Natural Gas & Propane Properties

What Natural Gas Is

Natural gas is a fossil fuel composed primarily of methane (CH4). On the Red Seal exam, when asked "What is the primary component of natural gas?" - the answer is always methane. Depending on the source, natural gas may also contain small amounts of ethane, propane, butane, and trace gases, but methane typically makes up 85-95% of the mixture.

Natural gas is processed before distribution to remove impurities and heavier hydrocarbons. The processed gas delivered to buildings through the utility distribution system is nearly pure methane with a consistent heating value that makes it predictable for appliance sizing.

Specific Gravity and Behavior

Natural gas has a specific gravity of approximately 0.60, meaning it is lighter than air (air = 1.0). This is critically important for safety because it means natural gas rises when released. Leaking natural gas will accumulate at the ceiling and upper areas of a room, and will eventually dissipate through openings at high points.

This rising behavior is the opposite of propane, and gasfitters must understand this difference when planning ventilation, leak detection, and gas detector placement.

The Odorant - Mercaptan

In its natural state, methane is odorless and colorless - making it impossible to detect by smell. To address this safety concern, a distinctive odorant called mercaptan (also known as tert-butyl mercaptan or ethyl mercaptan) is added to both natural gas and propane. Mercaptan produces a strong "rotten egg" smell that most people can detect at very low concentrations, well below the flammable range.

Natural gas is primarily methane, is lighter than air (specific gravity 0.60), and has mercaptan added as an odorant for leak detection. Both natural gas and propane receive this odorant.