DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
ACIDIC AND BASIC PROPERTIES
To make compounds easier to study, they have been divided into categories. Most of these
categorizations are based on the structure of the compound or how it acts. Some compound
categories are salts, anhydrides, hydrates, oxides, hydrocarbons, and acids and bases, among
many others.
Acids and bases are interesting because we come across them in our daily lives – indeed our
bodies manufacture some of them! We use them for cleaning and eating!!
The object of this activity is for you to find some distinguishing features of the compound
categories of acids and bases.
Several samples of acidic and basic solutions have been placed at this station. The acid bottles
are marked with A’s along with their chemical formula and name. The bases or alkalis (another
name for bases) have been marked with B’s.
1. Examine the chemical formulas for both sets. Is there anything common to the acids? To the
bases?
Acids
Bases
2. In the chemistry lab, it is NEVER a good idea to taste chemicals. But you have experience
(probably) tasting some acids and bases. Vinegar is an acid. Soap and baking soda have
basic properties. How would you describe the taste of each?
Acids taste
Bases Taste
3. INDICATORS are chemicals that tell us, usually by a color change, whether a class of
chemicals is present. Litmus is an indicator. You have both blue and red litmus paper. Place
some strips of litmus paper on a paper towel. Place your glass rod into one of the sample
bottles and then touch it to pieces of both colors of litmus. Clean the glass rod by rinsing it
in the beaker of distilled water. Repeat using several samples of both acids and bases.
Remember to clean the glass rod between each and every substance. Decide what the final
color of litmus is in both acids and bases.
Color of litmus in acid
Color of litmus in base
1
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
4. Phenolphthalein, a chemical that is the active ingredient in EX-LAX, is
another acid/base indicator. Plain phenolphthalein is colorless. You
have some of this chemical in a dropper bottle. Place several drops of
one sample into the well of a spot plate. Add one drop of
phenolphthalein and note the final color. Try this same procedure with
several samples of acids and bases. Make sure you wash the spot plate
and rinse them with distilled water. Decide what the final color of
acids and bases is in this indicator.
Spot plate
Phenolphthalein color in acids
Phenolphthalein color in bases
5. The pH scale is another measure chemists use to distinguish solutions that are acidic or basic.
At this station you have some pH paper. On the roll of pH paper is a color coded scale. Use
the glass rod once again to find the pH of the various samples. Compare the color of the
paper immediately after the sample has been applied to the scale and read the pH.
Remember to clean the rod between trials. Also take a pH reading for the distilled water.
Use a fresh sample of distilled water for this test.
pH’s of acid samples
pH’s of basic samples
pH’s of distilled water
6. Summarize the characteristics of acids:
Summarize the characteristics of bases:
Are there any questions you would like to ask?
2
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
IDENTIFYING HOUSEHOLD CHEMICALS AS ACIDIC, BASIC, OR NEUTRAL
As you have seen in the activity Acidic and Basic Properties, acids have low pH values and bases
have higher pH values. To be more precise, the pH scale ranges from 0 to 14. Numbers less
than 7 indicate acid solutions; greater than 7, basic solutions; and exactly 7, neutral solutions.
Water is written as H2O. If we write it as HOH, we can see that is made up of a H+ (hydrogen
ion) and a OH- (hydroxide ion). Since the H+ and OH- balance out, water is neutral. Any
solution that contains an equal number of H+ and OH- is neutral. If the H+ outnumber the OHthe solution is acidic. If the OH- outnumber the H+, then the solution is basic (alkaline).
-
More H+
More OH-
Using a pH indicator that has a multitude of colors is the most convenient way of estimating a
solution’s pH. Universal Indicator is one of the best for this purpose. A listing of the shades of
Universal Indicator along with its corresponding pH will help us determine whether our
household chemicals are acidic, basic, or neutral. Indicator paper is treated with universal
indicator
pH
Color
Description
0-2
3-4
5
6
7
8
9
10
11
12-14
red
red-orange
orange
yellow
yellow-green
green
dark green
green-blue
blue
violet
very acidic
moderately acidic
slightly acidic
slightly acidic
neutral
slightly basic
slightly basic
moderately basic
moderately basic
very basic
3
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
Look at the selection of household products on the dispensing table.
1. If the item is a liquid, place several drops in the well of a spot plate. If
the item is a solid or a thick liquid, add a small amount (about pea-size)
into a test tube and fill ½, way with distilled water. Allow to mix and
dissolve. Place several drops of this solution in the well of the spot
plate.
2. Test each solution with a small piece of universal indicator paper. Using information from
the previous page or on the indicator paper container, fill in summary chart below.
Product
pH
Acid,
base, or
neutral
Check one column
More
More Equal
+
H
OHH+ &
OH-
Active ingredient (See
list below) You may
have to check label.
vinegar
epsom salt
lye
aspirin
baking soda
windex
apple juice
bubble bath
washing soda
List of Active Ingredients: ammonium hydroxide, acetic acid, sodium carbonate, acetylsalicylic
acid, sodium hydroxide, citric acid, sodium bicarbonate, magnesium sulfate, sodium lauryl
sulfate
4
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
Questions:
1) Which solution had the highest number of hydrogen Ions (H+)?
