Extended-response Six-mark Questions: Chemistry

Six-mark extended-response questions appear on both chemistry papers of OCR Gateway Combined Science A, and, like biology, they are marked by levels of response. Chemistry 6-markers have their own flavour, though: they often ask you to describe a method, explain a property using bonding or particle theory, or compare and evaluate processes. The skill the examiner is really testing is your ability to connect what you observe (the macroscopic — a colour change, a high melting point) to why it happens (the sub-microscopic — ions, bonds, electrons, collisions). This lesson shows you how to structure a top-band chemistry answer.

By the end of this lesson you should be able to plan a chemistry 6-marker, link macroscopic observations to particle-level explanations, and know what separates a mid-band answer from a top-band one.

A chemistry six-marker targets AO2 above all — applying bonding and particle theory to explain what you observe — resting on AO1 knowledge and reaching AO3 when it compares or evaluates.

---

How Levels-of-Response Marking Works

Level	Marks	What it looks like
Level 3	5–6	Detailed and complete; accurate chemistry; well structured; correct terminology throughout
Level 2	3–4	Reasonable; some relevant chemistry; may lack detail or particle-level explanation
Level 1	1–2	Limited; fragmented; errors; little structure
No relevant content	0	Nothing creditworthy

Exam Tip: In chemistry, the difference between Level 2 and Level 3 is almost always whether you explain at the particle level. Describing that diamond is hard is Level 1–2; explaining it through the giant covalent structure and strong bonds is Level 3.

---

Two Reliable Structures

Chemistry 6-markers usually fall into one of two shapes, and each has a proven structure.

For "describe a method" questions — write in chronological order and name equipment:

flowchart TD
    A[Read the question] --> B{Method or explanation?}
    B -->|Describe a method| C[Steps in order; name apparatus; state observations]
    B -->|Explain a property| D[Point - Evidence - Link for each property]
    C --> E[Check: equipment named and observations stated]
    D --> E

For "explain a property" questions — use Point–Evidence–Link (P-E-L):

Step	Meaning	Example
Point	State the key fact	"Sodium chloride has a high melting point."
Evidence	Give the structure/mechanism	"It has a giant ionic lattice with strong electrostatic forces between oppositely charged ions."
Link	Connect to the question	"A large amount of energy is needed to overcome these forces, so the melting point is high."

---

Chemistry 6-mark Topics to Expect

Topic	Typical question
Particles (C1)	Explain changes of state using the particle model
Elements, compounds and mixtures (C2)	Explain properties from ionic, covalent or metallic bonding
Chemical reactions (C3)	Describe how to prepare a pure salt; explain electrolysis
Predicting and identifying reactions (C4)	Describe tests for ions and gases; predict products
Monitoring and controlling reactions (C5)	Explain factors affecting rate; describe a titration
Global challenges (C6)	Compare metal extraction methods; evaluate life-cycle impacts

---

A Three-band Model Answer

Question (6 marks): Explain, in terms of structure and bonding, why sodium chloride has a high melting point but only conducts electricity when it is molten or dissolved in water.

This is an "explain a property using bonding" question, so a top-band answer must link the giant ionic lattice to both properties.

Mid-band response: "Sodium chloride is made of sodium ions and chloride ions joined by ionic bonds. The ionic bonds are strong so it takes a lot of energy to break them, which is why it has a high melting point. It does not conduct when it is solid because the ions cannot move, but when it is melted or dissolved the ions can move so it can conduct electricity."

Examiner-style commentary: A sound Level 2 answer. It correctly identifies ionic bonding, links bond strength to the high melting point, and grasps that mobile ions are needed for conduction. To reach Level 3 it needs the term giant ionic lattice, the phrase electrostatic forces of attraction acting in all directions, and the idea that conduction requires charged particles (ions) that are free to move to carry charge. The reasoning is right but the terminology is not yet precise enough.

Stronger response: "Sodium chloride has a giant ionic lattice made of positive sodium ions (Na⁺) and negative chloride ions (Cl⁻) arranged in a regular pattern. There are strong electrostatic forces of attraction between the oppositely charged ions, and because these act throughout the lattice, a lot of energy is needed to overcome them — this gives sodium chloride a high melting point. When solid, the ions are held in fixed positions, so they cannot move to carry charge and it does not conduct. When it is melted or dissolved in water, the ions become free to move, so they can carry charge and it conducts electricity."

Examiner-style commentary: A strong Level 3 answer: it uses "giant ionic lattice" and "electrostatic forces", links the forces to the melting point, and explains conduction in terms of mobile ions. To make it watertight it could state that the electrostatic forces act in all directions (which is why the lattice is so strong) and specify that it is charged particles that are free to move that allow an ionic compound to conduct.

Top-band response: "Sodium chloride has a giant ionic lattice structure, made up of positive sodium ions (Na⁺) and negative chloride ions (Cl⁻) arranged in a regular, repeating three-dimensional pattern. There are strong electrostatic forces of attraction between the oppositely charged ions, and these forces act in all directions throughout the whole lattice. Because there are so many strong forces to overcome, a large amount of energy is required to separate the ions, so sodium chloride has a high melting point. When sodium chloride is solid, the ions are locked in fixed positions within the lattice; although they are charged, they cannot move, so there are no mobile charge carriers and solid sodium chloride does not conduct electricity. However, when it is melted or dissolved in water, the lattice breaks apart and the ions become free to move. These moving charged ions can now carry charge through the substance, so molten or dissolved sodium chloride does conduct electricity. In short, the giant ionic lattice explains both properties: the many strong electrostatic forces give the high melting point, and conduction depends on whether the charged ions are free to move."