Acids, Bases, and the pH Scale
If you look in your medicine cabinet and refrigerator and on your kitchen shelves, you will
find examples of groups of compounds known as acids and bases. Acids are found in aspirin,
vitamin C, and eyewash. Fruits such as oranges, grapes, lemons, grapefruit, and apples contain
acids. Milk and tea contain acids, as do pickles, vinegar, and carbonated drinks. Bases are found in
products such as lye, milk of magnesia, deodorants, ammonia, and soaps.
Name &
Uses
Formula
Strong Acids
Hydrochloric Pickling steel;
HCl
digesting food;
cleaning
bricks
Sulfuric
Manufacturing
H2SO4
paints, plastics
and fertilisers;
dehydrating
agent
Nitric
Removing
HNO3
tarnish;
making
explosives and
fertilisers
Weak Acids
Carbonic
Fizzy drinks
H2CO3
Boric
Washing eyes
H3BO3
Phosphoric
Making
H3PO4
fertilisers and
detergents
Acetic
Making
(vinegar)
cellulose
HC2H3O2
acetate used in
fibres and
films
Citric
Making soft
H3C6H5O7
drinks
Properties of Acids
As a class of compounds, all acids have certain physical and
chemical properties when dissolved in water. One of the physical
properties all acids share is a sour taste. Lemons taste sour because
they contain citric acid. Vinegar contains acetic acid. However, you
should never use taste to identify a chemical substance. You should use
other, safer properties.
Acids affect the colour of indicators. Indicators are compounds
that show a definite colour change when mixed with an acid or a base.
Litmus paper, a common indicator, changes from blue to red in an acid
solution. Another indicator, phenolphthalein, is colourless in an acid
solution.
Acids react with active metals to form hydrogen gas and a
metal compound. This reaction wears away, or corrodes, the metal and
produces a residue. For example, sulphuric acid in a car battery often
corrodes the terminals and leaves a residue.
Another property of acids can be identified by looking at a list
of common acids. What do all acids have in common? Acids contain
hydrogen. When dissolved in water, acids ionise to produce positive
hydrogen ions (H+). A hydrogen ion is a proton. So acids can often be
defined as proton donors.
The hydrogen ion, or proton, produced by an acid is quickly
surrounded by a water molecule. The attraction between the hydrogen
ion (H+) and the water molecule (H20) results in the formation of a
hydronium ion (H30+).
H+ + H20  H30+
The definition of an acid as a proton donor helps to explain why all hydrogen containing
compounds are not acids. Table sugar contains 22 hydrogen atoms, but it is not an acid. When
dissolved in water, table sugar does not produce H+ ions. Table sugar is not a proton donor so it does
not turn litmus paper red or phenolphthalein colourless.
Common Acids
The three most common acids in industry and the laboratory are sulphuric acid, nitric acid, and
hydrochloric acid. These three acids are strong acids. This means they ionise to a high degree in
water and produce hydrogen ions. The presence of hydrogen ions makes strong acids good for
conducting electricity.
Acetic acid, carbonic acid, and boric acid are weak acids. They do not ionise to a high
degree in water, so they produce few hydrogen ions. Weak acids do not conduct electricity as well.
Properties of Bases (Alkalis)
Bases are also known as alkalis. When dissolved in water, all bases
share certain physical and chemical properties. Bases usually taste
bitter and are slippery to the touch. However, bases can be poisonous
and corrosive so you should never use taste or touch to identify them.
Bases turn litmus paper from red to blue and phenolphthalein to
bright pink. Bases emulsify, or dissolve, fats and oils. They do this by
reacting with the fat or oil to form a soap. The base ammonium
hydroxide is used as a household cleaner because it “cuts” grease. The
strong base sodium hydroxide, or lye, is used to clean clogged drains.
All bases contain the hydroxide ion (OH-). When dissolved in
water, bases produce this ion. Because a hydroxide ion (OH-) can
combine with a hydrogen ion (H+) and form water (H20), a base is
often defined as a proton acceptor.
