Questions:
Natural source of methane: Natural gas (i don't really know if it's right)
Elements with lowest boiling point: Fluorine (had a look on Google)
2 observations that you will see in the test tubes for the reaction of sodium carbonate and ethanoic acid: effervescence in the 1st tube, the presence of carbon dioxide gas causes limewater to turn cloudy in 2nd test tube.
Combustion of ethanol: C2H50H + 302 -> 2CO2 + 3H20