ESOL
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.
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.
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
Here are a couple of definitions you should know:
Acid: A solution that has an excess of H+ ions. It comes from the Latin word acidus, which means
"sharp" or "sour".
Base: A solution that has an excess of OH- ions. Another word for base is alkali.
Aqueous: A solution that is mainly water. Think about the word aquarium. AQUA means water.
Strong Acid: An acid that has a very low pH (0-4).
Strong Base: A base that has a very high pH (10-14).
Weak Acid: An acid that only partially ionizes in an aqueous solution. This means that not every
molecule breaks apart. Weak acids usually have a pH close to 7 (3-6).
Weak Base: A base that only partially ionizes in an aqueous solution. This means that not every
molecule breaks apart. Weak bases usually have a pH close to 7 (8-10).
Neutral: A solution that has a pH of 7. It is neither acidic nor basic.
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? Scientists use something called the pH scale to measure how acidic or basic a liquid
is. Although there may be many types of ions in a solution, pH focuses on concentrations of
hydrogen ions (H+) and hydroxide ions (OH-). The scale measures values from 0 all the way up to
14. Distilled water is 7 (right in the middle). Acids are found between 0 and 7. Bases are from 7 to
14. Most of the liquids you find every day have a pH near 7. They are either a little below or a little
above that mark. When you start looking at the pH of chemicals, the numbers can go to the
extremes. If you ever go into a chemistry lab, you could find solutions with a pH of 1 and others
with a pH of 14. There are also very strong acids with pH values below 1, such as battery acid.
Bases with pH values near 14 include drain cleaner and sodium hydroxide (NaOH). Those
chemicals are very dangerous.
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