CHAPTER 6 Acids and Bases
Starting Points
(Page 234)
(Students’ answers will reflect their understanding at this point. There are no “correct” or
“incorrect” answers. Students will revisit their answers at the end of the chapter.)
Exploration: Consumer Products
(Page 235)
[Students’ responses will vary, depending on what products they find.]
Product
Brand X shampoo
pH information
“pH balanced”
Claims about
performance re pH
“maintains a normal pH
for your hair”
Questions
What is the pH of the
shampoo? What is the
“normal” pH of hair?
6.1 PROPERTIES OF ACIDS AND BASES
Investigation 6.1: Properties of Acids and Bases
(Page 236)
Purpose
The purpose of this investigation is to test previous knowledge about the properties of acids and
bases.
Problem
What properties are most useful for distinguishing acids and bases from other classes of
compounds?
Prediction
According to previous knowledge, the useful properties of acids and bases are:
Electrical conductivity
Effect on litmus
pH
Acids
conduct electricity
turn blue litmus red
<7
Bases
conduct electricity
turn red litmus blue
>7
Design
A sample of each solution is tested for conductivity, effect on litmus, and pH. The manipulated
variable is the choice of solution. The responding variables are their properties. The controlled
variables in this experiment are: volume of solution, temperature, and amount concentration. Pure
water is tested as a control.
Materials
Ɣ eye protection
Ɣ lab apron
Ɣ 8 - 50 mL beakers
Ɣ 25 mL each of 0.100 mol/L solutions of
ż CaCl2(aq)
ż C3H4OH(COOH)3(aq)
ż C6H12O6(aq)
ż Ca(OH)2(aq)
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Ɣ
Ɣ
Ɣ
Ɣ
ż NH3(aq)
ż NaHSO4(aq)
ż CH3OH(aq)
ż H2SO4(aq)
ż Na2CO3(aq)
pure water bottle
conductivity apparatus
red and blue litmus paper
pH meter (or pH paper)
Procedure
1. Place equal volumes of the sample solutions in labelled beakers.
2. Test and record the conductivity of each of the solutions and pure water using the
conductivity apparatus. Be sure to rinse the electrodes after each test to avoid contamination
of the subsequent solution.
3. Test each solution and pure water with both colours of litmus paper and record your results.
4. Test each solution and pure water with the pH meter or pH strips and record the pH value.
5. Dispose of solutions into the sink with lots of running water.
Evidence
Solution
H2O(l)
CaCl2(aq)
C3H4OH(COOH)3(aq)
C6H12O6(aq)
Ca(OH)2(aq)
NH3(aq)
NaHSO4(aq)
CH3OH(aq)
H2SO4(aq)
Na2CO3(aq)
Conductivity
no
yes
yes
no
yes
yes
yes
no
yes
yes
Effect on litmus
no change
no change
blue to red
no change
red to blue
red to blue
blue to red
no change
blue to red
red to blue
Approximate pH
7
7
3
7
12
9
2
7
1
11
Analysis
The conductivity of the compounds did not really help determine the difference between acids
and bases and the other compounds. All of the acids and bases conducted electricity. Some other
compounds (e.g., CaCl2(aq)) also conducted electricity, but are neutral and ionic, whereas others
such as C6H12O6(aq) and CH3OH(aq), were found to be neutral but did not conduct electricity. It
can be concluded that all acids and bases conduct electricity to some degree, but this property
cannot be used to distinguish them from neutral solutions. Litmus and pH tests were very
conclusive. All of the substances that were known to be acidic turned blue litmus red and had a
pH < 7. All substances that were known to be basic turned red litmus blue and had a pH > 7. Even
some compounds that seemed to be neutral were discovered to be basic, namely, NH3(aq) and
Na2CO3(aq). On the basis of the evidence, it can be concluded that litmus and pH readings are
properties of acids and bases that are very useful in distinguishing between them and
distinguishing them from other solutions.
Evaluation
This design was adequate in answering the problem, in that it provided evidence that could be
interpreted to answer the problem. The conductivity test was not useful in distinguishing acids
and bases from neutral substances, but it showed that all acids and bases conduct electricity. The
design could be improved by simplifying, so that it included only litmus or pH. Both the
materials and procedure were adequate with no obvious improvements (other than simplifying
based on an improved design). No specialized technological skills were necessary. Based on my
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evaluation of the experiment, I am very certain of the experimental evidence. Sources of
uncertainty or error include concentration and purity of the solutions.
The prediction is judged to be mostly verified because useful properties were predicted.
Clearly, previous knowledge was acceptable in this experiment.
The purpose was accomplished in a limited way because only a few properties were
identified, but they were mostly useful in answering the problem.
Section 6.1 Questions
(Page 237)
1. Empirical knowledge usually comes before theoretical knowledge.
2. Three contexts: consumer, commercial, and industrial.
3. pH is the most useful empirical property because differences in pH can easily be used to
distinguish between acids, bases, and neutral compounds.
4. In the early 1800s scientists recognized that the presence of hydrogen gave a compound its
acidic properties. This was later interpreted as salts of hydrogen, ionic compounds containing
hydrogen ions. The Arrhenius theory used the idea of hydrogen ions to explain acidic
solutions. Today, the presence of hydronium ions is thought to be responsible for the acidic
properties of a compound.
5. According to atomic theory, a hydrogen atom contains a proton (nucleus) and one electron.
Therefore, a hydrogen ion, H+, would represent a single proton. On the basis of molecular
theory, a hydronium ion, H3O+, contains a central oxygen atom with two shared pairs of
electrons, two hydrogen atoms, and a lone pair of electrons forming a covalent bond with a
hydrogen ion. This molecule is covalently bonded, like other polyatomic ions.
Hydrogen ions and hydronium ions are similar in that they both have a 1+ ion charge and
both would be surrounded by polar water molecules in an aqueous solution.
6. Acid: lime scale remover
Ɣ Contains sulfamic acid
Ɣ Keep out of reach of children
Ɣ Harmful if swallowed. Do not induce vomiting. Drink lots of water and go to the hospital.
Ɣ Causes eye and skin injury
Ɣ Dangerous fumes
Ɣ HHP symbol: Danger—Corrosive
Base: oven cleaner
Ɣ Contains sodium hydroxide
Ɣ Causes burns
Ɣ Produces dangerous fumes when mixed with other products
Ɣ Harmful if swallowed. Do not induce vomiting. Call poison control centre or doctor.
Ɣ Causes eye and skin injury. If in eyes, rinse well with water for 15 minutes.
Ɣ HHP symbol: Danger—Corrosive
7. (a) Litmus paper tells the user only whether the solution is acidic, basic, or neutral.
Measuring pH reveals how acidic or basic the solution is. pH is therefore a more accurate
and precise method of determining the acidity or basicity of a solution.
(b) Water is usually naturally slightly acidic because the atmosphere contains carbon dioxide
(CO2(g)). Carbon dioxide readily dissolves in water (e.g., in raindrops and other sources
of water exposed to air), forming a weak acid (H2CO3(aq), carbonic acid). Therefore,
natural events in the aquatic environment that affect CO2 concentrations also affect pH.
Outside of the natural acidity of aquatic environments, pollution from burning fossil fuels
increases the amounts of sulfur and nitrogen oxides introduced into the water, thereby
increasing their overall acidity.
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