O-Level Combined Science — Chemistry 3 — Questions and Answers
Question 1: What is the number of protons in an atom called?
- Mass number
- Atomic number (Correct answer)
- Neutron number
- Electron number
Correct answer: Atomic number
The atomic number is the number of protons in the nucleus of an atom, and it defines the element.
Question 2: Which property of metals explains why they can be drawn into wires?
- Malleability
- Conductivity
- Ductility (Correct answer)
- Lustre
Correct answer: Ductility
Ductility is the property that allows metals to be drawn into thin wires without breaking.
Question 3: What type of reaction is: CH₄ + 2O₂ → CO₂ + 2H₂O?
- Decomposition
- Synthesis
- Combustion (Correct answer)
- Displacement
Correct answer: Combustion
This is a combustion reaction — methane (CH₄) burns in oxygen to produce carbon dioxide and water.
Question 4: What is the name of CaCO₃?
- Calcium chloride
- Calcium carbonate (Correct answer)
- Calcium oxide
- Carbon calcium
Correct answer: Calcium carbonate
CaCO₃ is calcium carbonate, commonly found in limestone, marble, and chalk.
Question 5: Which gas is produced at the cathode during electrolysis of dilute sulfuric acid?
- Oxygen
- Hydrogen (Correct answer)
- Sulfur dioxide
- Chlorine
Correct answer: Hydrogen
During electrolysis of dilute sulfuric acid, hydrogen gas is produced at the cathode (negative electrode) as H⁺ ions are reduced.
Question 6: What is the purpose of a catalyst in a chemical reaction?
- To increase the amount of product
- To decrease the amount of reactant
- To provide an alternative pathway with lower activation energy (Correct answer)
- To permanently bond with reactants
Correct answer: To provide an alternative pathway with lower activation energy
A catalyst provides an alternative reaction pathway with lower activation energy, speeding up the reaction without being consumed.
O-Level Combined Science — Chemistry 3
What is the number of protons in an atom called?