Elements and Compounds - UNSOLVED PRACTICE SET

Q7. Define an element and a compound. Give one example of each from substances commonly found in your home.

Q8. How does a compound differ from a mixture in terms of: (i) formation process, and (ii) separation method?

Q9. When iron filings and sulphur powder are mixed, they form a mixture. When heated strongly, they form iron sulphide. List two differences between the mixture and the compound formed.

Q10. "All compounds are pure substances, but not all pure substances are compounds." Explain this statement with examples.

Q11. The formula of glucose is C₆H₁₂O₆. Calculate the ratio of carbon, hydrogen, and oxygen atoms in one molecule of glucose. Is this ratio the same for every molecule of glucose? Why?

Q12. A student says that air is a compound because it contains nitrogen, oxygen, and other gases combined together. Is the student correct? Give reasons for your answer.

Q13. With the help of a suitable example, explain the difference between an element and a compound. Describe an experiment to show that iron sulphide is a compound and not a mixture of iron and sulphur. Include the expected observations and conclusions.

Q14. "The discovery of elements and the understanding of compounds revolutionized chemistry and modern life." Discuss this statement by explaining: (i) how the periodic table organizes elements, (ii) how compounds like water and carbon dioxide are essential for life, and (iii) how the study of elements led to the development of new materials like alloys and plastics.

Q15. A teacher gives two samples to students — Sample X is a shiny grey solid that is attracted to a magnet, and Sample Y is a black solid that is not attracted to a magnet. When Sample X is heated with yellow sulphur powder, Sample Y is formed.

(a) Identify Sample X and Sample Y.

(b) Is the reaction between Sample X and sulphur a physical or chemical change? Give two reasons.

(c) Write the chemical equation for the formation of Sample Y.

(d) How would you prove that Sample Y is a compound and not a mixture?

Q16. The molecular formula of sulphuric acid is H₂SO₄.

(a) How many atoms of each element are present in one molecule of sulphuric acid?

(b) Calculate the total number of atoms in one molecule.

(c) If a sample contains 3.01 × 10²² molecules of H₂SO₄, calculate the number of hydrogen atoms, sulphur atoms, and oxygen atoms in the sample.

(d) Calculate the ratio by mass of hydrogen, sulphur, and oxygen in sulphuric acid. (Atomic masses: H = 1, S = 32, O = 16)

Q17. Ammonia (NH₃) is a compound formed by the chemical combination of nitrogen and hydrogen.

(a) Write the chemical formula of ammonia and identify the elements present.

(b) In one molecule of ammonia, what is the ratio of nitrogen atoms to hydrogen atoms?

(c) Calculate the percentage by mass of nitrogen in ammonia. (Atomic masses: N = 14, H = 1)

(d) If 10 g of nitrogen reacts completely with hydrogen to form ammonia, calculate the mass of ammonia produced. (Hint: Use the law of conservation of mass)

Q18. A chemistry teacher demonstrates the reaction between magnesium and oxygen. She burns 6 g of magnesium ribbon in air, and 10 g of magnesium oxide (MgO) is formed.

(a) Write the balanced chemical equation for this reaction.

(b) Calculate the mass of oxygen that combined with magnesium.

(c) Verify whether this reaction follows the law of conservation of mass.

(d) Is magnesium oxide an element, compound, or mixture? Justify your answer.

(e) If the teacher had used 12 g of magnesium, what mass of magnesium oxide would be formed? (Assume oxygen is in excess)