IGCSE Chemistry 0620: Comparing Metal Reactivity with Dilute Acid

A student investigates the reactivity of four metals by adding an identical-sized rolled strip of each metal (magnesium, zinc, iron and copper) to separate test tubes, each containing the same volume and concentration of dilute hydrochloric acid. She counts the number of gas bubbles released from each metal strip in the first 30 seconds: magnesium 46 bubbles, zinc 19 bubbles, iron 7 bubbles, copper 0 bubbles.

(a) Name the gas produced in these reactions, and describe a test that could be used to confirm its identity. [2]

(b) Using the bubble-count data, write the order of these four metals from most to least reactive. [1]

(d) Explain, in terms of the reactivity series, why no bubbles were observed in the copper test tube. [1]

(e) Suggest why the student used metal strips of the same surface area in each test tube. [1]

Part (a): Identifying and testing for the gas

The gas produced when a reactive metal reacts with dilute hydrochloric acid is hydrogen. To confirm this, a lit (burning) splint is held at the mouth of the test tube; hydrogen burns with a characteristic squeaky pop.

Part (b): Ranking the metals by reactivity

The more reactive a metal is, the faster it reacts with the acid, so more bubbles of hydrogen are released in the same time. Ordering the bubble counts from highest to lowest:

$\text{magnesium} (46) > \text{zinc} (19) > \text{iron} (7) > \text{copper} (0)$

So, from most to least reactive: magnesium, zinc, iron, copper.

Part (c): Word equation for magnesium

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

Part (d): Why copper produced no bubbles

Copper lies below hydrogen in the reactivity series. Only metals above hydrogen can displace hydrogen from an acid; since copper is below hydrogen, it has no tendency to do this, so it does not react with dilute hydrochloric acid at all, explaining the zero bubble count.

Part (e): Why the surface area was kept the same

If the metal strips had different surface areas, a larger strip would expose more metal atoms to the acid at once and could produce more bubbles even if it were less reactive than a smaller strip of a different metal. Keeping the surface area (along with the volume and concentration of acid) the same for every metal makes this a fair test, so the bubble count differences can be confidently attributed to differences in the metals’ reactivity alone.

Final answers

(a) Hydrogen; lit splint gives a squeaky pop.
(b) magnesium, zinc, iron, copper (most to least reactive).
(c) magnesium + hydrochloric acid → magnesium chloride + hydrogen
(d) Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid.
(e) Keeps the comparison a fair test, so differences in bubble count reflect reactivity only.