Chemistry Matters GCE 'O' Level Textbook PDF

# Chapter 22 Maintaining Air Quality ## What You Will Learn * What is air made of? * What are air pollutants? * What is the ozone layer? * What is the carbon cycle? * What are global warming and climate change? ## Do You Remember The Haze That Hits Singapore Nearly Every Year? This haze is cause...

# Chapter 22 Maintaining Air Quality ## What You Will Learn * What is air made of? * What are air pollutants? * What is the ozone layer? * What is the carbon cycle? * What are global warming and climate change? ## Do You Remember The Haze That Hits Singapore Nearly Every Year? This haze is caused by air pollution from neighboring countries when forests are burnt down for cultivation. The soot and ash cause the skies to look gray and reduce visibility. The haze also contains harmful pollutants like sulfur dioxide, carbon monoxide and oxides of nitrogen. Singapore has strict laws on the emission of air pollutants. Power plants are fitted with chemical scrubbers to remove most pollutants before the waste gases are released. While Singapore cannot prevent the haze from occurring, we can reduce its impact on our health. N95 masks, for example, filter out most of the harmful particles. How else can we reduce the impact of air pollution? ## 22.1 What Is Air Made Up Of? ### Learning Outcome * Describe the percentage composition of air. Most laboratories are equipped with a fire blanket that is meant to cover up and smother a small fire. It can also be used on a pot that has burst into flames. How does this happen? Flames occur when a substance undergoes combustion, which is the rapid oxidation of a substance. Combustion requires oxygen to occur and ceases once the oxygen supply is cut. A fire blanket prevents the oxygen in the air from getting to the flames, thus putting out the fire. Air is a mixture of gases. As seen in Figure 22.2, air's composition by volume is approximately 78% nitrogen and 21% oxygen. Water vapor, noble gases and carbon dioxide make up the approximately 1% that remains. Argon is the most common noble gas in clean air. | Gas | Percentage (%) | | --- | --- | | Nitrogen | 78% | | Oxygen | 21% | | Carbon Dioxide | 0.04% | | Noble Gases | 0.96% | Nitrogen is generally considered to be an inert gas as the nitrogen-nitrogen triple bond (N=N bond) is relatively strong and hard to break. Thus, reactions that involve nitrogen as a reactant have relatively high activation energies. Oxygen has many uses. Besides being essential for combustion, it is also used to support patients with breathing difficulties. The 21% of oxygen found in the air is insufficient to oxygenate their blood. Oxygen can also be used as the oxidiser in welding that joins pieces of metals together strongly. Argon and the other noble gases provide inert environments for high-temperature processes like in lightbulbs and in the thermal purification of copper and steel. These gases can be separated from each other by fractional distillation as they each have a unique boiling point. Atmospheric air is liquified at very low temperatures and then allowed to slowly warm up inside a chamber attached to a fractionating column. Nitrogen has the lowest boiling point, so it is the first to boil off and be collected. Table 22.1 shows the boiling points of some gases in air. | Component of Air | Boiling Point / °C | | --- | --- | | Nitrogen (N₂) | -196 | | Argon (Ar) | -186 | | Oxygen (O₂) | -183 | ## 22.2 What Are Air Pollutants? ### Learning Outcomes * Name and state the sources of common atmospheric pollutants. * Describe some of the problems caused by these pollutants. * Explain how some of these pollutants can be controlled. Air pollutants are also known as atmospheric pollutants. They are harmful gases that should not be found in a sample of clean air. They frequently cause harm to the environment and to living things. Many families in Singapore run air purifiers to provide good air quality at home. The air purifiers remove unwanted particles from the air. What are some common atmospheric pollutants that we seek to avoid? Air pollution is the introduction of unwanted and harmful chemicals into the atmosphere. Air pollution can come from natural causes, such as lightning strikes, wildfires and volcanic eruptions. It can also come from human activities, such as industrial emissions, combustion engine emissions and man-made fires. Table 22.2 on the next page shows some of the common air pollutants and their impacts. | Air Pollutant | Sources | Harmful Effects | | --- | --- | --- | | Sulfur dioxide (SO₂) | combustion of fossil fuels, volcanic eruptions | Sulfur dioxide causes breathing difficulties in humans and animals. Sulfur dioxide and nitrogen oxides react with oxygen in the air to form acidic compounds which dissolve in rain water, forming solutions of strong acids. This is called acid rain. Acid rain kills plants and aquatic life. It also corrodes buildings and