A materials engineer tests small offcuts of three metals (magnesium, zinc and iron) by dropping an identical-sized piece of each into separate beakers containing the same volume and concentration of dilute hydrochloric acid. Every beaker starts at 20 °C. The highest temperature…

IGCSE Chemistry 0620: Deducing Metal Reactivity from Temperature Data

Step 1: Link temperature change to reactivity

The reaction between a metal and dilute hydrochloric acid is exothermic. The more reactive a metal is, the faster and more vigorously it reacts, and the more thermal energy it releases into the acid in a given time. So a bigger temperature rise (in the same time, with the same volume and concentration of acid) is evidence of a more reactive metal.

Step 2: Compare the recorded values

Since every beaker started at $20\,°\text{C}$ , the temperature rises were:

Magnesium: $52 - 20 = 32\,°\text{C}$
Zinc: $38 - 20 = 18\,°\text{C}$
Iron: $26 - 20 = 6\,°\text{C}$

Step 3: Rank the metals

Ordering these rises from largest to smallest gives:

$\text{magnesium} (32\,°\text{C}) > \text{zinc} (18\,°\text{C}) > \text{iron} (6\,°\text{C})$

So, from most reactive to least reactive: magnesium, zinc, iron. This also matches their known positions in the reactivity series.

Why the other options are wrong

B (zinc, magnesium, iron): swaps zinc and magnesium, but magnesium produced the largest temperature rise, so it must be listed first.
C (iron, zinc, magnesium): this is the order reversed, least reactive to most reactive, not most to least.
D (magnesium, iron, zinc): swaps zinc and iron, but zinc’s rise ( $18\,°\text{C}$ ) was clearly greater than iron’s ( $6\,°\text{C}$ ).

Final answer

The correct order, most to least reactive, is magnesium, zinc, iron, option A.