2) Which solution had the highest number of hydroxide Ions (OH-)
3) Which solution was closest in having an equal number of H+ and OH- ?
5
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
WHAT MAKES ONE ACID STRONGER THAN ANOTHER?
The word “acid” to most people implies a dangerous chemical that is capable of disfiguring or
causing severe burns. There are acids that are extremely dangerous (ex. sulfuric, nitric, and
hydrochloric acids). There are also acids in our breakfast juice (citric acid), our soft drinks
(phosphoric acid), and our vitamin C tablets (ascorbic acid).
What makes one acid strong and another weak?
One clue lies in the fact that strong and weak acids differ in pH. The pH scale indicates the
relative number of hydronium ions that are in a solution. Hydronium ions, H3O+, are responsible
for many of the properties of acids.
Hydronium ions are formed when an H+ attaches to a H2O molecule. This is a more correct way
to write the hydrogen ion.
When an acid is mixed with water, it reacts, forming HYDRONIUM ions.
Ex.
HC1
Hydrochloric acid
+ H2O+ ---------- > H3O+
plus water
yields
+
C1-
hydronium ion plus chloride ion
The purpose of this activity is for you to determine the pH of several strong and weak acids.
From your results you can conclude how the pH scale relates to strength of acids.
1. At this station, you will find 3 containers of strong acids and 3 containers containing weak
acids, a spot plate, and universal indicator paper.
Place several drops of each solution in the wells of the spot plate. Test each solution with the
indicator paper and record the data in the table below.
.
Solution
1
pH
Strong or weak?
2
3
4
5
6
6
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
Describe the relationship between the strength of an acid and pH.
Describe the relationship between strength of acid and relative number of hydronium ions in
solution.
What is the difference between strong and weak acids?
Can you think of another factor that might change the pH of an acid? Explain your answer?
__________________________________________________________________________
__________________________________________________________________________
Are there any questions you would like to ask?
7
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
WHAT HAPPENS WHEN ACIDS AND BASES ARE MIXED?
Acids and bases are two categories of chemical compounds that have very different properties.
But one thing they have in common is that they always react in a similar way with one another.
Let’s mix some acids and bases and see if we can figure out what is going on!
1. Put a clean test tube in a test tube rack. Add 20 drops of hydrochloric acid from the marked
large beaker into the test tube.
Describe the hydrochloric acid
______
Add two drops of phenolphthalein from the dropper bottle to the acid in the test tube.
Color of phenolphthalein?
Color of phenolphthalein in the acid?
______
Phenolphthalein is ALWAYS the same color in acid (any acid). As long as there is acid present
in the test tube, phenolphthalein will be the same color.
2. Add 21 drops of the base, sodium hydroxide to the tube containing the acid and
phenolphthalein. After every few drops are added, shake the test tube.
Color of sodium hydroxide solution by itself?
______
Color of mixture?
Is there any acid left in the tube?
Why or why not?
______
Now the question is – What happened to all of that acid? This is the same kind of thing that
happens when you have too much acid in your stomach. You take a medicine (an antacid) to get
rid of it. Antacids are weak bases! A strong base would damage your tissues on its way down to
the stomach.
One of the by-products of the reaction of an acid and base is water. The formula for water is
H2O. One of the hydrogens comes from the acid the other hydrogen and the oxygen come from
the base. So there is water in your test tube. By boiling the mixture you have made, we can get
rid of the water in the tube and see what else was formed.
8
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
3. Put about ½ of the liquid from the test tube into a small beaker, and place the beaker on a hot
plate. Turn the dial up to high and allow all the water to boil away. Be careful not to touch
the hot beaker. Use the tongs to remove the beaker from the hot plate when the boiling is
complete.
Describe what is left in the beaker.
If the substance has a pink color, it may be due to the phenolphthalein and leftover base.
Remember, you added one more drop of base than acid. Although it is NEVER wise to taste
chemicals in the lab, the taste of this substance would be salty. (Take our word on it – do not
taste it.) As a matter of fact, it is the same salt that we sprinkle on French fries!
4. Fill in the blanks for the word equation for the reaction that took place between the two
chemicals.
Hydrochloric Acid + _________________ yields sodium chloride + ________________
(salt)
All acids will react in a similar manner with bases. Water is always a product plus a salt. The
identity of the salt will depend on which acid and which base are reacting. Some acids and bases
will also give a third product. Let’s try one of those.
5. Again place 20 drops of hydrochloric acid in a clean test tube. This time add about ¼ of a
wooden splint full of solid sodium bicarbonate (baking soda) to the acid.
Describe what happened when you added the base to the acid this time.
What was the physical state of one of the products? ______________________________
So a third product was formed here along with the salt and the water!
Where is salt physically located?
When you take baking soda to relieve an acid stomach, you may “burp.” Can you now
explain how?
9
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
Complete the word equation for the reaction that you just carried out.
hydrochloric acid + ____________ yields carbon dioxide + __________ + ___________
In general, acids and bases are said to NEUTRALIZE one another.
