How do two elements change properties when they form a compound

What reaction is taking place? 02 + c3h8 = h20 + co2

Which set of characteristics best describes igneous rock? a) largest type of rock, made of organic matter, hardest type of rock b) least abundant type of rock, made of other rocks, made mostly of minerals c) found on all continents, contains wavy bands of stripes, contains fossils d) most abundant type in earth's crust, made of magma/lava, contains no fossils

Why does his teacher ask him to balance the equation by including the correct coefficient