Assessment And Qualifications Alliance (AQA) GCSE Chemistry Paper 2 Practice Exam

Alkenes typically react with oxygen through a process called combustion. In the case of alkenes, they often undergo incomplete combustion when they react with limited oxygen supply, resulting in a smoky flame. This incomplete combustion occurs because alkenes, having a higher number of hydrogen atoms in their molecular structure, can produce carbon particles (soot) when there isn't enough oxygen to convert all the carbon to carbon dioxide. The smoky flame is a direct indicator of this incomplete combustion, which is characterized by a yellow, luminous flame due to the production of solid carbon particles. This contrasts with complete combustion, which would yield a blue flame and produce carbon dioxide and water. Other options suggest alternative reactions or behaviors that alkenes do not typically exhibit. For example, complete combustion with a clean flame is more representative of saturated hydrocarbons, while the idea of alkenes not reacting at all with oxygen is inaccurate, as they certainly do engage in combustion. The spontaneous explosion in the presence of oxygen is also not a characteristic reaction for alkenes under normal conditions. Thus, option C correctly captures how alkenes interact with oxygen in a combustible reaction setting.