Which of the following substances reacts explosively when combined with water?

• A. Carbon dioxide (CO_2)
• B. Calcium carbonate (CaCO_3)
• C. Copper(II) sulfate (CuSO_4)
• D. Sodium (Na)

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the nature of each substance listed and their typical reactions with water. Carbon dioxide (CO_2), calcium carbonate (CaCO_3), and copper(II) sulfate (CuSO_4) are generally stable when mixed with water and do not react explosively.
2. Step 2: Recall that sodium (Na) is an alkali metal known for its vigorous and exothermic reaction with water, producing sodium hydroxide (NaOH) and hydrogen gas (H_2). This reaction can be explosive due to the rapid release of hydrogen gas and heat.
3. Step 3: Write the chemical equation for sodium reacting with water: \(\mathrm{2Na + 2H_2O \rightarrow 2NaOH + H_2}\).
4. Step 4: Recognize that the hydrogen gas produced can ignite due to the heat generated, causing an explosive reaction, which is why sodium reacts explosively with water.
5. Step 5: Conclude that among the substances listed, sodium (Na) is the one that reacts explosively with water, while the others do not.