OH- + H+  H20
Name &
Uses
Formula
Strong Bases
Sodium
Making soap;
hydroxide
drain cleaner
NaOH
Potassium
Making soft
hydroxide
soap; battery
KOH
electrolyte
Calcium
Leather
hydroxide
production;
making plaster
Ca(OH)2
Magnesium
Laxative;
hydroxide
antacid
Mg(OH)2
Weak Bases
Ammonium
Household
hydroxide
cleaner
NH4OH
Aluminum
Antacid;
hydroxide
deoderant
Al(OH)3
Common Bases
Strong bases dissolve readily in water to produce large numbers
of ions. Therefore, strong bases are good electrolytes (conductors of
electricity). Examples of strong bases include potassium hydroxide, sodium hydroxide, and calcium
hydroxide.
Weak bases do not produce large numbers of ions when dissolved in water. Therefore, weak
bases on are poor electrolytes. Ammonium hydroxide, and aluminum hydroxide are weak bases.
Review Questions
1. What are 3 properties of acids? of bases?
2. Why are acids called proton donors?
3. Why are bases called proton acceptors?
4. What are 3 common acids? bases?
The pH Scale
As you know some substances are acids and some are bases. Other substances are neutral. But, what
does this mean? To measure the acidity of a solution, the pH scale is used. The pH of a solution is a
measure of the hydronium ion (H3O+) concentration. Remember, the hydronium ion is formed by
the attraction between a hydrogen ion (H+) from an acid and a water molecule. So, the pH of a
solution indicates how acidic the solution is.
The pH scale is a series of numbers from 0 to 14. The middle of the scale – 7 – is the neutral
point. A neutral solution has a pH of 7. It is neither an acid nor a base. Pure water is a neutral liquid.
A solution with a pH below 7 is an acid. Strong acids have low pH numbers. A solution with a pH
above 7 is a base. Strong bases have high pH numbers.
pH Indicators
The pH of a solution can be determined by using an indicator. You already know about two
indicators: litmus paper and phenolphthalein. Other indicators include pH paper, methyl orange, and
bromothyl blue. Each indicator shows a specific colour change as the pH of a solution changes.
Common household materials can be used as indicators. Red cabbage juice covers the entire
pH range. Grape juice is bright pink in the presence of an acid and bright yellow in the presence of a
base. Even tea can be an indicator. Have you ever noticed the colour of tea change when you add
lemon juice? For accurate pH measurements, a pH metre is used.
A universal indicator is used on pH indicator strips. A universal indicator is a combination of
dyes which gives a different colour for each level in the pH scale.
Indicator
Litmus paper
Phenolphthalein
Red cabbage juice
Grape juice
Universal indicator
In Acids
red
colourless
red/purple
bright pink
red/orange/yellow
In Bases
Blue
bright pink
blue/green/yellow
bright yellow
blue/purple
Review Questions
1. What is pH? Describe the pH scale.
2. What is the pH of an acid? a base? water?
3. How can the pH of a solution be determined?
4. What happens to litmus paper in an acid? a base?
Concentration of
Hydrogen ions
compared to distilled
water
pH Scale
Examples of solutions
and their respective pH
levels
pH Indicators Lab
Purpose ; How does the pH of a liquid show if that liquid is acidic or basic?
Materials List:
 1 test tube rack, with 6 test tubes
 About 25 mL of red cabbage juice, in a small beaker
 1 Pipette
 5 Paper Towels
 any 6 different testing liquids
 1 marker for test tubes
Procedure:
1. Read through the entire procedure.
2. Make an appropriate data table for collecting the data. (Title, How many rows & columns,
labels for rows & columns)
3. Collect everything except the testing liquids.
4. Choose your testing liquids, BUT do NOT collect them yet!
5. Label each test tube, with the appropriate number for the liquid you will test.
6. Pour some of the liquid into the graduated cylinder marked with the same number, so it will
be easier to pour into the test tube.(example – liquid #1 into grad. cylinder #1)
7. Fill each test tube ¼ full with the appropriate numbered liquid. Make sure you put the
correct liquid in the correct test tube.
8. Add one pipette of red cabbage juice to each test tube.
9. Gently swirl the test tube.
10. Let sit and move onto the next test tube.
11. When you finish testing all of your liquids, record the final colour of the liquids on your data
table.
12. Clean up your working area.
Performance in Experiment
 Stays focused and on task
yes
 Follows instructions given (including safety warnings) yes
 Works cooperatively with others
yes
 Collects data accurately
yes
 Treats equipment appropriately and respectfully
yes
 Cleans up work space consistently/completely
yes
Grade for PLAB:
partially
partially
partially
partially
partially
partially
/6 (not in academic grade)
no
no
no
no
no
no