Which one is the most pure substance?Rain water or Heavy water
Explain what you mean by:
1. Pure substance.
2. Rain water.
3. Heavy water.
Depending on the definitions and/or sources there is enough ambiguity in the question to make it meaningless.
Rain contains other compunds, including (but not limited to) NOx's and CO2.
Heavy water contains D instead of H....
If by pure substance you mean H2O, then both can be impure dependant upon the amounts of D or Co2 etc in solution.
I mean in comparison of heavy water and rain water,which one is most pure.
Seems like you still don't understand why the question doesn't make much sense.
Heavy water is D2O, but its purity is not something defined by the name. It can contain more or less of the normal hydrogen, it can be - depending on the source - contaminated with other things.
Stating "rain water" doesn't tell much about contamination. When the rain starts to fall after several weeks in the large city full of heavy industry it is much more contaminated than the rain falling for long in a place far from the sources of contamination (industry, volcanoes, deserts being source of dust and so on).
Basically you are trying to compare things not knowing what they are. It won't work.
Separate names with a comma.