Which metal among calcium, sodium, magnesium and potassium reacts most violently with cold water?

Introduction / Context:
This question tests your knowledge of metal reactivity, particularly the reaction of alkali and alkaline earth metals with cold water. Such reactions are important in safety considerations and in understanding periodic trends in chemical reactivity.

Given Data / Assumptions:

• The metals listed are calcium, sodium, magnesium, potassium and aluminium.
• We are considering their reaction with cold water.
• We must identify which metal reacts most vigorously.
• We assume pure metals and typical laboratory or textbook conditions.

Concept / Approach:
In the periodic table, reactivity of alkali metals (Group 1) increases as we move down the group. Lithium is least reactive, followed by sodium, then potassium, rubidium and caesium. Potassium reacts more vigorously with cold water than sodium. Alkaline earth metals (Group 2) like magnesium and calcium are generally less reactive with cold water than alkali metals. Magnesium reacts very slowly with cold water, whereas calcium reacts moderately, forming calcium hydroxide and hydrogen gas.

Step-by-Step Solution:
1) Sodium and potassium are alkali metals and known to react strongly with cold water, producing hydrogen gas and metal hydroxide.2) Potassium is lower in Group 1 than sodium, and its outer electron is more easily lost, making it more reactive.3) Magnesium is an alkaline earth metal and reacts only very slowly with cold water; its reaction is much less vigorous.4) Calcium reacts with cold water more than magnesium but still less violently than sodium and potassium.5) Therefore, among the given metals, potassium shows the most violent reaction with cold water.

Verification / Alternative check:
In standard school demonstrations, small pieces of sodium and potassium are placed in water. Sodium moves rapidly on the surface and may ignite hydrogen, while potassium often reacts so vigorously that the hydrogen catches fire with a lilac flame almost immediately. Magnesium is usually demonstrated with steam rather than cold water because its reaction with cold water is very slow. Calcium, although more reactive than magnesium with water, does not show as explosive a reaction as potassium. These observations confirm that potassium is the most reactive with cold water among the listed metals.

Why Other Options Are Wrong:
Calcium: Reacts with cold water but not as violently as sodium and potassium; the reaction is more gentle and produces milky calcium hydroxide.
Sodium: Very reactive with water but still less reactive than potassium in the same conditions.
Magnesium: Reacts only slightly with cold water; the reaction is slow and not violent.
Aluminium: Forms a protective oxide layer and does not react vigorously with cold water under normal conditions.

Common Pitfalls:
Students sometimes choose sodium because it is commonly used in textbook examples of vigorous metal water reactions. However, periodic trends show that reactivity increases down Group 1, making potassium more reactive than sodium. Another error is to ignore group trends and pick calcium or magnesium due to familiarity. To answer such questions correctly, always recall the order of reactivity within a group and remember that Group 1 metals are generally more reactive with water than Group 2 metals.

Final Answer:
The metal that reacts most violently with cold water among the options is potassium.