What kind of gas mixture is too rich to burn?

A gas mixture is considered too rich to burn when it contains an excessive amount of fuel relative to the amount of oxygen available for combustion. In the case of propane, if the concentration exceeds 9.6%, there isn't enough oxygen to support a complete combustion reaction. This leads to incomplete combustion, where the propane cannot effectively burn, producing too much unburned fuel and potentially hazardous conditions.

The rich mixture limits the flame’s ability to sustain itself effectively, which is crucial in practical applications like heating, cooking, or powering engines. For example, in an engine, running too rich may cause fouling of the spark plugs or produce excessive emissions.

The ranges specified in the other options either include areas where combustion can occur or are too low in propane to reach a state where combustion is not possible. Thus, a mixture that has more than 9.6% propane fails to produce a flame or heat efficiently, illustrating the essence of what it means for a mixture to be too rich to burn.