Level 3 — ProductionChemistry of Life Basics

Chemistry of Life Basics

⏱ 45 minutes60 marksprintable — key stays hidden on paper

Level 3 Paper: Production (From-Scratch Derivations & Explain-Out-Loud)

Time limit: 45 minutes Total marks: 60 Instructions: Answer ALL questions. Show full reasoning — you are being marked on the why, not just the what. Use chemical notation where helpful.

Question 1 — Build the atom from scratch (10 marks)

A neutral atom of phosphorus has atomic number 15 and mass number 31.

(a) From memory, define atomic number and mass number, then use them to state the number of protons, neutrons, and electrons in this atom. Show the arithmetic. (4)

(b) A second phosphorus atom has mass number 32. Explain, from first principles, what this atom is called relative to the first, what has changed in its nucleus, and why its chemical behaviour is essentially unchanged. (3)

(c) Phosphorus is one of the CHNOPS elements. Name all six CHNOPS elements and give one biological molecule that requires phosphorus specifically. (3)

Question 2 — Derive the bond hierarchy (12 marks)

Working only from the idea of electron behaviour:

(a) Explain, from scratch, how ionic, covalent, and hydrogen bonds each arise. For each, state whether electrons are transferred, shared, or not directly involved. (6)

(b) Rank the three bond types from strongest to weakest and justify the ranking. (3)

(c) Explain why NaCl (ionic) dissolves readily in water but oil (nonpolar covalent) does not. Link your answer to bond/charge behaviour. (3)

Question 3 — Polar vs nonpolar, explained out loud (9 marks)

(a) Define electronegativity and use it to explain the difference between a polar and a nonpolar covalent bond. (4)

(b) Water ( $H_2O$ ) is polar; methane ( $CH_4$ ) is nonpolar though both share electrons. Explain the difference in terms of electronegativity difference AND molecular shape. (5)

Question 4 — Water properties from first principles (12 marks)

Starting from the polar structure of a single water molecule and hydrogen bonding:

(a) Derive an explanation for cohesion and adhesion, and use them to explain how water rises in a narrow plant xylem tube (capillary action). (5)

(b) Explain why water has a high specific heat and give one biological consequence for a living organism. (4)

Question 5 — pH derivation and buffers (12 marks)

(a) Define pH from scratch, including the relationship between pH and hydrogen ion concentration $[H^+]$ . Write the defining equation. (3)

(b) A solution has $[H^+] = 1 \times 10^{-3}\ \text{mol/L}$ . Calculate its pH, classify it as acidic/basic/neutral, and state how many times more acidic it is than a neutral solution. Show all steps. (5)

Question 6 — Reactions, molecules, compounds, mixtures (5 marks)

(a) Define molecule, compound, and mixture, giving one example of each. (3)

(b) For the reaction $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ , identify the reactants and products, and state what the arrow represents. (2)

Answer keyMark scheme & solutions

Question 1 (10 marks)

(a) (4)

Atomic number = number of protons in the nucleus (defines the element). (1)
Mass number = number of protons + neutrons. (1)
Protons = 15; Electrons = 15 (neutral atom, so electrons = protons); Neutrons = mass − atomic number = $31 - 15 = 16$ . (2, all three correct)

(b) (3)

It is an isotope of the first atom. (1)
The nucleus now has 17 neutrons ( $32-15$ ) instead of 16 — the neutron number changed, proton number unchanged. (1)
Chemical behaviour depends on electrons/protons, which are unchanged, so reactivity is the same. (1)

(c) (3)

CHNOPS: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur. (2, all six)
Phosphorus-requiring molecule: ATP (or DNA/RNA backbone, phospholipids). (1)

Question 2 (12 marks)

(a) (6, 2 each)

Ionic: electrons transferred from one atom to another → oppositely charged ions attract. (2)
Covalent: electrons shared between atoms to fill outer shells. (2)
Hydrogen: electrons not directly involved/transferred; weak attraction between a partially positive H (already covalently bonded to N/O/F) and a partially negative atom. (2)

(b) (3)

