Understanding Metal Reactivity with Cold Water

Metals exhibit varying levels of reactivity when exposed to water. The reaction often involves the metal displacing hydrogen from water, leading to the liberation of hydrogen gas. The speed and intensity of this reaction depend on the specific metal and the temperature of the water (cold water, hot water, or steam).

Let's examine the reactivity of the given metals with cold water:

• Potassium (K): Potassium is a highly reactive alkali metal. It reacts vigorously and exothermically with cold water, producing potassium hydroxide and hydrogen gas. The reaction is so rapid that the heat generated can ignite the hydrogen gas.
• The reaction is: $\text{2K(s) + 2H}_2\text{O(l)} \rightarrow \text{2KOH(aq) + H}_2\text{(g)}$
• Iron (Fe): Iron is a less reactive metal compared to alkali and alkaline earth metals. Iron does not react with cold water. It reacts with steam at high temperatures to produce iron oxide and hydrogen gas.
• The reaction with steam is: $\text{3Fe(s) + 4H}_2\text{O(g)} \rightarrow \text{Fe}_3\text{O}_4\text{(s) + 4H}_2\text{(g)}$
• Calcium (Ca): Calcium is an alkaline earth metal. It reacts with cold water, but the reaction is less vigorous than that of sodium or potassium. Calcium hydroxide is formed, which is slightly soluble in water. The hydrogen gas produced may stick to the surface of the calcium, causing it to float temporarily.
• The reaction is: $\text{Ca(s) + 2H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq) + H}_2\text{(g)}$
• Sodium (Na): Sodium is another highly reactive alkali metal. It reacts vigorously and exothermically with cold water, producing sodium hydroxide and hydrogen gas. Like potassium, the reaction is fast and the heat generated can ignite the hydrogen gas.
• The reaction is: $\text{2Na(s) + 2H}_2\text{O(l)} \rightarrow \text{2NaOH(aq) + H}_2\text{(g)}$

Comparing Metal Reactions with Water

Based on their reactivity, we can categorize these metals' reactions with water:

Conclusion on Metal Reactivity

From the analysis, it is clear that Potassium, Calcium, and Sodium all react with cold water to liberate hydrogen gas, although at different rates. Iron, however, does not react with cold water; it requires steam to react and produce hydrogen gas.

Therefore, the metal that does NOT react with cold water to liberate hydrogen gas is Iron.

Revision Table: Metal Reactions

Additional Information on Metal Reactivity Series

The reactivity of metals is often summarized in the metal reactivity series. This series lists metals in order of decreasing reactivity. A more reactive metal can displace a less reactive metal from its compounds, including displacing hydrogen from water or acids. The position of a metal in the series determines its reaction with water and acids.

General trend of reactivity with water:

• Metals above hydrogen in the reactivity series can displace hydrogen from water or acids.
• Metals at the top (like K, Na, Ca) react with cold water.
• Metals in the middle (like Mg, Al, Zn, Fe) react with hot water or steam.
• Metals below hydrogen (like Cu, Ag, Au) do not react with water or dilute acids.

The reactivity order relevant to this question is roughly: Potassium > Sodium > Calcium > Magnesium > Aluminium > Zinc > Iron > Lead > Hydrogen > Copper > Silver > Gold.

Since Iron is below Calcium, Sodium, and Potassium in the reactivity series, it is less reactive and thus does not react with cold water.