structures made from limestone, marble and metals. | | Nitrogen oxides (NO and NO₂) | vehicle combustion engines, lightning | NO is colourless and odourless, NO₂ is red-brown and has a pungent smell. Sulfur dioxide causes breathing difficulties in humans and animals. Sulfur dioxide and nitrogen oxides react with oxygen in the air to form acidic compounds which dissolve in rain water, forming solutions of strong acids. This is called acid rain. Acid rain kills plants and aquatic life. It also corrodes buildings and structures made from limestone, marble and metals. | | Carbon monoxide (CO) | incomplete combustion of carbon-based fuels | Carbon monoxide binds irreversibly with the haemoglobin in our red blood cells. This lowers the ability of the haemoglobin to transport oxygen to the rest of our body. This can result in the loss of consciousness and even death. | | Methane (CH₄) | anaerobic (in the absence of oxygen) bacterial decay of organic substances, waste gases from cattle | Methane is a major greenhouse gas that leads to global warming. | | Unburnt hydrocarbons (CH) | vehicle combustion engines | Unburnt hydrocarbons can cause eye and respiratory tract irritation. Unburnt hydrocarbons react with nitrogen oxides in the presence of sunlight to form photochemical smog, which appears as a brown haze. The formation of smog also produces ozone, sulfur dioxide and nitrogen dioxide, which are other pollutants listed in this table. | | Ozone (O₃) | lighting, from reactions between oxygen molecules and sunlight in the upper atmosphere, from reactions between unburnt hydrocarbons and nitrogen oxides in the presence of sunlight in the lower atmosphere. | Ozone can cause eye and respiratory tract irritation, leading to breathing difficulties. When inhaled, it also causes chest pains and headaches. Ozone slows down photosynthesis in plants, which can damage crops. | ## 22.3 What Is The Ozone Layer? ### Learning Outcomes * Discuss the Importance of the ozone layer. * Describe how chlorine-containing compounds, chlorofluorocarbons (CFCs), can deplete the ozone layer. Have you ever had a sunburn? Singapore, being a country near the equator, receives a large amount of high-intensity sunlight on a regular basis. The sun's rays contain ultraviolet radiation that can cause sunburns, skin cancer, premature aging and cataracts in the eyes. Some sunglasses have a filter to block the harmful ultraviolet rays from the sun, protecting our eyes. Clouds do not do much to block out ultraviolet (UV) radiation. Luckily, there is a layer of ozone between 15 to 30 km above us. Although ozone is considered to be a pollutant when found at ground level, its presence in the atmosphere is beneficial. It can absorb ultraviolet radiation, thus reducing the amount of UV radiation that reaches the Earth's surface. The ozone layer can be found in the upper layer of the atmosphere called the stratosphere, which is nearly twice as high as the regular flight altitude of airplanes. Ozone molecules form when oxygen molecules interact with UV radiation from the sun. Ozone has the formula O₃ and is an allotrope of oxygen (O₂). Ozone has a pale blue color and a pungent smell that some of us would associate with electronics, particularly photocopiers. These same ozone molecules break up to reform oxygen when they absorb UV radiation. This cycle happens naturally in the stratosphere and maintains the ozone layer. Chlorofluorocarbons (CFCs) can be found in aerosol propellants, refrigerants and certain kinds of plastic. They contain the elements chlorine, fluorine and carbon. When they are released and rise up into the stratosphere, they can interact with UV radiation to produce chlorine atoms which react with ozone. In 1985, it was noticed that a hole had opened in the ozone layer above Antarctica. This hole has changed in size over the years and has also covered parts of Australia, New Zealand, Argentina and South Africa. It may have contributed to increases in skin cancer rates in these countries since. In 1987, the Montreal Protocol was signed by 46 countries. It restricts the production and release of CFCs into the atmosphere. The Protocol has since been adopted by nearly every country including Singapore which only allows the use of ozone-friendly refrigerant gases. The ozone layer is expected to fully heal sometime around 2075. ## 22.4 What Is The Carbon Cycle? ### Learning Outcomes * Describe the processes that make up the carbon cycle. * Describe how the carbon cycle regulates the amount of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas that contributes to global warming and climate change. You may have read about how reducing carbon dioxide emissions from burning fossil fuels and planting more trees can help to prevent the situation from worsening. How do these actions help the situation? The answer lies within the carbon cycle, shown in Figure 22.15. The element carbon is essential for life. All organic compounds are made of carbon skeletons that originate from carbon dioxide and water during photosynthesis. Most of the organic carbon on Earth is contained in the cells of living organisms, and as fossil fuels and the plastics we make from them. Oceans and other large bodies of water absorb carbon dioxide. Some marine plants use the dissolved gas to photosynthesize. It is also converted to carbonic acid and calcium carbonate that make up the shells of marine organisms and accumulate on the seabed when they die. Plants take in carbon dioxide and water to make glucose and oxygen in the presence of sunlight. 