Strongest → weakest: ionic (or strong covalent) > covalent > hydrogen. (1)
Ionic/covalent involve full transfer or sharing of electrons (strong), while hydrogen bonds are only partial-charge attractions (weak). (2)

(c) (3)

Water is polar; its charged ends pull apart Na⁺ and Cl⁻ ions and surround them → dissolves. (2)
Oil is nonpolar (no charged regions) so water molecules are not attracted to it → does not dissolve. (1)

Question 3 (9 marks)

(a) (4)

Electronegativity = an atom's tendency/ability to attract shared electrons. (2)
Nonpolar bond: electrons shared equally (small/zero electronegativity difference). Polar bond: electrons pulled toward the more electronegative atom, creating partial charges. (2)

(b) (5)

In water, O is much more electronegative than H → O–H bonds are polar (partial − on O, partial + on H). (2)
Water's bent shape means the polar bonds don't cancel → whole molecule is polar. (1.5)
In methane, C–H electronegativity difference is small AND the symmetrical tetrahedral shape causes bond dipoles to cancel → molecule is nonpolar overall. (1.5)

Question 4 (12 marks)

(a) (5)

Hydrogen bonds between water molecules cause cohesion (water sticks to water). (1.5)
Hydrogen bonds between water and other polar surfaces cause adhesion (water sticks to other things). (1.5)
In xylem, adhesion pulls water up the tube walls, cohesion drags the column of water along behind it → water rises against gravity (capillary action). (2)

(b) (4)

High specific heat: much energy is needed to break the many hydrogen bonds before temperature rises, so water resists temperature change. (2)
Biological consequence: stabilises body/cell/environmental temperature (e.g., maintains stable internal temperature; protects aquatic life from rapid change). (2)

(c) (3)

Water's polarity lets it surround and separate many polar/charged (hydrophilic) solutes, dissolving more substances than any other liquid → "universal solvent." (2)
Cell reactions occur in aqueous solution; dissolved reactants can move and react. (1)

Question 5 (12 marks)

(a) (3)

pH measures acidity via $[H^+]$ : $\text{pH} = -\log_{10}[H^+]$ . (2)
Lower pH = higher $[H^+]$ = more acidic; scale runs 0–14. (1)

(b) (5)

$\text{pH} = -\log_{10}(1\times10^{-3}) = 3$ . (2)
Classification: acidic (pH < 7). (1)
Neutral is pH 7, i.e. $[H^+]=10^{-7}$ . Difference $= 10^{-3}/10^{-7} = 10^4$ → 10 000 times more acidic. (2)

(c) (4)

A buffer contains a weak acid + its conjugate base (or weak base + conjugate acid). (1)
It absorbs added H⁺ (base component reacts) and releases H⁺ when needed (acid component reacts), keeping pH nearly constant. (2)
In blood, the bicarbonate buffer keeps pH ≈ 7.4; deviations disrupt enzyme/protein function → buffers maintain homeostasis. (1)

Question 6 (5 marks)

(a) (3)

Molecule: two or more atoms bonded together (e.g. $O_2$ ). (1)
Compound: substance of two or more different elements chemically bonded (e.g. $H_2O$ ). (1)
Mixture: two or more substances physically combined, not chemically bonded, retaining their properties (e.g. salt water / air). (1)

(b) (2)

Reactants: $CO_2$ and $H_2O$ ; Products: $C_6H_{12}O_6$ and $O_2$ . (1)
The arrow means "yields/produces" — direction of the chemical reaction from reactants to products. (1)

[
  {"claim":"Phosphorus-31 has 16 neutrons","code":"protons=15; mass=31; neutrons=mass-protons; result = (neutrons==16)"},
  {"claim":"P-32 isotope has 17 neutrons","code":"protons=15; mass=32; neutrons=mass-protons; result = (neutrons==17)"},
  {"claim":"pH of [H+]=1e-3 is 3","code":"import math; pH=-math.log10(1e-3); result = (abs(pH-3)<1e-9)"},
  {"claim":"[H+]=1e-3 is 10000x more acidic than neutral 1e-7","code":"ratio=(1e-3)/(1e-7); result = (abs(ratio-10000)<1e-3)"}
]