IGCSE Chemistry 0620: Reactions of Acids with a Metal, a Carbonate and a Base

AquaClear Maintenance services industrial hot-water tanks. Their technicians use a descaling solution of dilute hydrochloric acid to clean deposits found inside the tanks. During one visit, the acid is tested separately against three samples removed from a tank:

Sample 1: a strip of zinc metal from a corroded fitting
Sample 2: white scale, which is mostly calcium carbonate
Sample 3: a grey-black deposit of copper(II) oxide from an old valve

(a) Describe what would be observed when the descaling acid is added to Sample 1, and write a word equation for the reaction. [2]

(b) Write a word equation for the reaction between the descaling acid and Sample 2, and name the gas produced. [2]

(c) Sample 3 is a base. Write a word equation for its reaction with the descaling acid, and describe the colour change of the solid as it reacts. [2]

Part (a): Acid reacting with a metal (Sample 1)

Dilute acids react with reactive metals such as zinc to form a salt and hydrogen gas:

$\text{zinc} + \text{hydrochloric acid} \rightarrow \text{zinc chloride} + \text{hydrogen}$

Observation: bubbles of gas are seen forming at the surface of the metal, and the zinc strip gradually gets smaller as it reacts and dissolves. (The mixture may also feel slightly warm, since this reaction is exothermic.)

The salt formed is zinc chloride, because chloride ions come from the hydrochloric acid used.

Part (b): Acid reacting with a carbonate (Sample 2)

Dilute acids react with carbonates to form a salt, water, and carbon dioxide gas, three products, not two:

$\text{calcium carbonate} + \text{hydrochloric acid} \rightarrow \text{calcium chloride} + \text{water} + \text{carbon dioxide}$

Gas produced: carbon dioxide. (This could be confirmed by bubbling the gas through limewater, which would turn milky/cloudy.)

Part (c): Acid reacting with a base (Sample 3)

Copper(II) oxide is a base (a metal oxide). Dilute acids react with bases to form a salt and water only:

$\text{copper(II) oxide} + \text{hydrochloric acid} \rightarrow \text{copper(II) chloride} + \text{water}$

Colour change: the grey-black solid copper(II) oxide gradually disappears as it reacts, and a blue solution forms, since copper(II) chloride is blue when dissolved in water.

Final answers

(a) Bubbles form and the zinc dissolves; zinc + hydrochloric acid → zinc chloride + hydrogen.
(b) Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide; gas = carbon dioxide.
(c) Copper(II) oxide + hydrochloric acid → copper(II) chloride + water; the grey-black solid dissolves to give a blue solution.