Question

You may have noticed that water sometimes drips from the exhaust of a car as it is running. Is this evidence that there is at least a small amount of water originally present in the gasoline? Explain.

Short answer

The presence of water dripping from the exhaust of a car does not necessarily indicate that there was water in the original gasoline. Instead, it shows that water is a natural byproduct of the combustion process. The combustion of hydrocarbons in gasoline, such as octane, produces water vapor as shown in the chemical equation: \[2 \text{ C}_{8}\text{H}_{18} (l) + 25 \text{ O}_{2} (g) \rightarrow 16 \text{ CO}_{2} (g) + 18 \text{ H}_{2}\text{O} (g)\] This water vapor condenses into liquid water in the exhaust system and eventually drips out from the exhaust pipe.

Step-by-step solution

Understand the car's engine chemical reactions

In a car engine, gasoline (primarily composed of hydrocarbons like octane) reacts with oxygen in the air in a process called combustion. This combustion process releases energy in the form of heat and work, which powers the car. The balanced chemical reaction for the combustion of octane is given by: \[2 \text{ C}_{8}\text{H}_{18} (l) + 25 \text{ O}_{2} (g) \rightarrow 16 \text{ CO}_{2} (g) + 18 \text{ H}_{2}\text{O} (g)\] As seen in the above reaction, the products of octane combustion are carbon dioxide (CO₂) and water (H₂O).

Examine the water formation in the reaction

From the balanced chemical equation, 18 moles of water are produced for every 2 moles of octane molecules combusted. In reality, complete combustion may not occur in a car's engine due to various factors. However, even in incomplete combustion, a certain amount of water will still form as a byproduct. The water formed during the combustion process is initially in a gaseous state due to the high temperatures within the engine. As the exhaust gases move through the exhaust system and cool down, the gaseous water condenses into liquid form and accumulates in the exhaust. When enough liquid water has gathered, it drips out from the exhaust pipe.

Conclude if water was present in gasoline

Given the explanation above, the presence of water dripping from the exhaust of a car does not necessarily indicate that there was water in the original gasoline. Instead, it shows that water is a natural byproduct of the combustion process. Since the combustion of hydrocarbons in gasoline produces water vapor, which then condenses into liquid water, it is normal to observe water dripping from a car's exhaust.