Acids, bases and salts
Francine Taylor-Campbell, Contributor

Students from Bridgeport High School show the certificates they received at the school's 28th anniversary celebrations on Friday, September 29. - Photo by Anthony Minott

IMPORTANT POINTS

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.

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 mk)

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

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

(b) It has been suggested that the amount of polluting gases, such as 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 mk)

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

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

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

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

ii) Write ONE equation to show how EACH of the salts ZnSO4 and Na2 ZnO2 is formed from zinc oxide. (4 mks)

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 - 2 then 2 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) = CaSO 3 (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.

(ii) ZnO (s) + H2SO4 (aq) = ZnSO4 (aq) + H2 (g)

ZnO (s) + 2NaOH (aq) = Na2 ZnO2 (aq) + H2O (l)

Francine Taylor-Campbell is an independent contributor.