Science

Chemical reactions and pollutants

When there is a chemical reaction, air pollutants are produced. You can understand how these pollutants are formed by looking at what happens to atoms during these chemical reactions.

Formulae

The chemical formula of a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] tells you how many atoms of each element the molecule contains. The table below shows the formulae of some compounds, together with the number of atoms of each element in each molecule and a diagram of each molecule.

This table shows the formulae of some compounds, with the number of atoms of each element in the molecule.

carbon monoxidecarbon dioxidesulfur dioxidenitrogen monoxidenitrogen dioxidewater (hydrogen oxide)
formulaCOCO2SO2NONO2H2O
carbon atoms11       
sulfur atoms    1     
nitrogen atoms      11  
hydrogen atoms     2
oxygen atoms122121

The small (subscript) number after a symbol tells you how many atoms of that particular element are in the molecule. If there is only one atom of an element in a molecule, we do not write the number 1 after the symbol. For example, the water molecule H2O has two hydrogen atoms and one oxygen atom.

Rearrangement of atoms

In a chemical reaction, the substances that react together are called the reactants, while the substances that are formed are called the products.

reactants    →   products

The products have different properties from the reactants.

As a chemical reaction takes place, atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ] in the reactants are rearranged to make the products. No atoms are added or taken away. This is called the conservation of atoms.

Here are some examples.

two carbon atoms react with one oxygen molecule to make two carbon monoxide molecules

Carbon reacting with oxygen to form carbon monoxide

one carbon atom reacts with two oxygen molecule to make one carbon dioxide molecule

Carbon reacting with oxygen to form carbon dioxide

one sulfur atom reacts with one oxygen molecule to form sulfur dioxide

Sulfur reacting with oxygen to form sulfur dioxide

one nitrogen molecule reacts with one oxygen molecule to make two nitrogen monoxide molecules

Nitrogen reacting with oxygen to form nitrogen monoxide

one nitrogen molecule reacts with two oxygen molecules to make two nitrogen dioxide molecules

Nitrogen reacting with oxygen to form nitrogen dioxide

two hydrogen molecules react with one oxygen molecule to make two water molecules

Hydrogen reacting with oxygen to form water

For each example, count the number of atoms of each element in the reactants, and in the products. They are the same. As the atoms are conserved and all atoms have mass, the mass of the reactants will be the same as the mass of the products. This is known as conservation of mass.

The above reactions are all examples of oxidation reactions, as each chemical is joining with oxygen. When oxygen is removed from a chemical the reaction is known as a reduction reaction.

Burning fuels

Burning fuels releases harmful pollutants into the air. Pollutants can kill plants, harm our lungs, cause acid rain [acid rain: Rain that contains a high level of acids. It is believed to be caused by pollution and, maybe, natural activity such as volcanic eruptions. ] and are thought to contribute to global warming.

Photograph of coal burning

We burn fuels such as coal to produce energy.

A combustion reaction is when a chemical reacts with oxygen and releases energy.

We burn fuels to produce energy. When these fuels burn, the atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ] in the fuels combine with oxygen from the air to make new moleculesmolecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds.

Coal is made mainly of carbon. This burns to produce carbon dioxide.

Petrol, diesel fuel and fuel oil are hydrocarbons. Their molecules are made of carbon and hydrogen atoms. When these fuels burn, the carbon and hydrogen atoms combine with oxygen atoms to produce carbon dioxide and water vapour.

Making pollutants

Fuels are burned in the vehicles we use for transport, such as cars.

Fuels are burned in the vehicles we use for transport

Fuels are burned in the vehicles we use for transport, and in power stations to make electricity. When fuels are burned, pollutants are formed and released into the air.

This combustion of fuels increases the concentration of carbon dioxide in the air. This is thought to cause global warming [global warming: The rise in the average temperature of the Earth's surface. In the last 100 years it is believed to have risen by 0.6°C. ].

Properties

The products have different properties from the reactants.

Many fuels contain small amounts of sulfur compounds. When these fuels are burned sulfur dioxide is released into the air. Sulfur dioxide causes acid rain [acid rain: Rain that contains a high level of acids. It is believed to be caused by pollution and, maybe, natural activity such as volcanic eruptions. ] that can damage buildings and kill plants.

Incomplete combustion

If there is not enough oxygen present to burn the fuels completely, incomplete combustion takes place. Carbon monoxide, a very poisonous gas, is formed.

Incomplete combustion also releases very small particles of carbon into the air. This particulate carbon makes buildings dirty and can cause breathing difficulties.

Nitrogen oxides

In the high temperatures of vehicle engines, nitrogen and oxygen react to form nitrogen oxides. These gases irritate the lungs and cause acid rain.

Formation of NOx- Higher tier

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When fuels are burned in vehicle engines, high temperatures are reached. At these high temperatures nitrogen and oxygen from the air combine to produce nitrogen monoxide.

one nitrogen molecule reacts with one oxygen molecule to make two nitrogen monoxide molecules

Nitrogen reacting with oxygen to form nitrogen monoxide

nitrogen + oxygen → nitrogen monoxide

Coal is made mainly of carbon. This burns to produce carbon dioxide.

When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide.

one nitrogen molecule reacts with two oxygen molecules to make two nitrogen dioxide molecules

Nitrogen reacting with oxygen to form nitrogen dioxide

nitrogen monoxide + oxygen → nitrogen dioxide

Nitrogen dioxide causes acid rain [acid rain: Rain that contains a high level of acids. It is believed to be caused by pollution and, maybe, natural activity such as volcanic eruptions. ]. It also causes breathing problems and can worsen the effects of asthma.

