What is the number of moles of h2o produced if you combust two moles of ch4 according to the following balanced equation? ch4 + 2 o2 --> co2 + 2 h2o

The combustion of Methane produces CO₂ and water;

                         CH₄  +  2 O₂     →      CO₂  +   2 H₂O

According to balance equation,

        1 Mole of CH₄ on combustion give  =  2 Moles of H₂O
So,
                  2 Moles of CH₄ will produce  =  X Moles of H₂O

Solving for X,
                               X  =  (2 Moles × 2 Moles) ÷ 1 Mole

                               X  =  4 Moles
Result:
           4 Moles of H₂O is produced on combustion of 2 Moles of CH₄.

Yes it would be a physical change.