Which one of the following metals does NOT react with cold water ?

Options

A. Calcium (Ca)
B. Potassium (K)
C. Magnesium (Mg)
D. Sodium (Na)

Explanation

To determine which metal does not react with cold water, we can analyze the reactivity of the given metals: calcium, sodium, potassium, and magnesium.

### Step-by-Step Solution:

1. **Identify the Metals**: The metals in question are calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg).

2. **Reactivity of Calcium**: Calcium reacts with cold water, albeit less violently. The reaction can be represented as:
\[ \text{Ca (s)} + 2 \text{H}_2\text{O (l)} \rightarrow \text{Ca(OH)}_2 \text{(aq)} + \text{H}_2 \text{(g)} \]
This indicates that calcium does react with cold water.

3. **Reactivity of Sodium and Potassium**: Both sodium and potassium react violently with cold water. The reactions can be represented as:
\[ \text{Na (s)} + \text{H}_2\text{O (l)} \rightarrow \text{NaOH (aq)} + \frac{1}{2} \text{H}_2 \text{(g)} \]
\[ \text{K (s)} + \text{H}_2\text{O (l)} \rightarrow \text{KOH (aq)} + \frac{1}{2} \text{H}_2 \text{(g)} \]
Therefore, both sodium and potassium also react with cold water.

4. **Reactivity of Magnesium**: Magnesium does not react with cold water. Instead, it reacts with hot water to form magnesium hydroxide:
\[ \text{Mg (s)} + 2 \text{H}_2\text{O (g)} \rightarrow \text{Mg(OH)}_2 \text{(aq)} + \text{H}_2 \text{(g)} \]
This indicates that magnesium does not react with cold water.

5. **Conclusion**: Among the metals listed, magnesium is the only metal that does not react with cold water.

### Final Answer:
**Magnesium (Mg) does NOT react with cold water.**