If a substance cannot be broken down chemically into a simpler substance, what is it?

How do i complete this electrolysis of water lab? i’m at home, so i don’t have the materials, and the lab didn’t properly work and was incomplete at school.

Identify the missing numbers below to show the result of multiplying the numbers (1.6 × 10-19)(5.0 × 106) = c × 10d

If 90.0 grams of ethane reacted with excess chlorine,how many grams of dicarbon hexachloride would form