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9.3 The acidic environment 3. Acids

Syllabus reference (October 2002 version)
3. Acids occur in many foods, drinks and even within our stomachs
Students learn to: Students:
Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 11Jun 10]

Prior Learning: Preliminary module 8.4

Background: Acids such as ascorbic acid (vitamin C) and citric acid occur in many foods. Many drinks contain carbonic acid and some contain phosphoric acid. Other acids, such as benzoic acid and acetic acid, are added to drinks and food to act as preservatives. Hydrochloric acid is secreted into the human stomach to assist in the digestion of food, especially of proteins to amino acids.

define acids as proton donors and describe the ionisation of acids in water

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identify acids such as acetic (ethanoic acid), citric (2-hydroxypropane-1,2,3-tricarboxylic acid), hydrochloric and sulfuric acid

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describe the use of the pH scale in comparing acids and

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identify pH as -log10 [H+] and explain that a change in pH of 1 means a ten-fold change in [H+]

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process information from secondary sources to calculate pH of strong acids given appropriate hydrogen ion concentrations

Once you have checked secondary sources to see that the acid is a strong acid and to find out whether it is monoprotic, diprotic or triprotic you can process the information to calculate the hydrogen ion concentration.

pH = -log10 [H+] can then be used to calculate the pH from the hydrogen ion concentration.

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solve problems and perform a first-hand investigation to use pH meters/probes and indicators to distinguish between acidic, basic and neutral chemicals

Operation of a pH meter (external website) Dearborn Science Learning Center, University of Michigan, USA

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plan and perform a first-hand investigation to measure the pH of identical concentrations of strong and weak acids

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gather and process information from secondary sources to write ionic equations to represent the ionisation of acids

Summary: Writing ionic equations for acids

If an acid is a strong acid, the equation will usually be written with an arrow, right arrow, from left to right showing that ionisation of the acid molecules is almost complete.

If an acid is a weak acid, the equation will usually be written with the reversible arrows, Double arrows , that show that significant amounts of reactants (un-ionised molecules) as well as products ( H+ and an acid anion) are present in equilibrium.

Organic acids, such as acetic acid and citric acid, contain –COOH groups. The H can be attracted to a water molecule as a H+ .

Chemical equation

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describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions

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use available evidence to model the molecular nature of acids and simulate the ionisation of strong and weak acids

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describe acids and their solutions with the appropriate use of the terms strong, weak, concentrated and dilute

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compare the relative strengths of equal concentrations of citric, acetic and hydrochloric acids and explain in terms of the degree of ionisation of their molecules

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gather and process information from secondary sources to explain the use of acids as food additives

Acids as food additives

Some acidic food preservatives listed from strongest to weakest acid are:

  • sulfur dioxide    SO2
  • lactic acid    CH3CHOHCOOH
  • acetic (ethanoic) acid    CH3COOH
  • propionic (propanoic) acid    CH3CH2COOH

Food Standards Australia New Zealand (FSANZ) has only one acidic oxide listed as a food preservative. That oxide is sulfur dioxide, which can be added to food as the gas, as a solution in water or as a sulfite, bisulfite or metabisulfite salt.

Acidic food preservatives prevent micro-organisms from decomposing the food.

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identify data, gather and process information from secondary sources to identify examples of naturally occurring acids and bases and their chemical composition

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