The general equation for NEUTRALIZATION is
ACID + BASE -----------> ____________ + ____________
Place all used chemical in the waste container provided. Don’t worry about mixing them since
they will just neutralize one another.
Are there any questions you would like to ask?
10
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
BUFFERS – WHAT ARE THEY AND WHY CAN’T WE LIVE WITHOUT THEM?
You have probably heard of “buffered” aspirin, but did you also know that our body fluids are
also buffered? If they weren’t, drinking a Coke or Pepsi or eating a salad with Italian dressing
would probably kill us! Let’s find out what buffers do.
1. Using your graduated cylinder, place about 40 mL of distilled water in one flask and 40 mL
of the “buffer solution” in another flask. Add 5 drops of universal indicator to both flasks.
Both solutions should appear to be light green in color noting a neutral pH. This is the
approximate pH of our blood and body fluids (7.4). If the beaker with distilled water is not
green, heat it to boiling on the hot plate until green and cool it down before proceeding.
2. Now add 10 drops of vinegar to the flask of distilled water. What color does the solution
turn?
_____________
Look at the chart below for this indicator’s color pattern:
pH 1 2 3 4
5 6
7
8 9
10
11 12
13
14
-----------------------------------------------------------------------------------Red
orange gold olive
dark green
blue violet
What is the pH? ______________. Is it acidic, basic, or neutral? _____________
Add 10 drops of vinegar into the buffered solution. Result: __________________
3. Repeat #2 using 0.10 M NaOH instead of vinegar. What color does the solution turn?
_____________
4. Slowly pour vinegar into the buffer solution until you reach the same color that the distilled
water and vinegar shows. Pour the entire contents of the modified buffer into a large
graduated cylinder and measure the total volume of the buffer and vinegar.
The buffer solution could absorb __________ mL of vinegar before its pH was altered. The
unbuffered solution (distilled water) changed after only 20 drops.
Record this information in the data table below.
5. Repeat #4 using 0.1 M NaOH, matching the buffer and NaOH color to the distilled water and
NaOH color.
Record this information in the data table below.
11
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
DATA TABLE
Item tested
Amount added to distilled
water (unbuffered)
Amount added to buffer
solution
Vinegar
NaOH
6. Complete the statements below:
When acidic soda or vinegar is added to unbuffered water, the pH
When the same amount of soda or vinegar is added to a “buffered” solution the pH
Buffers are chemicals which prevent
The pH of your blood and body fluids is about 7.4. If this pH drops lower than 7.4, the condition
is known as acidosis. If the pH goes above 7.4, the condition is known as alkalosis.
If the pH of the blood and body fluids change by more than 0.2 points (ex: 7.2 or less or 7.6 or
greater), drastic effects can occur leading to coma or even death. Judging from this experiment,
are our blood and body fluids buffered or unbuffered? _______________
If our blood and body fluids contain these buffer chemicals, what would be the medical concern
if a person has excessive fluid loss through either heavy vomiting, diarrhea, or blood loss?
______
Are there any questions you would like to ask?
12
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
WHAT DOES CONDUCTIVITY TELL US ABOUT ACIDS AND BASES?
Let’s determine how strong a conductor of electricity some acids and bases are. You will be
using the Electrical Conductivity Apparatus (ECA) which permits a bulb to light when a
conductor (metal strip) or conducting solution is placed between the electrodes.
What has to be in a conducting solution for current to flow? ____________________________
Suppose you put a solution between the electrodes of the ECA. What are the possible things that
could happen?
______________________________________________________________________________
______________________________________________________________________________
If the bulb lights up, what does that mean? ___________________________________________
If the bulb just barely lights up, what can you conclude? ________________________________
______________________________________________________________________________
Test each of the solutions provided (in the wide mouth jars) by immersing the ECA electrodes in
the jar liquid. Be sure the electrodes are covered with the solution you are testing by swirling the
jars so the solution rinses and coats the electrodes. REMEMBER TO UNPLUG THE
APPARATUS BEFORE TOUCHING THE ELECTRODES TO CLEAN THEM WITH
DISTILLED WATER BETWEEN SAMPLES. Record how the bulb appears and what type of
solution is in the jar in the table below
Sample
Conductivity result
HCl
NaOH
H2SO4
H2O or HOH
NH3 (aq) (NH4OH)
HC2H3O2 or CH3COOH
13
Strong or weak electrolyte
or Non-electrolyte?
DAY 7
CHEMISTRY SUMMER SCIENCE INSTITUTE
Now, some of these solutions are strong acid or strong bases, while others are strong bases or
weak bases or non-electrolytes. Strong acids and bases are good conductors while weak acids
and bases are poor conductors.
Based on what you uncovered determine which solutions from the table match the terms below:
Strong acids ________________________________
Strong bases ________________________________
Weak acids _________________________________
Weak bases _________________________________
Non-electrolytes _____________________________
Now here’s a statement to ponder. Acids are not ionic compounds; they are polar covalent
compounds and have no charged particles in their structures like ionic compounds do. However,
their aqueous solutions do conduct electricity.
What is happening when these materials are placed in water?
14