A: `H_(2)` when allowed to expand at room temperature it causes heating effect.
R: `H_(2)` has inversion-tomperature much below room temperature.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question regarding the assertion and reason about the behavior of hydrogen gas (H₂) when allowed to expand at room temperature, we can break it down into the following steps:

### Step 1: Understand the Assertion
The assertion states that H₂, when allowed to expand at room temperature, causes a heating effect. This means that during the expansion process, the temperature of the gas increases.

**Hint:** Consider what happens to a gas when it expands and how temperature relates to the energy of the gas particles.

### Step 2: Understand the Reason
The reason provided states that H₂ has an inversion temperature much below room temperature. The inversion temperature is the temperature below which a gas cools upon expansion and above which it heats upon expansion.

**Hint:** Recall the concept of inversion temperature and its significance in the behavior of gases during expansion.

### Step 3: Analyze the Inversion Temperature of H₂
The inversion temperature for H₂ is given as 200 Kelvin. Room temperature is approximately 300 Kelvin (27°C). Since 300 K is above 200 K, this means that when H₂ expands at room temperature, it is above its inversion temperature.

**Hint:** Compare the given inversion temperature with room temperature to determine the effect on the gas during expansion.

### Step 4: Determine the Effect of Expansion on Temperature
When a gas expands above its inversion temperature, it absorbs heat from the surroundings, resulting in an increase in temperature. Therefore, the assertion that H₂ causes a heating effect upon expansion at room temperature is valid.

**Hint:** Think about the relationship between temperature, enthalpy, and the direction of heat flow during the expansion process.

### Step 5: Conclusion
Both the assertion (H₂ causes a heating effect upon expansion at room temperature) and the reason (H₂ has an inversion temperature much below room temperature) are true. Furthermore, the reason correctly explains the assertion.

**Final Answer:** Both assertion and reason are true, and the reason is the correct explanation of the assertion.