Acid/Base Lab (pre-lab to Elodea Lab)
LT:
I will be able to explain what is meant by a substance being an acid, base, or neutral and how this
relates to the pH scale. I will be able to identify common substances as either being an acid, a base, or
neutral.
Materials for the labs… (set up stations around the room, have groups rotate to music)
-pH indicator paper that goes by number
-toothpicks
-paper towels/newspaper
-pH chart
-substances to test (vinegar, orange juice, salt water, baking soda in DI water, Pepsi/Coke, Tap Water, DI
Water, Laundry detergent in DI water, glass cleaner, ammonia, bleach, milk, lemon juice, mouthwash,
tomatoe juice, rain water, black coffee, milk laxative (milk magnesia) drug, aspirin in DI water, Drano,
Lye/Lime Soap)
Protocol
1)
Make below data table in your lab-book… (add more rows as needed)
Substance Name
2)
3)
4)
5)
6)
pH Value & Color
Acid/Base/Neutral
Wait for teacher to assign your group a station.
When you are at each station, take a fresh toothpick dip it into the substance and place the
droplet on a pH strip. Wait a few seconds, then read the color and pH value by looking at your
chart.
Throw the used toothpick away and move to the next station.
Continue steps 3-4 until you have gone through all of the stations.
After you are done testing, go back to your table make the below pH scale line and then put the
substances where they need to go in your lab-book. (see below)
Locate each of the tested substances on the pH scale below. Place a vertical arrow to correspond to the
substance tested. Label each arrow with the name of the substance tested.
<--------------------------------------------------------------------------------------------->
0
2
4
ACID
7)
7
NEUTRAL
10
Read the background information on the backside.
12
BASE
14
As you have noticed substances with a pH of 0 to 6 are considered acid, while substances that have a
pH of 8 to 14 are considered basic. Substances at 7 or near to 7 are neutral. But what really is meant by
a substance being an acid or base. An acid is a substance that has a high concentration of hydrogen
ions (H+), while a base has a very high concentration of hydroxide ions (OH-). A neutral substance as
equal amounts of these ions. Take a minute to look at the below scale.
As you can see black coffee has about 100x more
hydrogen ions (H+) than pure water and drano has
about 10 million less times hydrogen ions than pure
water. So pH is way to measure the concentration of
hydrogen ions in a solution.
Real-world connection… An increase in C02 in our
atmosphere or C02 that is in our soda cans mixes with
the water to form a carbonic acid, making the solution
more acidic than it was before. Organic molecules
such as carbohydrates, lipids, proteins (enzymes), and
nucleic acids can be broken down by acidic
environments. This is sometimes good in the case of
digestion of carbohydrates and lipids, while in other
cases an enzyme that is broken down can no longer
due its job. Knowing that CO2 mixes with water to
form acid and how it could affect the molecules that
make up our cells will be useful information in our
next lab, Elodea Lab.
Reflect and Connect Questions (answer in your lab-book, 2 stars)
1. It is often recommended that aspirin be taken with a large glass of milk or water. Based on your results
in these lab exercises, explain why this is a good recommendation?
2. Would apple or orange juice be a good accompaniment to aspirin? Why or why not?
3. Enzymes function best at particular pH values. In the normal human stomach, a pH of 2.0-3.0 provides
the environment required for the proper functioning of the digestive enzymes found there. What effect
would the medicines that are basic have on these enzymes ability to digest (break down) food?
4. In some plants, soil pH affects the uptake of certain metals that bond with a pigment in the flower and
prevent its normal color expression. For instance, in hydrangeas, higher pH values prevent the uptake of
aluminum and the flowers appear pink whereas at lower pH values the flowers appear blue. Let’s say you
have a pink hydrangea plant.
Is the soil more acidic or basic in this situation?
Explain at least two ways to make the hydrangeas turn blue.
5. Buffers are aqueous solutions consisting of a mixture of a weak acid and its partner weak base. It has
the property that the pH of the solution changes very little when a small amount of strong acid or base is
added to it. Do you think human blood should act as a buffer? Why or why not?