Chemistry of Life Basics
Level 2 (Recall & Understanding)
Time Limit: 30 minutes Total Marks: 40
Instructions: Answer all questions. Show reasoning where calculation is required. Use correct scientific terminology.
Q1. Define the following terms: (4 marks) (a) Atomic number (b) Mass number (c) Isotope (d) Buffer
Q2. An atom of carbon has an atomic number of 6 and a mass number of 12. State the number of: (3 marks) (a) protons (b) neutrons (c) electrons (assuming the atom is neutral)
Q3. Name the six major elements found in living organisms represented by the acronym CHNOPS. (3 marks)
Q4. Explain why carbon is central to life, giving three specific structural reasons. (3 marks)
Q5. Distinguish between ionic bonds, covalent bonds, and hydrogen bonds by describing how each is formed. (6 marks)
Q6. Explain the difference between a polar covalent bond and a nonpolar covalent bond, giving one example of each. (4 marks)
Q7. Water has several properties vital to life. Briefly explain each of the following: (6 marks) (a) Cohesion (b) Adhesion (c) High specific heat
Q8. Define and distinguish between the terms molecule, compound, and mixture. Give one example of each. (6 marks)
Q9. Answer the following on the pH scale: (5 marks) (a) Define pH. (1) (b) State the pH value of a neutral solution. (1) (c) A solution has a pH of 3. Is it acidic or basic? (1) (d) If solution A has pH 4 and solution B has pH 6, how many times more acidic (higher [H⁺]) is A than B? (2)
Answer keyMark scheme & solutions
Q1. (4 marks — 1 each)
- (a) Atomic number = number of protons in the nucleus of an atom. (defines element identity)
- (b) Mass number = total number of protons + neutrons in the nucleus.
- (c) Isotope = atoms of the same element (same number of protons) but with a different number of neutrons (different mass number).
- (d) Buffer = a substance/solution that resists changes in pH by absorbing or releasing H⁺ ions.
Q2. (3 marks — 1 each)
- (a) Protons = atomic number = 6
- (b) Neutrons = mass number − atomic number = 12 − 6 = 6
- (c) Electrons = protons (neutral atom) = 6 Why: neutral atoms have equal protons and electrons; neutrons = mass − protons.
Q3. (3 marks — ½ each element)
- C – Carbon, H – Hydrogen, N – Nitrogen, O – Oxygen, P – Phosphorus, S – Sulfur.
Q4. (3 marks — 1 per valid reason)
- Carbon has 4 valence electrons, forming up to 4 stable covalent bonds. (1)
- Can bond with itself to form long chains, branches, and rings (catenation). (1)
- Forms bonds with many elements (H, O, N, etc.), giving huge molecular diversity / allows large macromolecules. (1)
Q5. (6 marks — 2 each)
- Ionic bond: formed by the transfer of electrons from one atom to another, creating oppositely charged ions that attract (e.g., Na⁺Cl⁻). (2)
- Covalent bond: formed by the sharing of electron pairs between atoms. (2)
- Hydrogen bond: a weak attraction between a partially positive H (bonded to O/N) and a partially negative atom of another molecule. (2)
Q6. (4 marks)
- Polar covalent: electrons shared unequally due to difference in electronegativity → partial charges (δ⁺/δ⁻). Example: water (H₂O). (2)
- Nonpolar covalent: electrons shared equally, no partial charges. Example: O₂ / methane (CH₄). (2)
Q7. (6 marks — 2 each)
- (a) Cohesion: attraction between water molecules (via hydrogen bonds); allows water to stick together (e.g., water column in plants, surface tension). (2)
- (b) Adhesion: attraction between water molecules and other (polar) surfaces; enables capillary action. (2)
- (c) High specific heat: water absorbs/releases much heat with little temperature change; stabilises temperature in organisms and environments. (2)
Q8. (6 marks — 2 each: definition + example)
- Molecule: two or more atoms chemically bonded together. Example: O₂. (2)
- Compound: substance of two or more different elements chemically combined in fixed ratio. Example: H₂O / NaCl. (2)
- Mixture: two or more substances physically combined, not chemically bonded, in variable proportions. Example: saltwater / air. (2)
Q9. (5 marks)
- (a) pH = a measure of the hydrogen ion (H⁺) concentration of a solution; = −log₁₀[H⁺]. (1)
- (b) Neutral = pH 7. (1)
- (c) pH 3 → acidic (below 7). (1)
- (d) Each pH unit = 10× change in [H⁺]. Difference = 6 − 4 = 2 units → 10² = 100 times more acidic. (2)
[
{"claim":"Carbon-12 neutron count = mass - atomic number = 6","code":"result = (12 - 6) == 6"},
{"claim":"Neutral carbon electrons equal protons = 6","code":"result = 6 == 6"},
{"claim":"pH 4 is 100x more acidic in [H+] than pH 6","code":"result = 10**(6-4) == 100"},
{"claim":"Neutral pH corresponds to [H+]=1e-7, pH=7","code":"result = int(-log(1e-7,10)) == 7"}
]