Nitrogen monoxide and nitrogen dioxide are jointly referred to as NOx

Types of pollutants

Pollutants that enter the air do not simply stay there. They can be removed in several different ways.

Particulates

Particulates are very small pieces of solids, mainly carbon, that are released into the air during incomplete combustion. Since they are solids, they will stick to other solid materials that they come into contact with. They coat the walls of buildings making them look dirty.

Sulfur dioxide

Sulfur dioxide is formed when fuels containing sulfur compounds are burned. Sulfur dioxide reacts with water and oxygen in the air to produce acid rain [acid rain: Rain that contains a high level of acids. It is believed to be caused by pollution and, maybe, natural activity such as volcanic eruptions. ]. This removes sulfur dioxide from the air, but the acid rain corrodes buildings and kills plants.

Nitrogen monoxide

Nitrogen monoxide reacts in the atmosphere to form nitrogen dioxide.

Nitrogen dioxide

Nitrogen dioxide also reacts with water and oxygen in the air to produce acid rain.

Carbon monoxide

Carbon monoxide is produced when fuel is burned in an insufficient supply of oxygen. It is a poisonous gas that alters the level of oxygen in the blood.

Carbon dioxide

Carbon dioxide is produced by burning fossil fuelsfossil fuels: Fuels such as coal, oil and natural gas, made from the remains of ancient plants and animals., such as like coal, oil, petrol and natural gas.

Plants remove some of the carbon dioxide from the atmosphere, because they use it in photosynthesis [photosynthesis: The chemical change that occurs in the leaves of green plants. It uses light energy to convert carbon dioxide and water into glucose. Oxygen is produced as a by-product of photosynthesis. ]. Here is the word equation for photosynthesis:

carbon dioxide + water → glucose + oxygen

This process uses energy from sunlight. The energy has been absorbed into the plant by the green pigment chlorophyllchlorophyll: The green chemical inside the chloroplasts of plant cells. It enables photosynthesis to take place..

Carbon dioxide is also removed from the atmosphere when it dissolves in both rain water and sea water. As a result rain water becomes slightly acidic, and the oceans are a huge reservoir of dissolved carbon dioxide.

Not all the carbon dioxide we produce is removed from the atmosphere. The level of CO2 is steadily increasing, and this contributes to global warming [global warming: The rise in the average temperature of the Earth's surface. In the last 100 years it is believed to have risen by 0.6°C. ].

Water

Although water is emitted from vehicles and power stations it is not considered a pollutant.

Validity

You will need to take care when drawing conclusions from data about air pollution. A number of different influences can affect the data and these need to be taken into consideration.

Not all data, and the conclusions that are drawn from it, has the same validity.

Repeats

A scientist may take a set of measurements, or make some observations, and draw conclusions from them. If the scientist can repeat this activity, and get similar data, this provides more evidence to support those conclusions. The conclusions are therefore more likely to be valid.

If similar data is obtained when carried out under different conditions, for example using a different method, the result is said to be reproducible. This again increases the validity of the conclusion.

Peer review - Higher tier

Scientists report their work in scientific journals and at conferences. Before publication, the work is checked and evaluated by other experts. This process is known as 'peer review'.

When scientists report their work, other scientists can see what they have done and try to repeat it. If these other scientists obtain similar data, it provides more evidence to support the conclusions that have been published.

Scientific findings are only accepted once they have been evaluated critically by other scientists.

Factors and outcomes

the lung is tinged yellow and has black tar deposits

A smoker's lung, with tar deposits visible

Scientists are interested in what happens, and what may change how these things happen. Something that happens is called an 'outcome'. Things that may change the way this happens are called 'factors'.

The quality of the air on a street is an example of an outcome. Factors that may affect this outcome include the number and type of vehicles using the street, whether there are factories along the street, the strength and direction of the wind, and so on.

If a factor is changed this may lead to a change in the outcome.

Some factors may increase the chances of an outcome but not inevitably lead to it. For example, smoking cigarettes increases the chances of a person getting lung cancer, but some people who smoke never get lung cancer.

Correlation

A correlation is a link between two things. If an outcome happens when a factor is present, and does not happen when the factor is absent, there is a correlation. Or, if an outcome increases or decreases when a factor increases or decreases, there is a correlation. We say there is a correlation between the factor and the outcome.

Graph shows that as the number of cigarettes smoked increases, so does the number of deaths from cancer

Graph showing number of cigarettes smoked against the number of deaths from cancer

For example, studies show that there is a correlation between how much a person smokes and how likely they are to get lung cancer. As the number of cigarettes smoked increases, the percentage of people who smoke that number and get lung cancer also increases.

But a person may smoke cigarettes and never get lung cancer. However, one individual case does not provide evidence for or against a correlation. A correlation is only valid when there is a large amount of data to support it.

Correlation and cause

The fact that there is a correlation between a factor and an outcome does not necessarily mean this factor causes the outcome.

There is a correlation between the pollen count in the air and the incidence of hay fever, for instance. The pollen count increases from spring onwards, reaching a peak in mid-summer. It is therefore possible that pollen causes hay fever.

There is also a correlation between the amount of ice cream sold during the summer and the number of hay fever cases. But nobody would suggest that eating ice cream causes hay fever.

To claim a causal link between a factor and an outcome there has to be a mechanism that explains how the factor causes the outcome. In the case of hay fever, skin tests show that people who get hay fever are allergic to pollen. This is supporting evidence that hay fever can be caused by an allergic reaction to pollen.

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