What is Meant by Acid Rain? Video Lecture - Class 8

In this video you will learn that in two
quite different ways the burning of
fossil fuels can cause our rain to
become acidified.
Sulfuric acid arises from the burning of
coal and oil, mainly in power stations.
Whereas nitric acid arises from the
exhaust pipes of our motor vehicles. You
will also see some of the effects that acid
rain has on our environment. Firstly it's
important to understand that rain is
naturally slightly acidic. Whilst falling
to the ground, the rain dissolves carbon
dioxide from the atmosphere to form
carbonic acid; a weak acid giving natural
rain a pH of 6. It's when we burn fossil
fuels that much stronger acids get into
our rain to form acid rain with a pH as
low as 3.
When the pH changes from 6 to 3
what do you think the acidity increases
by? 2 times? 3 times? 10 times a thousand times? Pause and think
Remember that each change of one in pH
is a tenfold change in acidity. So
changing from 6 to 3 is 10 times 10
times 10 or a thousand times more acidic.
We will consider sulfuric acid first. Plants
need a little sulfur to grow. They obtain
it from sulfates, for example potassium
sulfate, found naturally in the soil. The
sulfates are taken up by the plants
roots and sulfur atoms become bonded
to carbon atoms in the leaf's proteins.
When plants die and eventually form coal
and oil over millions of years, these
sulfur atoms remain bonded to the carbon
atoms. The sulfur content of coal and oil
is usually between 1% and 4%
by weight. What do you think is produced
when fossil fuels containing sulfur are
burnt? Pause the video whilst you think.
When the coal is burnt the sulfur and
the carbon atoms join with oxygen from
the air and are released into the
atmosphere as carbon dioxide and sulfur
dioxide.
In the presence of sunlight, a photochemical reaction will take place where sulfur dioxide
reacts with more oxygen to form sulfur
trioxide. This dissolves in the rainwater
forming sulfuric acid.
Let's now consider
how nitric acid gets into the rain. In
the high temperature conditions in the
cylinders of our Motor Vehicles, a small
amount of nitrogen and oxygen from the
air can react.
Remember that air taken in to combust
the fuel is nearly 80% nitrogen. So
exhaust gasses from vehicles contain
small but significant amounts of
nitrogen oxides. Once the oxides of
nitrogen are in the air they react with
more oxygen, just like we saw with sulfur
dioxide, and dissolve in the rainwater to
form nitric acid. If the rain falls on limestone soils,
which are alkaline, the acidity may be
neutralized. However if the rain lands on
neutral or acid soils or on vegetation
then it can cause damage. So why is this
acidity harmful to living things? Well
living things don't grow well in acid
conditions. It's easy to demonstrate
this: soak a slice of bread in vinegar
and another in pure water, leave them
open for a little while and then leave
them covered for a week. You will find
that the bread soaked in vinegar has not
gone mouldy whereas the other bread has. In fact we use vinegar to preserve food
in a process called pickling.
This is the case because certain enzymes
which are vital for growth are unable to
function in acid conditions. The other
main problem is in the soil. Whilst
naturally occurring toxic metals such as
aluminium are insoluble and therefore
fairly harmless in neutral soil, they
become soluble in acidic soil. They then
get taken up by living things and take
the place of essential metals such as
zinc and iron. To add to this the
hydrogen ions in the acid rain will
replace essential metal ions, such as
potassium, magnesium and calcium, which are normally held in the clay soil.
Causing these essential metals to be
washed deep into the subsoil away from
the roots of plants. So to summarise, normal rain is slightly
acidic due to the dissolved carbon
dioxide. But acid rain contains sulfuric
and nitric acid, making the rain a
thousand times more acidic and dangerous
for many living things. Sulfuric acid
arises from the burning of fossil fuels
containing sulfur. Nitric acid arises
from the combustion of atmospheric
oxygen and nitrogen, in the high
temperature conditions in the cylinders
of petrol and diesel engines.