Which of the following facts regarding boron and silicon in true?

Options

A. Boron is used to make boron steel or boron carbide control rods for nuclear reactor.
B. Boron and silicon form halides which are not hydrolysed.
C. Boron and silicon react with magnesium to form magnesium boride and magnesium silicide which are decomposed by acids to give volatile borane and silane, respectively.
D. Both boron and silicon react with alkali to form borates and silicates containing `BO_(3)^(3-)` and `SIO_(4)^(4-)` tetrahedral units, respectively.

Explanation

To determine which facts regarding boron and silicon are true, we will analyze each statement provided in the question.

### Step 1: Analyze the first statement
**Statement**: Boron is used to make boron steel or boron carbide controlled rods for nuclear reactors.

**Analysis**: This statement is true. Boron is indeed used in the production of boron steel and boron carbide, which are materials utilized in nuclear reactors for their ability to absorb neutrons.

### Step 2: Analyze the second statement
**Statement**: Boron and silicon form halides which are not hydrolyzed.

**Analysis**: This statement is also true. Boron halides (like BCl3) and silicon halides (like SiCl4) do not readily hydrolyze in water. The reaction of BCl3 with water produces boric acid (H3BO3) and hydrochloric acid (HCl), while SiCl4 reacts with water to form silicic acid (H2SiO3) and HCl.

### Step 3: Analyze the third statement
**Statement**: Boron and silicon react with magnesium to form magnesium boride and magnesium silicate, which are decomposed by acids to give volatile borane and silicon respectively.

**Analysis**: This statement is incorrect. When boron reacts with magnesium, it forms magnesium boride (Mg3B2), but the decomposition of magnesium boride does not yield volatile borane (B2H6) directly. Instead, it forms other products. Similarly, silicon does not yield a volatile product directly from magnesium silicate upon acid treatment.

### Step 4: Analyze the fourth statement
**Statement**: Both boron and silicon react with alkali to form borates and silicates containing BO3³⁻ and SiO4⁴⁻ tetrahedral units respectively.

**Analysis**: This statement is true. Boron reacts with sodium hydroxide to form sodium borate (Na3BO3), and silicon reacts with sodium hydroxide to form sodium silicate (Na4SiO4). Both products contain the respective tetrahedral units.

### Conclusion
Based on the analysis:
- **True Statements**: A, B, and D
- **False Statement**: C

### Final Answer
The true statements regarding boron and silicon are A, B, and D.

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