Physical changes are when things get changed without altering chemical consistencies, which is melting solid butter into liquid one, or boiling water. Chemical changes are things such as caramelizing sugar when making sweets, or when carbon dioxide is created and released when baking bread.
The reaction formula CH4 + 2O2 → CO2 + 2H2O shows the oxidation of 1 mole of CH4 (Methane) will yield 1 mole of CO2 (Carbon Dioxide). Since 1 mole of CH4 will weigh 12g (for the Carbon) + 4g (1g for each Hydrogen) = 16g, then 32g of CH4 will correspond to 32g / 16g/mole = 2 moles. Therefore the oxidation of 2 moles of CH4 will yield 2 moles of CO2.