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(s) + 6O₂(g) Glucose is used in other processes that sustain plant life. Living organisms gain the energy for life processes by breaking down glucose back into carbon dioxide and water, which are released into the atmosphere. C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l) Other sources of carbon present in the environment comes from inorganic origins. They can be found in rocks, sediments, dissolved in water, and as carbon dioxide in the air. The carbon cycle describes a set of processes that regulates the amount of carbon dioxide in the atmosphere. The carbon cycle describes how carbon-containing compounds are converted from one form to another. It involves land, oceans and lakes, and the atmosphere. Most fossil fuels contain mainly methane. They are burnt in the presence of oxygen to generate energy. Carbon dioxide is a product of combustion. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) ## 22.5 What Are Global Warming and Climate Change? ### Learning Outcomes * State that carbon dioxide and methane are greenhouse gases and contribute to global warming. * Describe how global warming can lead to effects like climate change and the melting of polar ice. Global warming has major implications for human civilization. It is predicted that extreme weather phenomena like typhoons and tornadoes will become stronger and occur more frequently. Parts of the world that are currently fertile will quickly become arid and barren. Island nations like Singapore are also likely to face challenges related to sea levels rising from melting polar ice sheets and the thermal expansion of ocean water. How does global warming occur and how does it cause climate change? The average temperature of the Earth has increased slightly more than 1 °C since the year 1900. This may not sound like much, but a huge amount of energy is needed to raise the average temperature of the Earth by 1°C. Furthermore, a large part of this increase occurred in the last 40 years, indicating that the trend is accelerating. Global warming is the increase in the average temperature of the Earth's surface due to increasing amounts of greenhouse gases in the atmosphere. Radiation from the sun reaches the Earth in a variety of wavelengths. Short wavelength radiation is absorbed by the Earth and converted to long wavelength radiation. Some of this radiation then escapes into space but the remainder is trapped by greenhouse gases in the atmosphere as heat. This is similar to how heat is trapped within a greenhouse to grow crops that require warmer temperatures. A certain level of greenhouse effect is essential to life on Earth. If the atmosphere did not trap heat, then Earth would be too cold to support life, like the planet Mars and the icy moons of the outer planets. On the other hand, too strong a greenhouse effect would make Earth searingly hot like the planet Venus, where the surface temperatures are hot enough to melt lead. A fine balance is needed to maintain planetary conditions that allow life to thrive. Carbon dioxide makes up 0.04% of clean air by volume. It is the most common greenhouse gas. The carbon cycle normally regulates the amount of carbon dioxide present in the atmosphere. However, human activities have caused the output of carbon dioxide to surpass its intake ever since the start of the Industrial Revolution in the 18th century. The combustion of fossil fuels in factories, power plants and vehicles is a major contributor of atmospheric carbon dioxide. These sources produced 36.7 billion tonnes of carbon dioxide, or about 4700 kg per human on Earth. The clearing of forests for agriculture is also a large contributor to carbon dioxide emissions. It has the added effect of reducing the number of plants available to absorb and retain carbon. Cattle are a significant source of methane, which is an even stronger greenhouse gas than carbon dioxide. The greenhouse effect of methane can be up to 80 times as strong as that of carbon dioxide. | Greenhouse Gas | Major Sources | |---|---| | Carbon dioxide (CO₂) | combustion of fossil fuels in power plants, industries and vehicles, deforestation | | Methane (CH₄) | waste gases from cattle, leakage from methane reservoirs under the arctic tundra and from oceanic methane ice | The word "climate" refers