O-Level Combined Science — Questions and Answers
Question 1: What is an atom and what are its three main subatomic particles?
- Atoms have only two particles
- An atom is the smallest unit of an element, consisting of protons (positive, in nucleus), neutrons (neutral, in nucleus), and electrons (negative, orbiting the nucleus) (Correct answer)
- Atoms are made of molecules
- Atoms have no internal structure
Correct answer: An atom is the smallest unit of an element, consisting of protons (positive, in nucleus), neutrons (neutral, in nucleus), and electrons (negative, orbiting the nucleus)
An atom has a dense nucleus containing positively charged protons and neutral neutrons, with negatively charged electrons orbiting in shells around the nucleus.
Question 2: What is Ohm's Law?
- V = I × R², where V is voltage, I is current, and R is resistance
- V = I × R, where V is voltage (volts), I is current (amps), and R is resistance (ohms) (Correct answer)
- V = I + R
- V = I / R
Correct answer: V = I × R, where V is voltage (volts), I is current (amps), and R is resistance (ohms)
Ohm's Law states that voltage (V) equals current (I) multiplied by resistance (R). This fundamental relationship applies to many electrical circuits and is essential for O-Level Physics.
Question 3: What is the difference between an exothermic and endothermic reaction?
- They are the same
- Exothermic reactions release heat to surroundings (temperature rises); endothermic reactions absorb heat from surroundings (temperature drops) (Correct answer)
- Exothermic reactions are always slow
- Endothermic reactions only occur in labs
Correct answer: Exothermic reactions release heat to surroundings (temperature rises); endothermic reactions absorb heat from surroundings (temperature drops)
Exothermic reactions release energy (e.g., combustion, neutralisation — surroundings get warmer). Endothermic reactions absorb energy (e.g., thermal decomposition, photosynthesis — surroundings get cooler).
Question 4: What is osmosis?
- Movement of any substance across a membrane
- The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane (Correct answer)
- Movement of solute molecules only
- Active transport of water
Correct answer: The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
Osmosis is the passive movement of water molecules through a partially permeable membrane from a dilute solution (high water potential) to a concentrated solution (low water potential).
Question 5: What is the law of reflection?
- Light always travels in straight lines
- The angle of incidence equals the angle of reflection, both measured from the normal to the reflecting surface (Correct answer)
- Light bends when it reflects
- Reflected light is always weaker
Correct answer: The angle of incidence equals the angle of reflection, both measured from the normal to the reflecting surface
The law of reflection states that the angle of incidence (incoming ray to normal) equals the angle of reflection (reflected ray to normal). The incident ray, reflected ray, and normal are all in the same plane.
Question 6: What is the role of enzymes in the human digestive system?
- They absorb nutrients
- They are biological catalysts that speed up the breakdown of food molecules into smaller, absorbable units (Correct answer)
- They kill bacteria in food
- They produce stomach acid
Correct answer: They are biological catalysts that speed up the breakdown of food molecules into smaller, absorbable units
Digestive enzymes are biological catalysts: amylase breaks down starch to maltose, protease breaks proteins into amino acids, and lipase breaks fats into fatty acids and glycerol.
O-Level Combined Science
What is an atom and what are its three main subatomic particles?