Methane — occurrence and preparation

• Methane, CH₄, is the simplest organic compound.
• It makes up about 50–90 % (often quoted ~93 %) of natural gas, is found with crude oil, in coal mines (where it forms explosive mixtures with air — "fire-damp"), and is released as bubbles from marshes where organic matter decays ("marsh gas").

Laboratory preparation: heat anhydrous sodium acetate with soda lime (a mixture of NaOH and CaO) in a hard-glass tube; the gas is collected by downward displacement of water (methane is almost insoluble in water, lighter than air):

CH₃COONa + NaOH —(CaO, heat)→ CH₄↑ + Na₂CO₃

• CaO is not a reactant; it keeps the NaOH dry (NaOH is hygroscopic and would attack the glass) and lowers the melting point of the mixture.
• Sodium acetate must be anhydrous because water would prevent the reaction.