(http://www.miamisci.org/)
(www.About.com)Objectives:
• to test household solutions with litmus paper
• to test household solutions using red-cabbage juice indicator
• to classify household substances as acids or bases
• to determine the pH range of red cabbage juice
•
Materials: (per group or station)
• Beaker of Red Cabbage Juice – (CHOP one large red cabbage into small pieces. Note: Blackberries, red onions, or even hibiscus flowers can be used as a substitute. SIMMER the cabbage pieces until the water turns a deep shade of purple. ALLOW the water to cool. REFRIGERATE when not in use.)
• Red and Blue Litmus Paper
• 6 Plastic cups
• Beakers of the following solutions: water, baking soda, vinegar, salt, sugar, lemon juice, soda, etc...
• Plastic spoon
• black marker
Motivation: How can we determine if a solution is an acid or a base?
What is the color of an acid solution when tested with a litmus paper? What about the bases? (assuming that students was already introduced to the topic)
(draw series of idea from students, write their idea on the board).
Can we still get the same result when we use homemade acid-base detector?
Activity: Procedure
1. Label your plastic cups #'s 1-7
2. Choose one solution and pour a small amount into plastic cup #1.
3. Write the name of the solution in your data table.
4. Place a drop of the solution onto red and then blue litmus paper.
Record results
5. Add 1 - 2 tablespoons of red cabbage juice to the cup. Record color change.
6. Determine if the solution is a acid, base or neutral
7. Repeat with solutions #2-7.
8. Clean up. Pour contents into sink and throw out used cups.
Observations:
Plastic Cup # Solution Red Litmus Blue Litmus Red Cabbage
Juice Acid/Base or Neutral
1 Water
2 Baking soda
3 Vinegar
4 Salt
5 Sugar
6 Lemon juice
7 Soda
Follow Questions:
1. Name the acids, name the bases, were there any neutral solutions?
2. What color did the cabbage juice turn to indicate a neutral substance?
3. Why is it important to use both blue and red litmus paper to determine pH?
Assimilation/Discussion:
Red cabbage water is an indicator of pH. Although it can't be used to determine exact pH, it can distinguish between acid (pH of 0 to 6), neutral (pH near 7), and base (pH of 8 to 14).
acid neutral base
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
In this activity:
• If the indicator turns the solution red or pink, the solution is an acid.
• A purple solution indicates that it is neutral, neither an acid nor a base.
• If the indicator turns the solution blue or green, the solution is a base.
Conclusion: ________________________________________________________________________
________________________________________________________________________________________________________________________________________________________________________________________________________________________
Assessment:
1: Which of the following statements is true concerning acids and bases?
acids and bases don't react with each other
acids mixed with bases neutralize each other
acids mixed with bases make stronger bases
acids mixed with bases make stronger acids
2: Which is the correct set of acid properties, as described by Boyle:
sour taste, corrosive, change litmus from red to blue
sour taste, corrosive, change litmus from blue to red
sweet taste, slippery, change litmus from blue to red
sour taste, slippery, change litmus from blue to red
3: Neutral solutions have a pH of:
0
7
14
4: Which is true?
pH of less than 7 is basic; pH of more than 7 is acidic
pH of less than 7 is acidic; pH of more than 7 is basic
5: Vinegar, fruit juice, and cola are examples of:
strong acids
weak acids
strong bases
weak bases
Assignment: What happens if two or more of the substances are combined? Once a solution has become green or pink, can you make it purple again?
Answers to the Assessment: 1: acids mixed with bases neutralize each other; 2: sour taste, corrosive, change litmus from blue to red; 3:7; 4: pH of less than 7 is acidic and pH of more than 7 is basic; 5: weak acids
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