Answering Describe and Explain Questions in IGCSE Chemistry

Describe and explain are the two most common command words on Paper 4, and they are the two most commonly confused. Treating them as interchangeable costs marks on nearly every structured question. The full list of command words is covered in command words; this guide focuses specifically on the describe/explain pair and how to structure answers for each.

Describe: what happens

A describe answer states observable facts: what you would see, measure or record. It does not include reasons or explanations.

What counts as describing:

• Observations: colour changes, gas production, temperature changes, dissolving, precipitate forming.
• Trends: “the melting point increases down the group.”
• Procedures: step-by-step methods, what equipment you would use, what you would measure.

What does NOT count:

• Reasons why something happens.
• Scientific explanations using particle theory, bonding or energy.

How to structure a describe answer

Match the number of distinct points to the number of marks. Each point should state one observable change or one step in a procedure.

Example (3 marks): Describe what you would observe when magnesium ribbon is added to dilute hydrochloric acid.

• The magnesium dissolves / decreases in size (1).
• Bubbles / fizzing / effervescence is observed (1).
• The solution remains colourless / gets warm (1).

Three observations, three marks. Writing a paragraph about why magnesium reacts with acid earns zero because the question asked for observations, not explanations.

The observation vocabulary

The exam awards marks for specific, precise observations. Use this vocabulary:

General term (scores poorly)	Precise term (scores well)
“It changes colour"	"It turns from orange to green"
"Gas is produced"	"Bubbles of a colourless gas"
"A solid forms"	"A white precipitate forms"
"It dissolves"	"The solid dissolves to form a colourless solution"
"It reacts"	"Effervescence / vigorous fizzing”

Colours matter. States matter. “A solid” is vague; “a white precipitate” scores.

Explain: why it happens

An explain answer gives the scientific reason for an observation or phenomenon. It must include a cause-and-effect link.

What counts as explaining:

• Particle-level reasoning: “the particles have more kinetic energy, so they collide more frequently.”
• Electron-level reasoning: “the outer electron is further from the nucleus, so it is lost more easily.”
• Energy reasoning: “the reaction is exothermic, so energy is released to the surroundings, raising the temperature.”

What does NOT count:

• Restating the observation in different words.
• Using circular reasoning: “it reacts because it is reactive.”

The explain structure

Every explain answer should follow: observation/fact -> because -> scientific reason.

Example (2 marks): Explain why sodium is more reactive than lithium.

• Both are in Group I and react by losing their outer electron (context) (1).
• Sodium has more electron shells than lithium, so the outer electron is further from the nucleus and more shielded, making it easier to lose (1).

The first mark is for establishing how the metals react. The second is for linking the structural difference (more shells, greater distance) to the outcome (easier electron loss, higher reactivity).

The “explain in terms of…” qualifier

When the question says “explain in terms of particles” or “explain in terms of electrons”, every part of your answer must use that language. An answer that explains in terms of energy when the question asks for particles earns zero.

“In terms of particles” examples:

• “The particles move faster and collide more frequently.”
• “The particles have more kinetic energy.”
• “There are more particles per unit volume.”

“In terms of electrons” examples:

• “The outer electron is lost more easily because it is further from the nucleus.”
• “Electrons are transferred from the metal to the non-metal.”
• “Delocalised electrons are free to move and carry charge.”

Describe and explain together

When both words appear in the same question, the marks are split. The describe marks are for the observation; the explain marks are for the reason.

Example (4 marks): Describe and explain what happens when a piece of zinc is placed in copper sulfate solution.

Describe (2 marks):

• The blue colour of the solution fades / becomes paler (1).
• A red-brown / brown deposit forms on the zinc (1).

Explain (2 marks):

• Zinc is more reactive than copper (1).
• Zinc displaces copper from the solution: zinc atoms lose electrons (are oxidised) and copper ions gain electrons (are reduced) (1).

Writing only the explanation without the observations earns 2 marks maximum. Writing only the observations without the explanation also earns 2 marks maximum. You need both for full marks.

Common errors

1. Describing when asked to explain. This is the most common error flagged in examiner reports. Read the command word before you start writing.
2. Circular reasoning. “The rate increases because the reaction is faster” says nothing. The mark is for the mechanism: more frequent collisions, more energetic collisions, lower activation energy.
3. Too few points. A 4-mark question needs four distinct ideas. Three good ideas earn three marks, not four.
4. Vague observations. “Something happens” or “it reacts” without specifying what you see earns nothing.
5. Not matching the qualifier. “Explain in terms of particles” requires particle language. “Explain in terms of bonding” requires bonding language.

More guidance on command words is in command words, and the technique for the highest-mark questions is in 6-mark extended response.

If describe/explain questions are consistently costing marks, the issue is usually about answer structure rather than content knowledge. A trial lesson can retrain the response pattern so the right language comes out under exam pressure.