Basic Concept of Chemistry Class 11: Pure Substances, Elements, and Compounds

Pure Substances, Elements, and Compounds

1. Which of the following is a characteristic of pure substances?

• Variable composition
• Fixed composition
• Mixed composition
• None of the above

Correct Answer: Fixed composition

2. Which of the following is an example of a pure substance?

• Air
• Water
• Tea
• Salt solution

Correct Answer: Water

3. What is the composition of glucose?

• Variable ratio of carbon, hydrogen, and oxygen
• Fixed ratio of carbon, hydrogen, and oxygen
• Only carbon and hydrogen
• Only oxygen and hydrogen

Correct Answer: Fixed ratio of carbon, hydrogen, and oxygen

4. Which of the following cannot be separated by physical methods?

• Mixtures
• Elements
• Compounds
• Both B and C

Correct Answer: Compounds

5. Which of the following is an example of an element?

• Water
• Copper
• Carbon dioxide
• Sugar

Correct Answer: Copper

6. How are the particles of an element arranged?

• Made up of only one type of atom
• Made up of two or more atoms of different elements
• Made up of molecules of compounds
• Made up of different types of atoms

Correct Answer: Made up of only one type of atom

7. What are the particles of hydrogen, nitrogen, and oxygen gases?

• Atoms
• Molecules
• Ions
• None of the above

Correct Answer: Molecules

8. What is formed when two or more atoms of different elements combine in a definite ratio?

• Molecule of a compound
• Element
• Mixture
• Atom

Correct Answer: Molecule of a compound

9. How are compounds formed?

• By physical mixing of elements
• By chemical combination of elements
• By heating of elements
• By cooling elements

Correct Answer: By chemical combination of elements

10. Which of the following is an example of a compound?

• Oxygen
• Water
• Copper
• Hydrogen

Correct Answer: Water

11. Which of the following can be separated into simpler substances only by chemical methods?

• Mixtures
• Compounds
• Elements
• All of the above

Correct Answer: Compounds

12. Which of the following is a characteristic of compounds?

• Same composition throughout
• Varying composition
• Easily separated by physical methods
• All of the above

Correct Answer: Same composition throughout

13. The particles of an element may exist as:

• Atoms
• Molecules
• Both A and B
• None of the above

Correct Answer: Both A and B

14. The molecule of a compound is formed when:

• Two or more atoms of the same element combine
• Two or more atoms of different elements combine
• One atom combines with one molecule
• None of the above

Correct Answer: Two or more atoms of different elements combine

15. Which of the following examples is a mixture?

• Pure water
• Salt solution
• Copper
• None of the above

Correct Answer: Salt solution

16. Which of the following cannot be separated by physical methods?

• Mixtures
• Elements
• Compounds
• All of the above

Correct Answer: Compounds

17. Which of the following elements exist as molecules?

• Sodium
• Copper
• Oxygen
• Both A and C

Correct Answer: Oxygen

18. Which of the following is an example of an element that exists as atoms?

• Helium
• Oxygen
• Nitrogen
• All of the above

Correct Answer: Helium

19. Which of the following is a characteristic of elements?

• Made up of atoms
• Made up of molecules
• Can be separated by physical methods
• Both A and B

Correct Answer: Made up of atoms

20. Which of the following is an example of a compound?

• Water
• Oxygen
• Copper
• Both A and B

Correct Answer: Water

Pure Substances, Elements, and Compounds

Pure substances are materials with a fixed composition, and their constituent particles are uniform in nature. Examples include copper, silver, gold, water, and glucose. Glucose, for instance, consists of carbon, hydrogen, and oxygen in a fixed ratio. Pure substances cannot be separated by physical methods.

Pure substances are classified into elements and compounds. Elements consist of only one type of atom, and these can exist as either individual atoms (like sodium and copper) or molecules (like hydrogen and oxygen). Compounds are formed when atoms of different elements combine in a definite ratio, and their constituents can only be separated by chemical methods. Examples of compounds include water, ammonia, and carbon dioxide.