Properties and effects of atmospheric pollutants: Carbon monoxide is a toxic gas. It is colourless and odourless and so is not easily detected.

The idea

Carbon monoxide, CO, is produced by the incomplete combustion of fuels such as petrol, diesel, natural gas and wood. It is toxic because it binds to haemoglobin in red blood cells more strongly than oxygen does, forming carboxyhaemoglobin and reducing the oxygen carried around the body; this can cause dizziness, unconsciousness and death. Its danger is increased because it is colourless and odourless, so people cannot see or smell it. This is why carbon monoxide detectors and good ventilation are used where fuel-burning appliances operate, and why faulty boilers and blocked flues are hazardous. The gas also has no warning irritation, so symptoms such as headaches may be mistaken for other illnesses.

What earns the marks

• State that carbon monoxide is formed by the incomplete combustion of carbon-containing fuels.
• Describe carbon monoxide as toxic and explain that it reduces the oxygen-carrying capacity of the blood.
• State that carbon monoxide is colourless and odourless, so it cannot be detected by sight or smell.
• Link these properties to the need for carbon monoxide detectors, adequate ventilation and regular servicing of fuel-burning appliances.
• Recognise the symptoms of carbon monoxide poisoning, such as headaches, dizziness and unconsciousness, and that they can be mistaken for other conditions.

Common mistakes and how to put them right

• Confusing carbon monoxide with carbon dioxide.

  Put it right by noting that carbon dioxide is a product of complete combustion and is not toxic in the same way, while carbon monoxide is toxic and forms in limited oxygen.

• Thinking carbon monoxide can be smelled or seen.

  Put it right by stating it is colourless and odourless, which is why detectors are needed.

• Believing carbon monoxide dissolves in the blood and simply blocks vessels.

  Put it right by explaining that it binds to haemoglobin and reduces oxygen transport.

Exam tips

• When asked why carbon monoxide is dangerous, give both parts: it is toxic and it is hard to detect because it is colourless and odourless.
• Use the correct spelling and formula CO, and avoid writing Co, which represents the element cobalt.
• Link the properties to a safety measure, such as using a carbon monoxide detector or ensuring rooms are ventilated, to show applied understanding.