Acid rain | Environmental Chemistry | Chemistry | FuseSchool


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.