to weather conditions over a period of time. The climate in an area is influenced by factors such as ocean temperature, wind speed and direction, and humidity. Since the temperature across the world is increasing, this leads to climate change because these factors are disturbed. Shifting wind patterns can lead to changes in rainfall patterns. Lush areas might begin to receive too little rain and turn into deserts while other areas may receive too much rain and flood more often than before. Fertile land would become barren and will no longer be able to produce food, potentially causing food shortages. Heat waves would also become more common. Some parts of the world would even become too hot for humans to live in. Higher global temperatures would also make destructive wildfires more common, which could destroy entire ecosystems. The heatwaves in India have killed thousands of people, especially the weak and the old, as temperatures rise beyond 40 °C. Deadly heatwaves are also becoming more common in other parts of the world, like Europe. Typhoons, hurricanes and cyclones are different names for the same kind of extreme weather—giant tropical storms that rotate around a clear "eye". They form over warm water and move westwards, bringing strong winds, sheets of rain, and an ultra high tide called the storm surge. Tropical storms can be very destructive when they hit a populated coastal area. Not only do they endanger human lives and destroy property, they also damage farmland due to the increase in salt content brought by the salt water. In 2005, Hurricane Katrina hit the south-east of the United States of America, causing US$ 150 billion worth of damage. The warmer the ocean, the stronger these storms become. There has been a notable increase in their strength and frequency since the 1980s. Marine organisms can be very sensitive to water temperatures. Coral reefs are a key marine habitat and rising temperatures have caused them to bleach and die. This greatly reduces marine biodiversity as habitats are lost. Population of fish species, many of which humans depend upon for food, are severely cut. More than half of the corals in the Great Barrier Reef, the world's largest coral reef off the eastern coast of Australia, has bleached and died since 1995. Warmer ocean waters also absorb carbon dioxide more quickly, forming carbonic acid which decreases the pH of the water. Crustaceans such as crabs, corals, oysters and plankton are negatively impacted. They depend on carbonate minerals to form their shells. Acidified waters remove carbonate minerals from the ocean via acid-carbonate reactions. Existing shells might also begin to dissolve, killing the organisms or making them more vulnerable to injury and predation. About 70% of the world's freshwater is locked up as ice in the polar ice caps. The entire continent of Antarctica is a giant sheet of ice. Increases in average global temperatures have caused glaciers and polar ice caps to melt and shrink. Many communities rely on a steady stream of melt-water from glaciers for drinking and for irrigation. Their lives will be greatly affected if the glaciers deplete faster than they can be replenished. The water level in these rivers may rise initially, but as the ice depletes, the rivers will eventually dry up. It has been estimated that sea levels around the world would rise by more than 60 m if the ice caps completely melted. This would permanently flood most coastal cities like Singapore since 60 m is about 20 storeys high. The sea level around Singapore has already risen by more than half a meter since 1975. Adding a large amount of fresh water to the ocean could also disrupt ocean currents. Ocean currents redistribute heat around the globe and help to stabilise global climate patterns. Their disruption would further amplify the effects of climate change. | Climate Change Phenomena | Effects | |---|---| | desertification of fertile land | The amount of food that can be produced globally would decrease. | | more frequent and severe heat waves | High temperatures can be fatal, especially for the elderly and frail. | | increase in frequency and intensity of extreme weather (e.g. typhoons and tornadoes) | Property is destroyed and lives may be lost. Cyclonic storms can permanently decrease the fertility of farmland near the coast due to saltwater deposition. | | ocean warming and acidification | Coral reefs are bleached and marine habitats are lost. Commercially important fish populations would be depleted. Acidified waters dissolve the shells of crustaceans. | | glacial retreat and melting of polar ice caps | Glacier-fed river levels would change. Sea levels would rise and permanently flood coastal areas. Climate-stabilising ocean currents would be disrupted. | ## Let's Practise 22.5 1. What is global warming? 2. Explain how an increase in greenhouse gas emission can lead to global warming. 3. State three consequences of climate change.