IGCSE Chemistry 0620: Reactions of Sodium, Calcium and Magnesium with Water and Steam

A chemistry teacher demonstrates reactions of Group I and Group II metals with water, using a large trough of cold water with a few drops of universal indicator added.

(a) A pea-sized piece of sodium is dropped onto the surface of the water. Describe two observations you would expect to make during this reaction. [2]

(b) Write a word equation for the reaction between sodium and water. [1]

(c) A similarly-sized piece of calcium is then dropped into a fresh trough of the same water. The reaction is noticeably less vigorous than with sodium. Explain this difference in terms of the reactivity series. [1]

(d) The teacher then heats a coil of magnesium ribbon and passes steam over it. State one observation you would expect to make, and write a word equation for this reaction. [2]

Part (a): Observing sodium in cold water

Sodium is a very reactive Group I metal. When dropped into cold water it:

Fizzes vigorously and moves rapidly across the surface, often melting into a small shiny/silvery ball because the reaction releases enough heat to melt the metal.
Causes the universal indicator to change colour from green to blue/purple, since the sodium hydroxide formed is an alkali.

Part (b): Word equation for sodium and water

$\text{sodium} + \text{water} \rightarrow \text{sodium hydroxide} + \text{hydrogen}$

Part (c): Comparing calcium to sodium

In the reactivity series, sodium is above calcium. Sodium is more reactive. A more reactive metal reacts more vigorously (faster, with more visible fizzing and heat) with cold water. Since calcium is less reactive than sodium, its reaction with cold water is noticeably calmer: steady bubbles of hydrogen form, but the metal does not melt or move rapidly across the surface.

Part (d): Magnesium and steam

Magnesium sits below calcium and sodium in the reactivity series, so it does not react noticeably with cold water. However, it reacts readily with the much more energetic steam:

$\text{magnesium} + \text{steam} \rightarrow \text{magnesium oxide} + \text{hydrogen}$

Observation: the magnesium burns with a brilliant, dazzling white flame, leaving behind a white solid (magnesium oxide) and releasing hydrogen gas.

Final answers

(a) Fizzes and skates across the surface (may melt into a ball); indicator turns blue/purple.
(b) sodium + water → sodium hydroxide + hydrogen
(c) Calcium is less reactive than sodium (lower in the reactivity series), so its reaction with cold water is slower and less vigorous.
(d) Bright white flame, white solid forms; magnesium + steam → magnesium oxide + hydrogen.