Fig. 8.1 shows apparatus used to react a metal with steam. The gas produced is collected over water. Fig. 8.1 (a) Name the gas collected over water and desc...

This apparatus passes steam over a heated metal; the metal reacts to form its oxide and releases hydrogen, which is collected over water. The observations rank reactivity and test practical technique.

(a) Gas collected and its test [2]. The gas is hydrogen [1]. To confirm it, hold a lighted splint to the gas: it burns with a squeaky pop [1].

(b) Why the wet mineral wool is heated [1]. To boil off the water and produce steam (water vapour), which then passes over and reacts with the metal [1].

(c) Word equation for magnesium and steam [1]. magnesium + steam → magnesium oxide + hydrogen [1].

(d) Balanced symbol equation [2].

Mg + H₂O → MgO + H₂

Award [1] for correct formulae and [1] for balancing. One Mg, two H and one O appear on each side.

(e) Colour of zinc oxide hot and cold [2]. Yellow when hot [1] and white when cold [1]. This reversible colour change is a well-known property of zinc oxide.

(f) What copper not reacting shows [1]. Copper is less reactive than zinc [1], since it will not even react with steam.

(g) Safety step when heating stops [1]. Remove the delivery tube from the water before you stop heating [1]. This prevents cold water being sucked back into the hot tube (suck-back), which could crack it.

Exam tip: the squeaky-pop test identifies hydrogen, and the "remove the tube before removing the heat" rule is a recurring safety mark whenever a gas is collected over water.