The correct option is **C. Iron**. ### Explanation of the Correct Answer To understand why iron cannot replace hydrogen from water or steam, we need to look at the reactivity series of metals. The reactivity series is a list of metals arranged in order of decreasing reactivity. The more reactive a metal is, the more easily it can displace hydrogen from compounds like water (H₂O) or acids. 1. **Reactivity Series Overview**: - The reactivity series generally ranks metals from most reactive to least reactive as follows: Potassium (K), Sodium (Na), Calcium (Ca), Magnesium (Mg), Aluminum (Al), Zinc (Zn), Iron (Fe), Tin (Sn), Lead (Pb), Copper (Cu), Silver (Ag), and Gold (Au). - In this series, metals that are higher can displace hydrogen from water or steam, while those lower down cannot. 2. **Analyzing Each Option**: - **A. Sodium (Na)**: Sodium is very reactive and can easily react with water to produce sodium hydroxide and hydrogen gas. The reaction is vigorous and exothermic. \[ 2 \text{Na} + 2 \text{H}_2\text{O} \rightarrow 2 \text{NaOH} + \text{H}_2 \uparrow \] - **B. Magnesium (Mg)**: Magnesium can also react with steam to produce magnesium oxide and hydrogen gas. The reaction occurs at high temperatures. \[ \text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2 \uparrow \] - **C. Iron (Fe)**: Iron does not react with water at room temperature. It can react with steam, but the reaction is much slower and requires higher temperatures. The reaction produces iron(II) oxide and hydrogen gas. \[ \text{Fe} + \text{H}_2\text{O} \rightarrow \text{FeO} + \text{H}_2 \uparrow \quad (\text{at high temperature}) \] However, in practical terms, iron is not considered a metal that can readily replace hydrogen from water or steam under normal conditions. - **D. Calcium (Ca)**: Calcium is also very reactive and can displace hydrogen from water, forming calcium hydroxide and hydrogen gas. \[ \text{Ca} + 2 \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \uparrow \] ### Why the Other Options are Incorrect or Weaker - **Sodium (A)**: Sodium is one of the most reactive metals and can easily displace hydrogen from water, making it a strong candidate for replacing hydrogen. - **Magnesium (B)**: Magnesium is also reactive enough to displace hydrogen from steam, especially at elevated temperatures, which makes it a strong candidate as well. - **Calcium (D)**: Calcium is highly reactive and can displace hydrogen from water, making it a strong candidate for replacing hydrogen. ### Common Pitfalls - **Misunderstanding Reactivity**: Students often confuse the ability of metals to react with water or steam with their general reactivity. Just because a metal is reactive does not mean it can displace hydrogen from water under all conditions. - **Temperature Considerations**: Some metals may require high temperatures to react with water or steam, which can lead to confusion about their reactivity. ### Revision Summary - The reactivity series ranks metals based on their ability to displace hydrogen from water or acids. - Sodium, magnesium, and calcium can all displace hydrogen from water or steam, while iron cannot do so readily at room temperature. - Iron requires high temperatures to react with steam, making it less reactive in this context. - Understanding the conditions under which metals react is crucial for predicting their behavior in chemical reactions.