1. Which of the following is a pure substance? A. Distilled water B. Air C. Sea water D. Petroleum 2. Which of the following diagrams can represent a mixture of two compounds? (In these diagram, and O represent a nitrogen and an oxygen atom respectively) A. B. C. D. 3. Which of the following gases is used to fill airships nowadays? A. Helium B. Hydrogen C. Neon D. Air 4. How many atoms are present in a glucose molecule, C6H12O6 ? A. 1 B. 3 C. 12 D. 24 5. Which of the following statements concerning a nucleus is correct? A. All atoms of the same element must have the same number of protons and neutrons in their nuclei. B. The nucleus of an oxygen atom is about 81837 times heavier than the rest of the atom. C. A nucleus is about 1001 the size of the atom. D. The nucleus of a certain element is different from that of any other element. 6. Which of the following combinations about the atomic structure of a sulphur atom (Z=16) is correct