
-
1 bottle labeled BASE X
-
1 bottle labeled ACID A
-
1 bottle labeled ACID B
-
1 bottle labeled ACID C
-
3 plastic cups
-
1 placemat
-
Safety goggles
-
Paper towels for spills
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.
UNIVERSAL INDICATOR COLOR GUIDE
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
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.
To see how this is done, follow these steps.
Step 1: Gently squeeze 7 drops of Indicator into
Cup 1.
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.
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.
You will NOT be given directions to follow to answer
this question. Instead, use the materials you have been given
to plan your own experiment to tell which is stronger - Acid B
or Acid C.
3a. Now carry out your experiment. Use the space
below to make a table or chart to record the results and then
fill it in as you work.
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
|
|
|
3b. Write down and number the steps you followed
as you conducted your experiment. Be specific, so another student
in your class could conduct the experiment exactly as your did.
For example, if your mixed solutions together, tell how much of
each solution you used.
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:
4f. How do you know this?