An oxide is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of -2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water. Hydrocarbon combustion affords the two principal carbon oxides: carbon monoxide and carbon dioxide.

Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure.

Acid rain is caused by emissions of sulfur dioxide and nitrogen oxides, which react with the water molecules in the atmosphere to produce acids. Governments have made efforts since the 1970s to reduce the release of sulfur dioxide into the atmosphere with positive results. Nitrogen oxides can also be produced naturally by lightning strikes and sulfur dioxide is produced by volcanic eruptions. The chemicals in acid rain can cause paint to peel, corrosion of steel structures such as bridges, and erosion of stone statues.

In this unit you will:

  • Identify and distinguish between acidic, basic, amphoteric and neutral oxides
  • Explain how acid rain forms and the impact it has on the environment
  • Perform stoichiometric calculations involving volumes of gases


An oxide is a chemical compound that contains at least one oxygen atom and one other element. There are four types of oxides of the elements: acidic, basic, neutral and amphoteric.

Basic Oxides

These oxides are metal oxides that react with acids. They also react with water to produce a basic solution.

Common basic oxides Na2O, MgO, CaO, FeO, CuO, Cu2O
Reaction with an acid Na2O (s) + H2SO4 (aq) →  Na2SO4 (aq) + H2O (l)
Reaction with water MgO (s)  +  H2O (l) →  Mg(OH)2 (l)

Acidic Oxides

These are non-metal oxides that react with bases. They also react with water to produce an acidic solution.

Common acidic oxides CO2, SO2, SO3, NO2, N2O5, P4O10 
Reaction with a base CO2 (g) + NaOH (aq) → Na2CO3 (aq) + H2O (l)

SO2 (g) + 2NaOH (aq) → Na2SO3 (aq) + H2O (l)
Reaction with water CO2 (g) + H2O (l)→ H2CO3 (aq)

SO3 (g) + H2O (l) → H2SO4 (aq)

SO2 (g) + H2O (l) → H2SO3 (aq)

Amphoteric Oxides

Amphoteric oxides are oxides that can act as an acid or a base. These oxides react as acids when reacted with a base and act as bases when reacted with an acid.

Common amphoteric oxides Al2O3, Fe2O3, ZnO, H2O, Ga2O3 
Reaction with an acid Al2O3 (s) + 6HCl (aq) → 2AlCl3 (aq) + 3H2O (l) 
Reaction with a base Al2O3 + 2NaOH → 2NaAlO2 + H2O

Neutral Oxides

These oxides do not react with water or acids or bases.

Common neutral oxides NO, CO, N2O


Acidic Oxides in the Atmosphere

The main acidic oxides present in the atmosphere are sulfur dioxide and oxides of nitrogen. Carbon dioxide is also present to a large extent but is not a major factor causing acid rain.

Some coal or oil reserves contain considerable quantities of sulfur compounds. Most sulfur dioxide (SO2) released into the atmosphere comes from the burning of such coal or oil in electric power stations. Sulfur dioxide is the major contributor to acid rain that can affect places thousands of kilometres from the source. The low sulfur content of Australian coal is one reason why Australia is the world's largest exporter of coal.

Smelters where metal sulfides are heated in air to remove the sulfur as SO2 are becoming a smaller source of atmospheric SO2. Most of the sulfur dioxide released is now used to make sulfuric acid. The huge amount of sulfur dioxide that used to be released from Mt Isa mine's smelter in Queensland is now changed to sulfuric acid and transported by rail 150 km away to make ammonium sulfate and superphosphate fertilisers at Phosphate Hill.

Volcanoes are an unpredictable source of sulfur dioxide. After Mt Pinatubo erupted in the Philippines in 1991, oxidation of the emitted sulfur dioxide and reaction with water formed an aerosol of sulfuric acid droplets. This aerosol reduced global temperatures by about 0.5oC.

Oxides of nitrogen are produced in car engines and other high temperature combustion environments. This is because the temperature is high enough to cause the nitrogen  and oxygen in air to react. Some oxides of nitrogen are also produced from lightning during electrical storms.

Equations Showing Formation of Acidic Oxides

In a metal sulfide smelter, the ore is heated in air and converts to a metal oxide, releasing sulfur dioxide.

2ZnS (s) + 3O2 (g) → 2ZnO (s) + 2SO2 (g)

When coal is burnt, sulfur in the coal reacts with oxygen and is converted to sulfur dioxide.

S (s) + O2 (g) → SO2 (g)

Lightning strikes cause reaction between the two most common gases in the atmosphere.

N2 (g) + O2 (g) → 2NO (g)

High temperature combustion reactions in furnaces and internal combustion engines in cars produce significant amounts of nitrogen monoxide at temperatures above 1300oC.

N2 (g) + O2 (g) → 2NO (g)

Colourless, neutral nitrogen monoxide reacts with oxygen in the air to form brown, acidic nitrogen dioxide [nitrogen(IV) oxide].

