The Chemistry of Acids
and Bases
For thousands of years people have known that vinegar, lemon juice and many
other foods taste sour. However, it was not until a few hundred years ago that it
was discovered why these things taste sour - because they are all acids.
The term acid, in
fact, comes from
the Latin term
acere, which
means "sour".
In the seventeenth century, the Irish writer and amateur chemist Robert Boyle first
labeled substances as acids or bases.
Acids taste sour, are corrosive to metals, change litmus (a dye extracted from
lichens) red, and less acidic when mixed with bases.
Bases feel slippery, change litmus paper blue and become less basic when mixed
with acids.
While Boyle and others tried to explain why acids
behave the way they do, the first reasonable
definition of acids and bases would not be
proposed until 200 years later.
Acids are compounds that break into hydrogen (H+) ions and another
compound when placed in an aqueous solution.
Bases are compounds that break up into hydroxide (OH-) ions and
another compound when placed in an aqueous solution.
If you have an ionic compound and you put it in water, it will break apart
into two ions. If one of those ions is H+, the solution is acidic.
If one of the ions is OH-, the solution is basic. There are other ions that
make acidic and basic solutions, but we won't be talking about them
here.
The pH scale is actually a measure of the number of H+ ions in a solution. If
there are a lot of H+ ions, the pH is very low. If there are a lot of OH- ions,
that means the number of H+ ions is very low, so the pH is high.
Acids
Have a sour taste. Vinegar is a solution of acetic acid. Citrus
fruits contain citric acid.
React with metals to produce hydrogen gas.
Can produce carbon dioxide gas
Bases
Have a bitter taste.
Feel slippery. Many soaps contain bases.
Some Properties of Acids
Produce H+
Taste sour
Corrode metals
Conduct Electricity
React with bases to form a salt and water
pH is less than 7
Turns blue litmus paper to red
Some Common Acids
Vinegar
Soda
Oranges
Some Properties of Bases
Produce OH- ions
Taste bitter
Feel soapy, slippery
pH greater than 7
Turns red litmus paper to blue
“Basic Blue”
Some Common Bases
NaOH sodium hydroxide lye
KOH potassium hydroxide liquid soap
Mg(OH)2 magnesium hydroxide Milk of magnesia
Al(OH)3 aluminum hydroxide Maalox (antacid)
Acid/Base definitions
Acids – Has H+ ions
Bases – Has OH- ions (hydroxide)
What is the difference between a strong acid and a weak
acid?
A strong base and a weak base?
The pH scale is a way of
expressing the strength of acids
and bases. Instead of using very
small numbers, we just use the
NEGATIVE power of 10 on the
Molarity of the H+
pH
Over 7 = base
7
= neutral
Under 7 =acid
Paper testing
Paper tests like litmus paper and pH paper
place a drop of the solution from the end of the
stirring rod onto a piece of the paper
Read and record the color change.
color indicates
Note what the
pH meter
Tests the voltage of the electrolyte
Converts the voltage to pH
Very cheap, accurate
pH indicators
Indicators are dyes that can be added that will
change color in the presence of an acid or base.
Some indicators only work in a specific range of
pH
Some dyes are natural, like radish skin or red
cabbage
Acid: A solution that has an excess of H+ ions. It comes from the Latin word acidus that
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. That means not
every molecule breaks apart. They usually have a pH close to 7 (3-6).
Weak Base: A base that only partially ionizes in an aqueous solution. That means not
every molecule breaks apart. They usually have a pH close to 7 (8-10).
Neutral: A solution that has a pH of 7. It is neither acidic nor basic.
The
pH
Scale
THE pH SCALE
pH is something that effects your life alot! Look on shampoo and conditioner bottles, and you ’ll see that some are “pH
formulated” for the best hair care! We want drinking water and soil in the garden and a multitude of other stuff “pH correct”!
So what’s this pH stuff all about? Good question.
pH is a number used by chemists to indicate the concentration of hydrogen ions in a solution. The pH scale tells us how
acidic or basic a substance is. The scale ranges from 0 (maximum acidity) to 14 (maximum alkalinity).
The middle of the scale, 7, represents the neutral point. The acidity increases from neutral toward 0, with 1-6 being acidic
and and the alkalinity increases from neutral toward 14, with 8-14 being basic (or alkaline).
A substance with a pH of 7 is neutral, meaning that it is not an acid or a base. Mixing acids and bases can result in a neutral
solution because the two neutralize each other.
The pH scale is logarithmic. This means that a difference of one pH unit represents a tenfold change. For example,
the acidity of a sample with a pH of 5 is ten times greater than that of a sample with a pH of 6. A difference of 2 units, from 6 to
4, would mean that the acidity is one hundred times greater, and so on.
pH Lab Procedure:
1.
Put your safety glasses on You will be testing several items to determine the pH of them. Be sure you can identify
each item that is to be tested. Make a prediction as to whether each item is an acid or a base. Be sure to record you
predictions on the data sheet.
2. Choose one of the liquid test items to test first. Measure 10 ml of the liquid to be tested into one of the small plastic
testing cups.
3. Use one of the eye droppers and add cabbage indicator solution to the test liquid, drop by drop. Continue this until
there is no further color change.
4. Use the pH chart and determine where the color “fits” onto the chart. Determine the pH number that corresponds to
the color on the scale. This is the pH of your test liquid. Record your results on the data sheet provided.
5. Repeat this process for all test items.
6 . What do you think happens when you mix an acidic substance and an alkaline substance together? Try it! Be sure
to record your experiments and results!
7. When you are finished testing the liquids, clean up and answer the lab questions.
Due Thursday is your lab packet. Cover sheet, procedure, data sheet, lab questions answered in complete
sentences.
