Home > Chemistry > Core > The acidic environment > The acidic environment: 2. Acid oxides in the atmosphere

9.3 The acidic environment: 2. Acid oxides in the atmosphere

Syllabus reference (October 2002 version)
2. While we usually think of the air around us as neutral, the atmosphere naturally contains acidic oxides of carbon, nitrogen and sulfur. The concentrations of these acidic oxides have been increasing since the Industrial Revolution
Students learn to: Students:
Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 11 Jun 10]

Prior learning: Preliminary modules 8.3.3.

Background: Just as elements show a pattern in properties in the Periodic Table so metal oxides and non-metal oxides show a pattern in properties. Metal oxides are usually basic and non-metal oxides usually acidic. The extent of the acidity or basicity of an oxide can often be predicted from the element's position in the Periodic Table.

identify oxides of non-metals which act as acids and describe the conditions under which they act as acids

Go To Top

analyse the position of these non-metals in the Periodic Table and outline the relationship between position of elements in the Periodic Table and acidity/basicity of oxides

Further information

A substance is said to be basic if it:

  • dissolves in water to produce a solution that turns litmus blue, conducts electricity and (don't try this!) has a bitter taste


  • reacts with an acid removing the acid properties.

A substance is said to be acidic if it:

  • dissolves in water to produce a solution that turns litmus red, conducts electricity and (once again, don't try this!) has a sour taste


  • reacts with a base removing the base properties.

A base can remove acid properties and an acid can remove base properties because of a reaction called neutralisation:

acid + base right arrow salt + water

At the end of the 1800s, chemists pictured acid solutions as containing hydrogen ions, H+, and base solutions as containing hydroxide ions, OH-. The acidic properties were due to the hydrogen ions and the basic properties were due to the hydroxide ions. When an acid solution and base solution were mixed, the hydrogen ions and hydroxide ions combined to form water molecules.

H+ + OH- right arrow H2O

The reaction between the base sodium hydroxide and hydrochloric acid can be represented by:

  • a word equation
  • a full ionic equation
  • an ionic equation (showing no spectator ions) or
  • a full formula (also known as neutral formula or balanced formula) equation.

    For example:

    sodium hydroxide + hydrochloric acid right arrow sodium chloride + water

    Na+ + OH- + H+ + Cl- right arrow Na+ + Cl- + H2O

    OH- + H+ right arrow H2O

    NaOH + HCl right arrow NaCl + H2O

    The salt ions, Na+ and Cl-, are called spectator ions because they don't actually react. These ions are floating around separately in the base and acid solutions before mixing and are floating around in the neutralisation mixture after mixing. If the salt solution formed is evaporated, the salt ions will come together to form solid salt, but this is not a chemical reaction.

A term that is sometimes used instead of basic solution is alkaline solution. A basic solution and an alkaline solution refer to solutions with pH > 7.

An alkali is a water soluble base; usually a group 1 or group 2 metal hydroxide. Group 1 and group 2 refer respectively to the elements in the first and second columns of the Periodic Table. Group 1 elements are called the alkali metals and group 2 elements are called the alkaline earth metals.

Alkalis are just one type of base. The Venn diagram below illustrates that alkalis are a subset of bases.

Venn Diagram depicting alkalis set within the set of bases

However, in some texts, all basic solutions are called alkaline solutions.

Go To Top

define Le Chatelier's principle

Go To Top

identify factors which can affect the equilibrium in a reversible reaction

Further information

Change in concentration

The principle can be illustrated by the changes you observe in a solution of indicator, such as litmus. An indicator is a carbon compound that can be represented by HIn. H is an hydrogen atom that can be released as an hydrogen ion H+.

In represents the rest of the carbon compound.

In a neutral purple litmus solution, there is a mixture of red HIn molecules and blue In- ions. There is an equilibrium between the red HIn and the blue In-.

HIn double arrows H+ + In-
red           blue

An acid is a substance that produces H+ ions in solution. If an acid is added to a purple litmus solution, the solution turns red. Using Le Chatelier's principle, the higher concentration of hydrogen ions is predicted to cause an equilibrium shift to the left, producing the red form of litmus. This is what is actually observed when acid is added.