2NO (g) + O2 (g) → 2NO2 (g)

Evidence Regarding Concentrations of Oxides of Sulfur and Nitrogen in the Atmosphere

The annual emissions of oxides of sulfur and nitrogen into the
atmosphere since the 1940s. Emissions have stabilised since the
1980s due to more strict evironmental controls by governments.
Emissions of oxides of nitrogen have increased since the 1950s as
a result of more cars in use around the world.

Evidence regarding the changing concentration of these oxides is outlined below.

  • Measurement of atmospheric concentrations of these oxides began in the 1950s and were significantly improvemed in the 1970s with the development of infrared spectroscopy. Levels of these oxides usually stays around 0.01 ppm but it becomes difficult to measure their concentrations with a high degree of confidence at these concetrations.

  • Oxides of sulfur and nitrogen can also exist in aqueous form in the atmosphere as acid rain and eventually wash out into rivers and lakes where it does the most damage by lowering pH. There are no known ways to reliably produce accurate measurements of these aqueous concentrations.

  • There has been increasing incidence of acid rain in localised areas where sulfur and nitrogen pollution is high. There has been direct evidence of increasing incidence of heavy metal contamination, an effect of decreasing pH in water ways which leech heavy metal contaminates from rocks.

  • There has been increased corrosion of the built environment since the Industrial Revolution.

Although we do not have evidence to suggest a global increase in the concentration of these oxides, a combination of the above evidence would seem to suggest that there are localised concentrations of oxides of sulfur and nitrogen and that they have been increasing since the industrial revolution.

Formation of Acid Rain

Acid rain, or more precisely acid precipitation, is the word used to describe rainfall that has a pH level of less than 5.6.  Acid rain is caused by gaseous acidic oxides permeating the atmosphere and reacting with the water present there to form acidic solutions. Unpolluted rainwater is slightly acidic because of the presence of carbon dioxide in the atmosphere and will have a pH of about 5.6. Rain is usually described as acid rain if it has a pH of less than this. In Australia, rain with a pH of 3.6 has been recorded in the lower Hunter Valley north of Sydney due to a number of coal burning power stations in the area.

Reactions of Sulfur Dioxide to Produce Acid Rain

Once in the atmosphere, sulfur dioxide can itself be oxidised by atmospheric oxygen to form sulfur trioxide.

SO2 (g) + O2 (g) → SO3 (g)

Both sulfur dioxide and sulfur trioxide are irritating gases that can produce respiratory distress and choking at reasonably low concentrations.

In the atmopshere, both sulfur dioxide and sulfur trioxide react with water to produce acidic solutions.

SO2 (g) + H2O (l) → H2SO3 (aq)

SO3 (g) + H2O (l) → H2SO4 (aq)

In the first equation sulfurous acid (H2SO3) is produced and in the second equation sulfuric acid (H2SO4) is produced. You should remember that the presence of the hydrogen ions in an aqueous solution tells us that these substances are acids. The second equation producing sulfuric acid could easily be continued as

H2SO4 (aq)2H+ (aq) + SO42- (aq)

Both sulfurous and sulfuric acid in rainwater would significantly lower its pH and are major causes of acid rain.

Reactions of Oxides of Nitrogen to Produce Acid Rain

As we have seen above, once nitrogen monoxide gets into the atmosphere it reacts further with oxygen to produce nitrogen dioxide. Nitrogen dioxide then reacts with waterin two different reactions, both of which produce nitric acid (HNO3) and the first of which produces nitrous acid (HNO2)as well.

2NO2 (g) + H2O (l) → HNO2 (aq) + HNO3 (aq)

4NO2 (g) + O2 (g) + 2H2O (l) → 4HNO3 (aq)


Effects of Acid Rain

Acid rain causes acidification of lakes and streams and contributes to damage of trees at high elevations (for example, red spruce trees above 600 m) and many sensitive forest soils. In addition, acid rain accelerates the decay of building materials and paints, including irreplaceable buildings, statues,and sculptures that are part of our nation's cultural heritage. Prior to falling to the earth, sulfur dioxide and nitrogen oxide gases and their particulate matter derivatives, sulfates and nitrates, contribute to visibility degradation and harm public health.

Lakes & Streams

The ecological effects of acid rain are most clearly seen in the aquatic, or water, environments, such as streams, lakes, and marshes. Acid rain flows to streams, lakes, and marshes after falling on forests, fields, buildings, and roads. Acid rain also falls directly on aquatic habitats. Most lakes and streams have a pH between 6 and 8, although some lakes are naturally acidic even without the effects of acid rain. Acid rain primarily affects sensitive bodies of water, which are located in watersheds whose soils have a limited ability to neutralize acidic compounds (called "buffering capacity"). Lakes and streams become acidic (pH value goes down) when the water itself and its surrounding soil cannot buffer the acid rain enough to neutralize it. In areas where buffering capacity is low, acid rain also releases aluminium from soils into lakes and streams; aluminium is highly toxic to many species of aquatic organisms.