Testing Items
BS Baking Soda Solution
A Apple Juice
Am Ammonia
F Fantastic/409 Cleaner
L Lemon Juice
MM Milk of Magnesia
W Distilled Water
V Vinegar
B Bleach
1 Rain water sample 1
2 Rain water sample 2
Before going
to the lab table.....
Predict for each item:
Acid
Neutral
Base
What happens when you put an acid
with a base?
Be sure and do some experiments to determine the answer to this
question that you will be answering after completing your lab work.
What happens when you put an acid
with a base?
pH Values of Common Household Materials
1.0 battery acid (sulfuric acid)
1.8-2.0 limes
2.2-2.4 lemon juice
2.2 vinegar (acetic acid)
2.8-3.4 fruit jellies
2.9-3.3 apple juice, cola
3.0-3.2 acid rain
3.0-3.5 strawberries
3.7 orange juice
4.0-4.5 tomatoes
5.6 unpolluted rain
5.8-6.4 peas
6.0-6.5 corn
6.1-6.4 butter
6.4 cow’s milk
6.5-7.5 human saliva
6.5-7.0 maple syrup
7.0 distilled water
7.3-7.5 human blood
7.6-8.0 egg whites
8.3 baking soda
9.2 borax
10.5 milk of magnesia
11.0 laundry ammonia
12.0 lime water
13.0 lye
14.0
When fossil fuels are burned, sulfur dioxide and nitrous oxides are released.
When these gases react with water, they produce sulfuric and nitric acids,
respectively.
Pull here
Fossil fuels are fuels that
originated from decaying plants
and animals such as oil, coal
and natural gas.
sulfur dioxide and
nitrous oxides
+
H
sulfur dioxide
and
nitrous oxides
O
O
+
O
O
H
sulfuric acid
nitric acid
acid
rain
Rearrange the sentences to describe how acid rain is
caused.
Pull here
Sulfuric and nitric acid are produced.
pH is a measure of acidity.
The lower the pH the more
acidic.These
The higher
the pH
the with water in the atmosphere.
toxic gases
react
more alkaline. A pH of 7 is
considered neutral.
The rain is now has a lower pH and is acidic. This is known as acid rain.
When fossil fuels are burned, sulfur dioxide and nitrous oxides are released.
Click here
for the
answer
Description of how acid rain is formed.
When fossil fuels are burnt sulfer dioxide and nitrous oxides are released.
These toxic gases react with water in the atmosphere.
Sulfuric and nitric acid are produced.
The rain is now has a lower pH and is acidic. This is known as acid rain.
Click here for
causes of acid
rain.
Which of these cause acid rain? Click on the image to see if
you are correct.
Windmills
Car
Volcanoes
Lightning
Plants
Power stations
This does not cause acid
rain.
Click
Click here
here
to
to go
go
back
back
This causes acid rain.
Click
Click here
here
to
to go
go
back
back
This causes acid rain, but
its effect is minor.
Click
Click here
here
to
to go
go
back
back
Arrange the following effects of acid rain in order of significance.
Deterioration of buildings
Damage to lakes, streams and rivers
Soil and tree-root damage
Effects on human health
Acid rain lowers the pH of lakes and ponds.
Pull here
pH is a measure of acidity.
The lower the pH the more
acidic. The higher the pH the
more alkaline. A pH of 7 is
considered neutral.
Pull here
An ecosystem is a community
of organisms that interact
with each other and the
environment they live in.
Explain how this can have adverse effects on an entire
ecosystem.
Click here
for the
answer
One of the most dangerous effects of acid rain is on the human respiratory system.
Sulfur dioxide and nitrous oxides are toxic gases and can cause respiratory
problems such as asthma.
Why do think the respiratory system is at high risk from
these gases?
Click here
for the
answer
Acid rain can have a detrimental effect on buildings. Over time, it gradually
dissolve parts of the building. The building material most effected by this is
limestone.
Pull here
Limestone is made from
calcium, carbon and oxygen.
When it reacts with acid it
produces carbon dioxide.
How do you think the effect of acid rain on limestone
could effect quality of life?
Click here
for the
answer
Pull here
Many historic buildings and statues are made from limestone. The effects of a
rain can wear
away
these historic structures.
The greenhouse
effect
is
a
greenhouse effect is a
natural
natural phenomenon
phenomenonwhere
where
heat
heat is
is trapped
trappedinside
inside the
the
Earth's
Earth's atmosphere.
atmosphere.
Limestone is made from calcium carbonate. When it reacts with acid, it
produces carbon dioxide. This adds to the greenhouse effect.
Click
Click here
here
to
to go
go
back
back
In an ecosystem like a pond, food webs are intertwined. If the pH level falls
too low, plants cannot survive. The fish that eat the plants then begin to die
off. When the fish die off, the predators that feed on them will also have
less food.
Click
Click here
here
to
to go
go
back
back
Through the respiratory system, humans inhale gases. Since the gases
produced from burning fossil fuels are toxic, it is the respiratory system that is
most at risk from direct contact with these gases.
Click
Click here
here
to
to go
go
back
back
Where do sulphur dioxide emissions come from?
Sulphur dioxide (SO2) is generally a byproduct of industrial
processes and burning of fossil fuels. Ore smelting, coalfired power generators and natural gas processing are the
main contributors.
In 2000, for instance, U.S. SO2 emissions were measured at
14.8 million tonnes - more than six times greater than
Canada's 2.4 million tonnes. But the sources of SO2
emissions from the two countries are different. In Canada,
68% of emissions come from industrial sources and 27%
comes from electric utilities (2000). In the U.S., 67% of
emissions are from electric utilities (2002).