Which of the following is not correctly matched ?

Options

A. Heavy water- Used as moderator in nuclear reactors.
B. Zeolites-Used for removing permanent hardness of water.
C. "Syn gas"- Mixture of `N_(2)` and `H_(2)`.
D. Hydrolith - On hydrolysis with water produces `H_(2)`.

Explanation

To determine which of the following statements is not correctly matched, we will analyze each option one by one.

1. **Heavy water used as a moderator in nuclear reactors**:
- Heavy water (D2O) is indeed used as a moderator in nuclear reactors. Its purpose is to slow down the neutrons produced during nuclear fission, allowing for a sustained chain reaction. Therefore, this statement is **correct**.

2. **Zeolites used for removing permanent hardness of water**:
- Zeolites are compounds that can remove permanent hardness from water, which is caused by the presence of calcium and magnesium ions. They work by exchanging these ions with sodium ions, effectively precipitating the hardness-causing ions out of solution. Thus, this statement is also **correct**.

3. **Syngas is the mixture of nitrogen and hydrogen**:
- This statement is **incorrect**. Syngas, or synthesis gas, is actually a mixture of carbon monoxide (CO) and hydrogen (H2), not nitrogen and hydrogen. Therefore, this is the statement that is not correctly matched.

4. **Hydrolith on hydrolysis with water produces H2**:
- Hydrolith refers to calcium hydride (CaH2). When it reacts with water, it produces calcium hydroxide (Ca(OH)2) and hydrogen gas (H2). The balanced chemical equation for this reaction is:
\[ \text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + 2\text{H}_2 \]
- Thus, this statement is **correct**.

In conclusion, the only statement that is not correctly matched is the third one: **Syngas is the mixture of nitrogen and hydrogen**.

### Final Answer:
The statement that is not correctly matched is: **Syngas is the mixture of nitrogen and hydrogen**.