Acid rain causes a cascade of effects that harm or kill individual fish, reduce fish population numbers, completely eliminate fish species from a waterbody, and decrease biodiversity. As acid rain flows through soils in a watershed, aluminium is released from soils into the lakes and streams located in that watershed. So, as pH in a lake or stream decreases, aluminum levels increase. Both low pH and increased aluminum levels are directly toxic to fish. In addition, low pH and increased aluminum levels cause chronic stress that may not kill individual fish, but leads to lower body weight and smaller size and makes fish less able to compete for food and habitat.

Some types of plants and animals are able to tolerate acidic waters. Others, however, are acid-sensitive and will be lost as the pH declines. Generally, the young of most species are more sensitive to environmental conditions than adults. At pH 5, most fish eggs cannot hatch. At lower pH levels, some adult fish die. Some acid lakes will have no fish at all.


Over the years, scientists, foresters, and others have watched some forests grow more slowly without knowing why. The trees in these forests do not grow as quickly at a healthy pace. Leaves and needles turn brown and fall off when they should be green and healthy. In extreme cases, individual trees or entire areas of the forest simply die off without an obvious reason.

Researchers now know that acid rain causes slower growth, injury, or death of forests. Acid rain has been implicated in forest and soil degradation in many areas of the eastern United States, particularly high elevation forests of the Appalachian Mountains from Maine to Georgia that include areas such as the Shenandoah and Great Smoky Mountain National Parks. Of course, acid rain is not the only cause of such conditions. Other things that add stress, such as air pollutants, insects, disease, drought, or very cold weather also harm trees and plants. In most cases, in fact, the impacts of acid rain on trees occur due to the combined effects of acid rain and these other environmental stressors. After many years of collecting information on the chemistry and biology of forests, researchers are beginning to understand how acid rain works on the forest soil, trees, and other plants.

Acid rain does not usually kill trees directly. Instead, it is more likely to weaken trees by damaging their leaves, limiting the nutrients available to them, or exposing them to toxic substances slowly released from the soil. Quite often, injury or death of trees is a result of these effects of acid rain in combination with one or more additional threats.

Scientists know that acidic water dissolves the nutrients and helpful minerals in the soil and then washes them away before trees and other plants can use them to grow. At the same time, acid rain causes the release of substances that are toxic to trees and plants, such as aluminum, into the soil. Scientists believe that this combination of loss of soil nutrients and increase of toxic aluminum may be one way that acid rain harms trees. Such substances also wash away in the runoff and are carried into streams, rivers, and lakes. More of these substances are released from the soil when the rainfall is more acidic.

However, trees can be damaged by acid rain even if the soil is well buffered. Forests in high mountain regions often are exposed to greater amounts of acid than other forests because they tend to be surrounded by acidic clouds and fog that are more acidic than rainfall. Scientists believe that when leaves are frequently bathed in this acid fog, essential nutrients in their leaves and needles are stripped away. This loss of nutrients in their foliage makes trees more susceptible to damage by other environmental factors, particularly cold winter weather.

Automotive Coatings

Over the past two decades, there have been numerous reports of damage to automotive paints and other coatings. The reported damage typically occurs on horizontal surfaces and appears as irregularly shaped, permanently etched areas. The damage can best be detected under fluorescent lamps, can be most easily observed on dark colored vehicles, and appears to occur after evaporation of a moisture droplet. In addition, some evidence suggests damage occurs most frequently on freshly painted vehicles. Usually the damage is permanent; once it has occurred, the only solution is to repaint.

The general consensus within the auto industry is that the damage is caused by some form of environmental fallout. "Environmental fallout," a term widely used in the auto and coatings industries, refers to damage caused by air pollution (e.g., acid rain), decaying insects, bird droppings, pollen, and tree sap. The results of laboratory experiments and at least one field study have demonstrated that acid rain can scar automotive coatings. Furthermore, chemical analyses of the damaged areas of some exposed test panels showed elevated levels of sulfate, implicating acid rain.

The auto and coatings industries are fully aware of the potential damage and are actively pursuing the development of coatings that are more resistant to environmental fallout, including acid rain. The problem is not a universal one - it does not affect all coatings or all vehicles even in geographic areas known to be subject to acid rain - which suggests that technology exists to protect against this damage. Until that technology is implemented to protect all vehicles or until acid deposition is adequately reduced, frequent washing and drying and covering the vehicle appear to be the best methods for consumers who wish to minimize acid rain damage.

