Which of the following substances reacts vigorously when mixed with water?

• A. Silicon dioxide (SiO_2)
• B. Sodium (Na)
• C. Copper (Cu)
• D. Carbon dioxide (CO_2)

Answer: (B)

Verified step by step guidance

1. Understand the nature of each substance listed: Silicon dioxide (SiO_2), Sodium (Na), Copper (Cu), and Carbon dioxide (CO_2).
2. Recall that substances that react vigorously with water are typically highly reactive metals or compounds that produce a noticeable chemical reaction such as heat, gas, or a change in solution.
3. Consider Sodium (Na), an alkali metal known for its vigorous reaction with water, producing sodium hydroxide (NaOH) and hydrogen gas (H_2), often with heat and sometimes flames. The reaction is: \(\mathrm{2Na + 2H_2O \rightarrow 2NaOH + H_2}\).
4. Evaluate Silicon dioxide (SiO_2) and Copper (Cu), which are generally inert or very slow to react with water under normal conditions, meaning they do not react vigorously.
5. Consider Carbon dioxide (CO_2), which dissolves in water to form carbonic acid (H_2CO_3) but does not react vigorously; this is a mild, equilibrium process rather than a vigorous reaction.