Hydrogen accounts for approximately `75%` of the mass of the universe. Hydrogen serves as the nuclear fuel of our Sun and other stars, and these are mainly composed of hydrogen. On the earth, though hydrogen is rarely found in the uncombined state. Since the earth's gravity is too weak to hold such light molecules, nearly all the `H_2` originally present in the earth's atmosphere has been lost to space. In the earth's crust and oceans, hydrogen is found in water, petroleum, proteins, carbohydrates and other compounds and it is the ninth most abundant element on a mass basis. Hydrogen has three isotopes : hydrogen or protium `()`, deuterium or heavy hydrogen (D or ), tritium `(T or )`. The physical properties of the three isotopes are different due to the difference in their masses, i.e. isotope effect. The chemical properties of the three isotopes are similar as they have the same electronic configuration. Reaction between hydrogen and oxygen is highly exothermic, and gas mixtures that contain as little as `4%` by volume hydrogen in oxygen (or in air) are highly flammable and potentially explosive.
`2H_(2(g))+O_(2(g)), DeltaH^(ɵ)=-485 kJmol^(-1)`
As hydrogen is environmentally clean it is an enormously attractive fuel. 'Hydrogen economy' is an emerging field in which it is thought that our energy needs can be met by gaseous, liquid and solid hydrogen. As hydrogen is no a naturally occuring substance such as coal, oil or natural gas, energy must be exploaded to produce hydrogen before it can be used.
Which of the following fuel produces least environmental pollution?

Options

A. Hydrogen
B. Coal
C. Wood
D. Gasoline

Explanation

To determine which fuel produces the least environmental pollution based on the provided information, we can follow these steps:

### Step 1: Understand the Context
The paragraph discusses hydrogen's prevalence in the universe, its isotopes, and its role as a clean fuel. It highlights that hydrogen is not found in its uncombined state on Earth due to weak gravity and that it is mainly found in compounds like water and hydrocarbons.

**Hint:** Focus on the properties of hydrogen mentioned in the paragraph, particularly its environmental impact.

### Step 2: Identify the Isotopes of Hydrogen
Hydrogen has three isotopes:
- Protium (H or ¹H)
- Deuterium (D or ²H)
- Tritium (T or ³H)

While these isotopes differ in mass, they share similar chemical properties due to having the same electronic configuration.

**Hint:** Remember that isotopes have the same chemical behavior but different physical properties.

### Step 3: Analyze the Reaction of Hydrogen with Oxygen
The reaction between hydrogen and oxygen is highly exothermic, releasing a significant amount of energy. The reaction is represented as:
\[ 2H_{2(g)} + O_{2(g)} \rightarrow 2H_2O_{(g)} \]
with a ΔH of -485 kJ/mol, indicating that it releases energy.

**Hint:** Exothermic reactions typically indicate that energy is released, which can be beneficial for fuel applications.

### Step 4: Assess Environmental Impact
The paragraph states that hydrogen is "environmentally clean" and is an "enormously attractive fuel." This suggests that hydrogen produces minimal environmental pollution compared to traditional fuels like coal, oil, or natural gas.

**Hint:** Look for keywords in the paragraph that indicate environmental cleanliness or pollution levels.

### Step 5: Conclusion
Based on the information provided, hydrogen is identified as the fuel that produces the least environmental pollution. This conclusion is supported by the statement in the paragraph that emphasizes hydrogen's clean nature as a fuel source.

**Final Answer:** Hydrogen produces the least environmental pollution.