Air and water - Chemistry - page 193

Checklist
After studying Chapter 11 you should know and understand
the following terms.
• Artificial fertiliser A substance added to soil to increase
the amount of elements such as nitrogen, potassium and
phosphorus. This enables crops grown in the soil to grow
more healthily and to produce higher yields.
• Atmosphere (air) The mixture of gases that surrounds
the Earth.
• Bulk chemical A chemical that, because of its large usage
across a range of uses, is produced in large quantities.
• Carbon dioxide A colourless, odourless gas, soluble in
water, producing a weak acid called carbonic acid. It makes
up 0.04% of air. It is produced by respiration in all living
things and by the burning of fossil fuels. It is taken in by
plants in photosynthesis.
• Chemical equilibrium A dynamic state. The
concentrations of the reactants and products remain
constant because the rate at which the forward reaction
occurs is the same as that of the back reaction.
• Eutrophication A process that occurs when fertiliser
drains into lakes and rivers, causing algae to multiply rapidly
and the water to turn green. it results in fish and other
organisms suffocating and dying through lack of oxygen in
the water.
• Flue gas desulfurisation (FGD) The process by which
sulfur dioxide gas is removed from the waste gases of
power stations by passing them through calcium hydroxide
slurry.
• Fractional distillation of air The process used to extract
individual gases from the air. Air is a major raw material. The
mixture of gases is separated by first liquefying the mixture
at low temperature and high pressure. The temperature is
then allowed to rise and the gases collected as they boil off.
The gases so produced have many and varied uses.
• Haber process The chemical process by which ammonia
is made in very large quantities from nitrogen and
hydrogen.
• Hardness of water This is caused by the presence of
calcium (or magnesium) ions in water, which form a 'scum'
with soap and prevent the formation of a lather. There are
two types of hardness:
• temporary hardness - caused by the presence of
dissolved calcium (or magnesium) hydrogencarbonate
• permanent hardness - this results mainly from dissolved
calcium (or magnesium) sulfate.
• Hydrogen bond A weak intermolecular force (bond) that
occurs between water molecules because the bonds within
the molecules are polar. It is the strongest of these weak
forces.
• Limestone A form of calcium carbonate (CaCO 3 ). Other
forms include chalk, calcite and marble.
• Nitrogen cycle The system by which nitrogen and
its compounds, both in the air and in the soil, are
interchanged.

• Nitrogen fixation The direct use of atmospheric nitrogen
in the formation of important compounds of nitrogen.
Bacteria present in root nodules of certain plants are able to
take nitrogen directly from the atmosphere to form essential
protein molecules.
• Optimum temperature A compromise temperature used
in industry to ensure that the yield of product and the rate
at which it is produced make the process as economical as
possible.
• Ozone (trioxygen) A colourless gas produced in the
stratosphere by the action of high-energy ultraviolet
radiation on oxygen gas, producing oxygen atoms. These
oxygen atoms then react with further oxygen molecules to
produce ozone. Its presence in the stratosphere acts as a
screen (ozone layer) against dangerous ultraviolet radiation.
• Particulates Very small particles found in the atmosphere,
such as certain types of smoke emitted from diesel engines,
as well as dust.
• Photosynthesis The chemical process by which green
plants synthesise their carbon compounds from atmospheric
carbon dioxide using light as the energy source and
chlorophyll as the catalyst.
• Pollution The modification of the environment caused by
human influence. It often renders the environment harmful
and unpleasant to life. Atmospheric pollution is caused by
gases such as sulfur dioxide, carbon monoxide and nitrogen
oxides being released into the atmosphere by a variety of
industries and also by the burning of fossil fuels. Water
pollution is caused by many substances, such as those found
in fertilisers and in industrial effluent.
• Raw material A basic material from which a product is
made. For example, the raw materials for the Haber process
are nitrogen and hydrogen.
• Removal of hardness Temporary hardness is removed
by boiling. Both temporary and permanent hardness are
removed by:
• addition of washing soda (sodium carbonate)
• ion exchange
• distillation.
• Reversible reaction A chemical reaction which can go
both ways. This means that once some of the products have
been formed they will undergo a chemical change once
more to re-form the reactants. The reaction from left to
right, as the equation for the reaction is written, is known
as the forward reaction and the reaction from right to left is
known as the back reaction.
• Stratosphere A layer of the atmosphere above the
troposphere in which the ozone layer exists.
• Transpiration The process of evaporation of water from
leaves.
• Troposphere A layer of the atmosphere closest to
the Earth which contains about 75% of the mass of the
atmosphere. The composition of dry air is relatively constant
in this layer of the atmosphere.
• Water cycle The cycle by which water circulates around
the Earth. The driving force behind the water cycle is the
Sun.