Change in temperature

When an acidic oxide gas such as carbon dioxide dissolves in water heat is released (the forward reaction as written is exothermic).

H2O(l) + CO2(g) double arrows H2CO3(aq) + heat

If the carbonic acid solution formed is heated the equilibrium shifts to the left and carbon dioxide gas is released. This happens whenever a solution of a gas is heated. Raising the temperature of a solution of a gas in water lowers the solubility of the gas.

Change in gas pressure

If the pressure of the gas above a water solution of the gas is raised, then more gas goes into solution. If the pressure of gas above a solution of the gas in water is decreased, then gas comes out of solution.

You have probably noticed that when you take the lid off a bottle of carbonated-water (water containing dissolved carbon dioxide) soft drink that bubbles of carbon dioxide gas form and escape the solution.

H2O(l) + CO2(g) double arrows H2CO3(aq)

When you took the lid off the bottle, the concentration of CO2 above the solution decreased. The equilibrium shifted to the left to produce more CO2 gas.

Go To Top

identify data, plan and perform a first-hand investigation to decarbonate soft drink and gather data to measure the mass changes involved and calculate the volume of gas released at 25oC and 100kPa

Methods that could be used to decarbonate soft drink

What you might need:

  • A means of weighing to at least the nearest gram.

  • 250 or 300 mL bottles or cans of soda water; buy unopened bottles with the liquid level as low as possible.

  • For the warming method: a source of dry heat, such as an electric hotplate or a saucepan for gently warming the soda water, a dry towel and a thermometer.

  • For the salting-out method: 1g of table salt for each 50 mL of soda water.

  • It is important that heating or addition of salt is gradual so that the soda water does not foam or spray out of the container. Such loss of mass would require you to start all over again.

  • To achieve a more accurate result in these two methods incorporate a control that will allow you to subtract the amount of water lost due to evaporation.

Calculations required to determine the volume of gas released

  • Loss of mass due to escape of carbon dioxide gas.

  • Conversion of grams of CO2 lost to moles of CO2.

  • Use of the knowledge that one mole of gas at 25oC and 100kPa occupies 24.8 L.
Go To Top

describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier's principle

Go To Top

calculate volumes of gases given masses of some substances in reactions, and calculate masses of substances given gaseous volumes, in reactions involving gases at 0oC and 100kPa or 25oC and 100kPa


When acid is added to certain anions, gas is formed. For example:

  • acid + carbonate 2HCl + CaCO3 right arrow CaCl2 + CO2 + H2O
  • Net ionic equation: 2H+ + CO32- right arrow CO2 + H2O

  • acid + sulfide 2HBr + FeS right arrow FeBr2 + H2S
  • Net ionic equation: 2H+ + S2- right arrow H2S

1 mole of gas at 100kPa has a volume of 22.71 L at 273.15 K (0oC) or 24.79 L at 298.15 K (25oC).

Go To Top

identify natural and industrial sources of sulfur dioxide and oxides of nitrogen

National Pollutant Inventory Database for Australia (external website) 2008/2009 data within Australia - Sulfur dioxide from All Sources, Department of Environment, Water, Heritage and the Arts, Australia.

The figures for Oxides of Nitrogen Department of Environment, Water, Heritage and the Arts, Australia.

Go To Top

describe using equations, examples of chemical reactions which release sulfur dioxide and chemical reactions which release oxides of nitrogen

Go To Top

analyse information from secondary sources to summarise the industrial origins of sulfur dioxide and oxides of nitrogen and evaluate reasons for concern about their release into the environment

Specific information on industrial origins of pollutants in Australia can be found in the National Pollutant Inventory Database. (external website) Reasons for concern about release of these gases into the environment can be found by:

Go To Top

assess the evidence which indicates increases in atmospheric concentration of oxides of sulfur and nitrogen

Go To Top


 Acid rain effects described (external website) United States Environmental Protection Agency, US

(These web sites last checked 27 June 2008)

Go To Top

Neals logo | Copyright | Disclaimer | Contact Us | Help