Question

5 why is water both a compound and a pure substance? a it is made of all the same kind of atoms b it is made of all the same kind of molecules, but each molecule contains more than one type of atoms c it is made of different molecules, but each molecule contains only the same type of atoms d it can undergo a chemical change to become pure hydrogen and pure oxygen

Explanation:

• Option A: A pure substance made of the same kind of atoms is an element (e.g., O₂), not a compound. Water has H and O atoms, so A is wrong.
• Option B: A pure substance has uniform composition (same molecules). Water molecules (\(H_2O\)) each have 2 H and 1 O (different atoms), so it's a compound (made of different atoms in a fixed ratio) and a pure substance (same molecules). This matches.
• Option C: Water has only \(H_2O\) molecules, not different molecules. So C is wrong.
• Option D: The ability to decompose into H₂ and O₂ shows it's a compound, but doesn't explain why it's a pure substance. So D is not the best answer.

Answer:

B. It is made of all the same kind of molecules, but each molecule contains more than one type of atoms