Forwards and Backwards
Your Activity
Learn about pH and the pH scale and about acids and base solutions.
Materials
Microplate
125-ml flask
Household grade ammonia
Two 150-ml dropper bottles
Vinegar
Universal Indicator
One 30-ml dropper bottle
pH Scale Chart
Create
1. Always wear safety goggles
2. Rinse the microplate and the flask
3. In the microplate wells, combine the following solutions
Well 1
No ammonia
10 drops vinegar
Well 2
2 drops ammonia
8 drops vinegar
Well 3
4 drops ammonia
6 drops vinegar
Well 4
6 drops ammonia
4 drops vinegar
Well 5
8 drops ammonia
2 drops vinegar
Well 6
10 drops ammonia
No vinegar
4. Add 1 drop indicator to each well. Compare the colors to those using the colors on the pH scale chart.
a. Is vinegar basic (high pH) or aciding (low pH)?
b. How about ammonia?
5. Add 75 ml water to the flask. Add 10 drops of indicator solution to the flask.
6. Add 1 drop ammonia to the flask; swirl to mix. Note the color change. Add 1 drop vinegar to the flas; swirl to mix. Note the
color change.
7. Add 2 more drops of vinegar. Can you get the color back to blue with 1 drop of ammonia? Two drops of ammonia?
Science Topics
Chemistry, Chemical Reactions
What’s going on?
The pH scale measures the amount of hydrogen ions in solution. A low pH or acidic solution has a high number of hydrogen
ions. A high pH or basic solution has a low number of hydrogen ions. Both strong acids (low pH) and strong bases (high pH)
are corrosive. Chemical reactions go forwards and backwards depending on the chemical environment, such as the pH level. In
this experiment, an indicator reacts with hydrogen ions in solution to produce different colors. The changing colors identify
the changing pH levels.
Forwards and Backwards
Activity Lead Notes
Introduction
This activity is an extension of “Acids and Bases” activity where the students used the red cabbage indicator solution to
test whether a substance is acid or base. In this activity, students prepare six solutions combining vinegar and ammonia
that range incrementally from acid (all vinegar) to base (all ammonia). They add an indicator to each solution and
compare the resulting colors to those on a pH scale chart. Students then use a flask containing water and indicator
solution. They add drops of ammonia and vinegar, experimenting with the number of drops it takes to change the color
of the water (indicating pH) from acid to base and back again.
Learning Objectives
Students learn about pH and the pH scale and about acid and base solutions. They learn that they can test for pH with
an indicator solution. They also learn that some chemical reactions are reversible.
Background
The pH scale measures the amount of hydrogen ions in solution. A low pH or acidic solution has a high number of
hydrogen ions. A high pH or basic solution has a low number of hydrogen ions. Both strong acids (low pH) and strong
bases (high pH) are corrosive.
Chemical reactions go forwards and backwards depending on the chemical environment, such as the pH level. In this
experiment, an indicator reacts with hydrogen ions in solution to produce different colors. The changing colors identify
the changing pH levels.
The word “acid” comes from the Latin word for “sour”. Vinegar is sour, so is citric acid in lemons and oranges.
For thousands of years people have known that vinegar, lemon juice and many other foods taste sour. However, it was
not until a few hundred years ago that it was discovered why these things taste sour - because they are all acids. The
term acid, in fact, comes from the Latin term acere, which means "sour". While there are many slightly different
definitions of acids and bases, in this lesson we will introduce the fundamentals of acid/base chemistry. In the
seventeenth century, the Irish writer and amateur chemist Robert Boyle first labeled substances as either acids or bases
(he called bases alkalies) according to the following characteristics:
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Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and become less
acidic when mixed with bases.
Bases feel slippery, change litmus blue, and become less basic when mixed with acids.
Materials
The materials are required for each student/group. You can also make it a completion, so the child with the most
inflated balloon at the end of the class wins a prize.
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One microplate – six wells across
125-ml flask
Household grade ammonia
Two 150-ml dropper bottles
Vinegar
Universal Indicator
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One 30-ml dropper bottle
Preparing Materials
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Label the 150-ml dropper bottles “Vinegar” and “Ammonia”.
Label the 30-ml dropper bottle “Universal Indicator”.
Make sure the microplate and flask is clean.
Daily Setup
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Set out the activity instructions in a sign holder
On the table, set out the following:
o Labeled dropper bottles with ammonia, vinegar and universal indicator
o Microplate
o Flask
Refill all chemical bottles
Ammonia and vinegar may be diluted with a small amount of water
Use the universal indicator at full strength
Procedure
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Ask kids to identify few materials as acids, bases or neutral
Ask kids to follow the steps in the activity instruction.
Takedown
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Clean the flask and microplate.
Return the bottles to the storage.
Discuss Student Observations
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What is the pH level of neutral solution? (7)
What is pH of less than 7? (acidic)
What is pH of more than 7? (basic)
Extensions
http://chemistry.about.com/od/demonstrationsexperiments/ss/disappearink.htm
http://chemistry.about.com/od/chemistryhowtoguide/ht/colorbubble.htm
Source
Oregon Museum of Science and Industry – Experiencing Chemistry – Part 1
Forwards and Backwards
pH Scale Chart
Forwards and Backwards
Student Worksheet
Describe the solution in each of following as: 1) Acid 2) Base 3) Neutral
A. Soda ___________________
B. Soap ___________________
C. Coffee _________________
D. Wine __________________
E. Water _________________
F. Grapefruit ______________
G. Milk ___________________
H. Baking Soda _____________
Answers: A1, B2, C1, D1, E3, F1, G1, H2