Chapter-8 Elements — Online MCQ Test
SCIENCE · Grade 8 · CBSE(NCERT)
Practice Chapter-8 Elements with a free chapter-wise online MCQ test.
This chapter covers: Compounds.
AI-generated questions from basic to board-exam level, with instant results and explanations.
Chapter-8 Elements — Important Questions & Answers
What is a compound?
- A. A pure substance made of two or more elements chemically combined in a fixed ratio
- B. A mixture of different elements that are not chemically bonded
- C. A single element in pure form
- D. A substance that cannot be broken down further
Answer: A. A pure substance made of two or more elements chemically combined in a fixed ratio
A compound is formed when two or more elements combine chemically in fixed proportions with new properties different from the constituent elements.
A compound is formed when two or more elements combine chemically in fixed proportions with new properties different from the constituent elements.
Which of the following is a compound?
- A. Oxygen
- B. Iron
- C. Water
- D. Nitrogen
Answer: C. Water
Water (H₂O) is a compound made of hydrogen and oxygen elements chemically bonded. The others are elements.
Water (H₂O) is a compound made of hydrogen and oxygen elements chemically bonded. The others are elements.
What is the chemical formula of common salt?
- A. NaCl
- B. NaOH
- C. KCl
- D. CaCl₂
Answer: A. NaCl
Common salt is sodium chloride, represented by the formula NaCl, composed of sodium and chlorine elements.
Common salt is sodium chloride, represented by the formula NaCl, composed of sodium and chlorine elements.
How many elements are present in sugar (C₁₂H₂₂O₁₁)?
- A. Two
- B. Three
- C. Four
- D. Five
Answer: B. Three
Sugar contains three elements: carbon (C), hydrogen (H), and oxygen (O).
Sugar contains three elements: carbon (C), hydrogen (H), and oxygen (O).
What happens to the properties of elements when they form a compound?
- A. The properties remain exactly the same
- B. The properties combine in a simple average
- C. Completely new properties emerge that are different from the constituent elements
- D. Only the color changes, other properties remain the same
Answer: C. Completely new properties emerge that are different from the constituent elements
When elements combine to form compounds, they exhibit entirely new properties different from their parent elements, such as hydrogen and oxygen forming water.
When elements combine to form compounds, they exhibit entirely new properties different from their parent elements, such as hydrogen and oxygen forming water.
Why is salt (NaCl) different from sodium metal and chlorine gas?
- A. Because salt is heavier than both elements
- B. Because the elements have chemically combined to form a new substance with different properties
- C. Because salt is always white in color
- D. Because salt can only be found in the ocean
Answer: B. Because the elements have chemically combined to form a new substance with different properties
Sodium is a reactive metal and chlorine is a toxic gas, but when chemically combined, they form edible sodium chloride (salt) with completely different properties.
Sodium is a reactive metal and chlorine is a toxic gas, but when chemically combined, they form edible sodium chloride (salt) with completely different properties.
A student burns magnesium in oxygen to form magnesium oxide. Which property is NOT likely to change?
- A. Color
- B. Chemical composition
- C. Melting point
- D. Position in the periodic table
Answer: D. Position in the periodic table
When a compound forms, the constituent elements' position in the periodic table remains fixed; however, their properties (color, melting point, etc.) change significantly.
When a compound forms, the constituent elements' position in the periodic table remains fixed; however, their properties (color, melting point, etc.) change significantly.
If compound A contains elements X and Y in a 2:1 ratio by mass, and compound B contains the same elements in a 1:2 ratio by mass, what can be concluded?
- A. A and B are the same compound
- B. A and B are different compounds with different properties
- C. A and B will have identical properties
- D. A and B cannot exist as separate substances
Answer: B. A and B are different compounds with different properties
Different mass ratios of the same elements form different compounds (law of multiple proportions), which will have distinctly different properties.
Different mass ratios of the same elements form different compounds (law of multiple proportions), which will have distinctly different properties.
In a comprehensive analysis of two substances, scientists find that substance X always contains carbon and hydrogen in a 1:4 atomic ratio, while substance Y contains the same elements in a 1:6 atomic ratio. What is the relationship between X and Y?
- A. They are isotopes of the same element
- B. They are different compounds with the law of multiple proportions demonstrated
- C. They are mixtures of the same elements
- D. They cannot both exist in nature
Answer: B. They are different compounds with the law of multiple proportions demonstrated
Different compounds (like CH₄ methane and C₂H₆ ethane) exemplify the law of multiple proportions where the same elements combine in different whole-number ratios, forming distinct compounds.
Different compounds (like CH₄ methane and C₂H₆ ethane) exemplify the law of multiple proportions where the same elements combine in different whole-number ratios, forming distinct compounds.
A compound with molecular formula C₃H₈O has how many total atoms, and if 5 molecules are present, what is the total atom count?
- A. Total in one molecule: 12 atoms; in 5 molecules: 60 atoms
- B. Total in one molecule: 11 atoms; in 5 molecules: 55 atoms
- C. Total in one molecule: 14 atoms; in 5 molecules: 70 atoms
- D. Total in one molecule: 10 atoms; in 5 molecules: 50 atoms
Answer: A. Total in one molecule: 12 atoms; in 5 molecules: 60 atoms
C₃H₈O has 3 carbon + 8 hydrogen + 1 oxygen = 12 atoms per molecule. For 5 molecules: 12 × 5 = 60 total atoms.
C₃H₈O has 3 carbon + 8 hydrogen + 1 oxygen = 12 atoms per molecule. For 5 molecules: 12 × 5 = 60 total atoms.