Name:
D:
P:
pH & Organisms
Introduction: What do lemons and vinegar have in common? They both
taste sour on your tongue. This is because they are acids and acids
taste sour. What do shampoo and ear wax have in common?
They feel soapy and taste bitter. They are bases.
Lots of washing powders are bases ( WARNING: Do not taste these! ).
Another word for a base is an alkali and we say that bases are alkaline
compounds.
Some properties of acids and bases are:
*acids have a sour taste
*acids are corrosive
*acids lose their acidity when they are combined with bases
*bases feel soapy
*bases taste bitter
*bases can be corrosive
Acids react with bases and weak acids only really react with very strong
bases. The problem with strong acids and strong bases is that they are so strong
that they can always persuade even the weakest of bases or acids to react with
them. There usually is something around to fill that role since all substances
lie somewhere on a scale between being strong acids and being strong bases. Most
natural substances lie in the middle zone where they're neither particularly
acidic or basic.
Scientists use something called the pH scale to measure how acidic or basic
something is. The pH scale measures acidity in the same way that a millimetre
scale measures length, for example. A low pH indicates an acidic compound (an
acid), whilst a high pH indicates a basic compound (a base).
Things that are neither acidic nor basic are neutral and come in the middle
of the pH scale – for example, pure water in the illustration above. Knowing
whether something is acidic or basic can be very useful. For example, wasp stings
are alkaline but bee stings are acidic. So if you get stung you need to put
bicarbonate of soda on a bee sting and vinegar on a wasp sting. Hair conditioner
is slightly alkaline as shampoo tends to be slightly acidic, so any traces of
shampoo can be neutralized by the conditioner. What does this tell you about allin-one shampoo/conditioners?
In our lab, we will learn how to identify acids and bases by using both
qualitative and quantitative tests. In addition, we will investigate some of the
effects acids and bases can have on organisms.
PART A:
Identifying Acids and Bases
1. Obtain one dropper full of each of the assigned solutions.
a. Place each dropper full into a different well in your spot plate
(KEEP THEM STRAIGHT!!! MAKE SURE YOU KNOW WHICH SOLUTION IS WHICH)
2. Return to your table and test each substance with…
a. Cabbage juice and record the color change in the chart on the next
page.
b. pH paper and record the pH number in the chart on the next page.
c. Identify whether each substance is an acid, a base, or neutral
Substance=
Substance=
Substance=
Substance=
Substance=
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
pH=
pH=
pH=
pH=
pH=
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Substance=
Substance=
Substance=
Substance=
Substance=
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
pH=
pH=
pH=
pH=
pH=
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Substance=
Substance=
Substance=
Substance=
Substance=
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
Cabbage Juice =
pH=
pH=
pH=
pH=
pH=
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Acid/Base/Neutral?
Questions for PART A:
1. Using your results from the lab, which substance is the most acidic?
substance is the most basic?
Which
2. Make a pH scale which places the substances you tested in order from most
acidic to most basic.
3. What is the difference between a qualitative test for acids/bases and a
quantitative test? What will each type of test tell you about the solution?
4. Knowing that your stomach has a very strong acid in it; if you had a
“heartburn” (acid reflux) should you take aspirin to make it feel better?
Why or why not?
5. When you add acids to bases they neutralize each other (remember H+ + OH- =
H2O). Therefore, what do you think happens in your stomach when you take an
“ant-acid” like Mylanta?
PART B: Organisms & pH: Individual organisms must maintain an internal
homeostasis. Many factors can affect that stability—for example, the relative
concentrations of hydrogen ions (H+) and hydroxide ions (OH-). The biochemical
activities of living tissues frequently affect and are affected by pH. Let’s
investigate!
Water &
1.
2.
3.
Acid
Pour 25mL of tap water into a beaker.
Record the initial pH
Add 5 drops of 0.1M Hydrochloric Acid (HCl)
to the water. Swirl and record the pH.
4. Add another 5 drops (you have now added
10 TOTAL drops) to the water. Swirl and
record the pH.
5. Continue adding 5 drops, swirling and
recording pH until you have added 30 TOTAL
drops.
6. Wash the beaker thoroughly and dry it.
Water &
1.
2.
3.
Base
Pour 25mL of tap water into a beaker.
Record the initial pH
Add 5 drops of 0.1M Sodium Hydroxide (NaOH)
to the water. Swirl and record the pH.
4. Add another 5 drops (you have now added
10 TOTAL drops) to the water. Swirl and
record the pH.
5. Continue adding 5 drops, swirling and
recording pH until you have added 30 TOTAL
drops.
6. Wash the beaker thoroughly and dry it.
Liver Homogenate & Acid
1. Pour 25mL of Liver Homogenate into a beaker.
2. Record the initial pH
3. Add 5 drops of 0.1M Hydrochloric Acid (HCl)
to the liver. Swirl and record the pH.
4. Add another 5 drops (you have now added
10 TOTAL drops) to the liver. Swirl and
record the pH.
5. Continue adding 5 drops, swirling and
recording pH until you have added 30 TOTAL
drops.
6. Wash the beaker thoroughly and dry it.
Liver Homogenate & Base
1. Pour 25mL of Liver Homogenate into a beaker.
2. Record the initial pH
3. Add 5 drops of 0.1M Sodium Hydroxide (NaOH)
to the liver. Swirl and record the pH.
4. Add another 5 drops (you have now added
10 TOTAL drops) to the liver. Swirl and
record the pH.
5. Continue adding 5 drops, swirling and
recording pH until you have added 30 TOTAL
drops.
6. Wash the beaker thoroughly and dry it.
Effects of HCl (ACID)
Substance
Tested
Tap
Water
Liver
Homogenate
Qualitative
Observations
of Tap Water
and Liver
Homogenate
0 drops
5 drops
Substance
Tested
Tap
Water
Liver
Homogenate
Qualitative
Observations
of Tap Water
and Liver
Homogenate
0 drops
5 drops
10 drops
15 drops
20 drops
25 drops
30 drops
20 drops
25 drops
30 drops
Effects of NaOH (BASE)
10 drops
15 drops
Questions for PART B:
1. Make a graph which shows the changes in pH in the tap water with
both acid and base. Make a second graph which shows the changes
in pH in the liver with both acid and base.
2. Summarize the effects of HCl and NaOH on the tap water.
3. What was the total pH change for the 30 drops of HCl added to the liver.
What was the total change for the 30 drops of NaOH?
4. How does the liver respond to the changes in pH?