Acids, bases and salts
Francine Taylor-Campbell, Contributor

REAL BIG MAN! A boy points to his friend, Conrad White (right), who plays in the Manning Cup for Waterford High School. - Anthony Minott/Freelance Photographer

You should be able to: Define acid, acid anhydride, base, alkali, salt, acidic, basic, amphoteric and neutral oxides.

Relate acidity and alkalinity to the pH scale.

Discuss the strengths of acids and alkalis on the basis of their completeness of ionisation.

Points to note

• Non-metals form acidic oxides while metals form basic oxides.
  
  
• Amphoteric oxides show both basic and acidic properties, while neutral oxides show neither.
  
  
• Non-metallic oxides dissolve in water to form acids.
  
  
• Metallic oxides are basic oxides and dissolve in water to form alkalis.
  
  
• Acids combine with bases to form salt and water only.
  
  
• Salts can be formed from the reaction of acids with carbonates, hydrogen-carbonates, alkalis and metals.
  
  
• The pH of a substance indicates how acidic or basic (alkaline) its aqueous solution is. The pH scale goes from 0 - 14 with a pH less than seven described as acidic and a pH greater than seven described as basic or alkaline. One can use pH paper (litmus) or indicator solutions, such as methyl orange and phenolphthalein, to determine whether substances are alkaline or acidic.

An acid is a substance which dissolves in water or aqueous solution to produce H+ions. An acid is formed when an acid anhydride (acidic oxide) dissolves in water.

Some acids are stronger than others. The strength of an acid depends on how completely it ionises or breaks up into ions when in solution. So the mineral acids, such as hydrochloric, sulphuric and nitric acids, completely dissociate in solution and are called strong acids.

HCl (aq) H+ (aq) + Cl- (aq) There are no undissociated molecules.

For weak acids, a large number of their molecules remain undissociated in solution, hence ionisation is incomplete. Organic acids are generally weak, such as ethanoic (acetic) acid.

Alkalis can also be strong or weak. NaOH is a strong alkali, while aqueous ammonia is weak as it is only slightly dissociated in aqueous solution.

Let us examine the question below:

1. (a) Sulphur (IV) oxide, a primary pollutant, can dissolve in water droplets in the atmosphere.

(i) Write the formula for sulphur (IV) oxide. (1 mark)

(ii) Write an equation for the reaction of sulphur (IV) oxide with water. (1 mark)

(iii) What happens to the pH of water when sulphur (IV) oxide dissolves in it? (2marks)

(b) It has been suggested that the amount of polluting gases, like the oxides of sulphur and nitrogen, could be reduced by passing exhaust air from factories, through beds of powdered reagents.

(i) Give the name of ONE reagent that could be used for this purpose. (1 mark)

(ii) Explain why the reagent should be used in a powdered form. (1 mark)

(iii) Explain how the reagent could work to reduce the quantity of polluting gases in the atmosphere. (1 mark)

(c) Zinc is one of the metals that forms an amphoteric oxide.

(i) What is meant by the term, 'amphoteric oxide'? (2 marks)

Answers

A. (i) Sulphur (IV) oxide has the formula SO2.

Comment: The # IV represents the oxidation state of sulphur. Since oxygen generally has an oxidation # of - two then two oxygen atoms contribute - 2*2 = - 4 and sulphur is + 4.

- 4 + 4 = 0.

(ii) SO2 (g) + H2O (l) = H2SO3 (aq). SO2 is an acidic oxide and dissolves in water to form acids.

(iii) The pH of water decreases, that is, becomes more acidic. On the pH scale, acids are from 0-6 and bases are from 8-14, thus an acidic substance would lower the pH.

B. (i) Calcium carbonate or calcium oxide could be used.

Comment: The acidic oxides of sulphur and nitrogen can be neutralised, by reacting with a basic oxide or a carbonate.

(ii) The reagent should be used in powdered form to increase the surface area in contact with the gas. A larger surface area means an increase in reaction rate.

(iii) By reacting with the gases to form a neutral product, the quantity of polluting gases in the atmosphere could be reduced. SO2 (g) + CaO (s) = CaSO3 (s) CaCO3 (s) + SO2 (g) = CO2 (g) + CaSO3 (s)

C. (i) An amphoteric oxide is one that shows both basic and acidic properties. It can react with both acids and bases.

Francine Taylor-Campbell is an independent contributor.