The Built Environment

Acid rain and the dry deposition of acidic particles contribute to the corrosion of metals (such as bronze) and the deterioration of paint and stone (such as marble and limestone). These effects seriously reduce the value to society of buildings, bridges, cultural objects (such as statues, monuments, and tombstones), and cars.

Dry deposition of acidic compounds can also dirty buildings and other structures, leading to increased maintenance costs. To reduce damage to automotive paint caused by acid rain and acidic dry deposition, some manufacturers use acid-resistant paints.

To observe the effects of acid rain on marble and limestone, two building materials commonly used in monuments, ancient buildings, and in many modern structures you can place a piece of chalk in a bowl with white vinegar and another piece in a bowl of tap water. Leave the dishes overnight and the next day you will see that the piece of chalk is more worn away.

This experiment with chalk allows you to see the effect of acid rain on marble and limestone because chalk is made of calcium carbonate, a compound occurring in rocks, such as marble and limestone, and in animal bones, shells, and teeth. You should remember and be able to write equations showing the reaction of an acid with a carbonate as well as the reaction of an acid with an active metal.


Sulfates and nitrates that form in the atmosphere from sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions contribute to visibility impairment, meaning we can't see as far or as clearly through the air. Sulfate particles account for 50 to 70 percent of the visibility reduction in the eastern part of the United States.

Human Health

Acid rain looks, feels, and tastes just like clean rain. The harm to people from acid rain is not direct. Walking in acid rain, or even swimming in an acid lake, is no more dangerous than walking or swimming in clean water. However, the pollutants that cause acid rain (sulfur dioxide and nitrogen oxides) also damage human health. These gases interact in the atmosphere to form fine sulfate and nitrate particles that can be transported long distances by winds and inhaled deep into people's lungs. Fine particles can also penetrate indoors. Many scientific studies have identified a relationship between elevated levels of fine particles and increased illness and premature death from heart and lung disorders, such as asthma and bronchitis.


Calculations Involving Gas Volumes

In 1808, Joseph Gay-Lussac, a French chemist who lived from 1778-1850, his observations on gaseous reactions together to form a law which bears his name. Gay Lussac's Law that states that the relation between the combining volumes of gases and the volume of their products (if they are two gases) may be expressed in small, whole numbers. This law, also called the Law of Combining Gases, is one of the most important in chemistry. Restated it says that:

Volumes of gases in any chemical change bear a simple ratio to one another under the same conditions of temperature and pressure.

In 1811, the gas law of Gay-Lussac led an Italian, Amedeo Avogadro, to advance an explanation or hypothesis as to why gases combine in simple volume ratios according to the coefficients of their equations. Specifically, his hypothesis explained why two volumes of hydrogen unite with one volume of oxygen to make two (not three) volumes of water vapor and why three volumes of hydrogen and one volume nitrogen make two volumes of ammonia gas. Up until this time, no one had considered that a gas molecule might consist of two atoms. But Avogadro reasoned that if it did, Gay Lussac's law of combining volumes could be explained. Thus graphically measurements of other gas combinations confirmed Avogadro's assumption that the molecules of common gases are composed of two atoms each.

This assumption, combined with Boyle's and Charles' gas laws, led Avogadro to state that:

All gases behave alike because equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules.

Much later the actual number of molecules in a given volume of any gas was determined.

Because equal volumes of gas contain the same number of particles at the same temperature and pressure, it follows that one mole of any gas should have the same volume at the same temperature and pressure.

  • The volume of one mole of gas at 0°C and 100 kPa = 22.71 L
  • The volume of one mole of gas at 25°C and 100 kPa = 24.79 L

The molar volume of a gas is the volume occupied by 1 mole of the gas at 298 K (25°C) and 100 kPa of pressure is 24.79 L. This is known as Room Temperature and Pressure or RTP.  For example, 1 mole of oxygen gas would occupy 24.79 L at 25°C and half a mole of oxygen gas would therefore occupy 12.4 L.

The number of moles of a gas is
the sample volume of the gas
divided by the molar volume of the

Because gases contract when cooled, different temperatures will have different values for volume. The volume of one mole of any gas at 273 K (0°C) and 100 kPa of pressure is  22.71 L. This is known as Standard Temperature and Pressure or STP. For example, 1 mole of oxygen gas, would occupy 22.71 L at 0°C and half a mole of oxygen gas would therefore occupy 11.35 L.

There are values for the molar volumes of gases at STP and RTP on the data sheet that is provided in the HSC exam.

Worked Example

If 10 million kg of coal containing 0.1% sulfur is burned in a power station, what volume of sulfur dioxide is released at 25°C and 100 kPa?

mass sulfur = (0.1 x 10 000 000 000 g) / 100 = 10 000 000 g

number moles sulfur = 10 000 000 g / 32.065 g mol-1 = 311 867 mol

volume sulfur dioxide = 311 867 mol x 24.79 L mol-1 = 7 731 183 L = 7.7 ML