Acids and Bases
Vanderbilt Student Volunteers for Science
Training Presentation
Fall 2015 VINSE/TnScore
Important!!!
• Please use this resource to reinforce your
understanding of the lesson! Make sure you
have read and understand the entire lesson prior
to picking up the kit!
• We recommend that you work through the kit
with your team prior to going into the classroom.
• This presentation does not contain the entire
lesson—only selected experiments that may be
difficult to visualize and/or understand.
I. Intro Demo—Defining acids and
bases
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Have 2 VSVS volunteers hold the
3 clear plastic glasses up so
students can see them.
Tell the students that you have
three liquids.
Pour the contents of the bottle
labeled V (acid – vinegar) into the
first glass.
Pour the contents of the bottle
labeled W (water) into the second
glass.
Pour the contents of the bottle
labeled B (base) into the third
glass.
Have a student describe the three
liquids they see. Describe liquids
based on visual cues only.
I. Intro Demo—Defining acids and
bases (cont.)
•
Ask students if they have ever
heard of litmus paper. If so, what
is it used for?
– Litmus paper is used to test
whether something is acidic or
basic.
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•
Test each glass of clear liquid by
dipping first the red and then the
blue litmus paper into the liquid
and noting what changes, if any,
occur. (You may want to record
the changes on the board for later
reference.)
Do not identify the liquids at
this point. The vinegar should
turn the blue litmus paper pink;
the water should not change the
color of either paper, and the
baking soda water should turn
pink litmus paper blue.
II. Discussion of Acids and Bases
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•
Note: Select the information that is appropriate
for the class you are teaching. Try to keep the
information on a level that the students can
understand. Feel free to add other appropriate
information. Keep this discussion brief.)
ACIDS
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Weak acids like the natural acids in food give foods a
sour, sharp flavor.
Strong acids can burn your skin.
Many acids are corrosive. They eat away metals and
other substances.
Some acids can be helpful. The acid in your stomach
aids in digestion.
Two acids (sulfuric acid and nitric acid) cause damage
in acid rain.
BASES
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Bases taste bitter and feel slippery.
Weak bases are used to settle upset stomachs.
Detergents and many cleaning solutions are basic.
Strong bases can burn the skin.
III. Testing Acids and Bases
• Ask: How can scientists tell
which solutions are acidic or
basic?
– (Based on the first experiment,
some students may be able to
reason that scientists could
use the different color of litmus
to tell how acidic or basic a
solution is.)
• Ask students, “When blue pH
paper is used as the indicator,
what color do the acid
solutions turn it?” (pink)
• Ask students, “When pink pH
paper is used as the indicator,
what color do the base
solutions turn it? (blue)
There are many
indicators that can be
used to show different
pH’s.
•
Tell students to look at the
handout. Point out a few
examples of how different
indicators can be useful at
different pH’s.
For example:
• Litmus can tell us if something
is either a base or an acid
• Methyl orange indicator
changes color at ph3/4 . It is red
below pH 4 and yellow above
pH 4.
• Universal indicator has a large
number of color changes at
different pH’s and can more
precisely tell us what the pH of
a solution is.
Some pH Indicators
Acid
pH
Base
Neutral
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Universal
indicator
cyanidin
(red cabbage)
blue litmus
red litmus
phenolphthalein
thymol blue
phenol red
bromo-thymol
blue
methyl red
bromo-cresol
green
methyl orange
bromo-phenol
blue
cresol red
pH
Adapted from http://www.ausetute.com.au/indicata.html
III. Testing Acids and Bases (cont.)
•
Put the Testing Household Items
Results chart (see worksheet) on the
board.
•
Organize the class into 8 groups.
Students will conduct the experiments in
pairs but will share dropper bottles.
•
Distribute the following materials to
each group:
– 2 well trays (1 per pair) , 4
observation sheets and 4 Results
sheets (1 per student).
– 1 bag containing dropper bottles of
household substances (1 per group
of 4)
– 2 containers of each: red litmus
paper, blue litmus paper, and pH
paper (at least 15 of each)
III. Testing Acids and Bases
(cont.)
• Tell the students to:
– Arrange the bottles of
household chemicals in
numerical order (1-12: there
will not be #s 2, 9, or 10).
– Place their well-plate on the
instruction sheet on top of the
diagram, so that they can see
the numbers through the
bottom of each well.
III. Testing Acids and Bases
(cont.)
• Squirt a small amount of
liquid from the #1 dropper
bottle (vinegar) into the first
well.
• Test its acidity and basicity
by dipping a red and blue
piece of litmus paper into the
liquid in the well.
• Test its pH by dipping a piece
of universal indicator paper
into the liquid in the well. The
color of the pH paper should
immediately be compared to
the label on the vial to figure
out the correct pH number.
III. Testing Acids and Bases
(cont.)
• Place the 2 litmus papers
and the pH paper in the
rectangles marked #1
(vinegar).
• Report findings
– Circle the color change and
record the pH number on
the Results sheet.
– Tell VSVS student who is
completing the chart on the
board.
VSVS students need to
monitor students for correct
results.
• Repeat with all the liquids
IV. Neutralization of an Acid with a
Base
• Ask students if they know what happens when
an antacid is taken for an upset stomach
– Stomachs contain hydrochloric acid, with a resting
pH of 4-5.
– This pH drops after eating because HCl and
proteases are released to aid in digestion.
– pH can get down to 1-2 after a high-protein meal.
– Too much acid in stomach  indigestion. Antacids
react with stomach acid to neutralize it.
IV. Neutralization of an Acid with a Base (cont.)
• Pass out dropper bottles of HCl, NaOH and Universal Indicator liquid.
• Tell students to:
– 1. Put a squirt of 0.1 M HCl in the first well on the bottom row (D1), add a
few drops of universal indicator, and note the color (red).
– 2. Do the same for 0.1 M NaOH in D2 (color should be purple).
– 3. Do the same in D3 with distilled water.
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D1 D2 D3
IV. Neutralization of an Acid with a
Base (cont.)
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Add a squirt of Universal Indicator to the 2 jars of distilled water. Note the color.
Add 20 drops (counting carefully) of .1M HCl into one of the jars. Note the color.
What is the pH?
Add .1M NaOH one drop at a time, counting the drops added, and observing
the color change in the universal indicator. Stir with the coffee stirrer when the
color starts to change back to neutral. Stop when the universal indicator color is
that of neutral pH7. Record the # drops needed to neutralize the acid.
Add 20 drops of .1M NaOH to the other jar and note the color. What is the pH?
Add .1M HCl one drop at a time, counting the drops added, and observing the
color change in the universal indicator. Stir with the coffee stirrer when the color
starts to change back to neutral. Stop when the universal indicator color is that of
neutral pH7. Record the # drops needed to neutralize the base.
Discussion
• Ask students:
• Are the foods that we have tested acidic or basic?
– Mostly acidic.
– Are the cleaners that we have tested acidic or basic?
– Mostly Basic (cleaners used on your body are sometimes
slightly acidic).
• Is there any difference in pH of distilled water or rain
water?
– Rain water usually tests slightly acidic because of dissolved
carbon dioxide. However, pollutants from burning fossil fuels
such as sulfur oxides and nitrogen oxides can cause the rain
water to be more acidic (hence acid rain).
– Distilled water should be neutral.