Mark Scheme

Section A: 1. B; 2. C; 3. B; 4. B; 5. B. (5)

6 (a) any two: higher temperature; higher concentration; smaller pieces/larger surface area; catalyst (2). (b) more particles in a given volume (1); more frequent collisions (1); so more successful collisions per second (1). (c) provides a different pathway (1) with a lower activation energy (1). (7)

7 (a) a compound of hydrogen and carbon only (1). (b) oil is heated and evaporates (1); vapours rise up a column that is hot at the bottom, cooler at the top (1); each fraction condenses at its boiling point (1). (c) carbon dioxide (1) and water (1). (d) produces carbon monoxide (1) which is toxic / reduces oxygen carried by the blood (1). (8)

8 (a) products can react to reform the reactants (1). (b) forward and reverse reactions occur at the same rate (1); concentrations of reactants and products stay constant (1). (3)

9 (a) spot the mixture on a pencil line (1); solvent moves up the paper carrying substances different distances (1); more soluble substances travel further (1). (b) Rf = distance moved by substance ÷ distance moved by solvent (1); between 0 and 1 (1). (c) lighted splint (1); squeaky pop (1). (7)

10 (a) carbon dioxide (1). (b) algae/plants evolved and photosynthesised (1), producing oxygen (1). (c) carbon dioxide — burning fossil fuels/deforestation (2); methane — cattle farming/landfill/rice fields (2). (7)

11 (a) water that is safe to drink (1); low levels of dissolved salts and microbes (1). (b) filter to remove solids (1); sterilise (chlorine/ozone/UV) to kill microbes (1). (c) requires large amounts of energy (to distil or reverse osmosis) (1); making it costly (1). (6)

12 — Levels (6): Indicative: greenhouse gases (CO₂, methane, water vapour) absorb infrared. Short-wave solar radiation is absorbed by Earth; Earth re-emits longer-wavelength infrared; greenhouse gases absorb and re-radiate it back, warming the surface. Human activities (burning fossil fuels, deforestation, farming) increase greenhouse gas concentrations, enhancing the effect and raising global temperatures.

• L3 (5–6): Correct mechanism (infrared absorbed and re-emitted) and human causes linked to rising temperature.
• L2 (3–4): Some of the mechanism and/or causes.
• L1 (1–2): Simple statements.

Sample Answers with Examiner Commentary

Question 12 — Sample Answers

Grade 9 answer "Greenhouse gases such as carbon dioxide, methane and water vapour trap heat. Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface. The Earth then re-emits energy as longer-wavelength infrared radiation. Greenhouse gases absorb this infrared and re-radiate it in all directions, including back to the surface, which keeps the Earth warm enough for life. Human activities such as burning fossil fuels and deforestation increase the concentration of carbon dioxide, and farming increases methane. This enhances the greenhouse effect, so more heat is trapped and global temperatures rise." Mark: 6/6. Examiner commentary: The mechanism is precise (infrared absorbed and re-radiated) and clearly linked to specific human causes and rising temperature. Top band.

Grade 6 answer "Greenhouse gases like carbon dioxide trap heat from the Sun and keep the Earth warm. Humans burn fossil fuels which makes more carbon dioxide, so the Earth gets hotter." Mark: 4/6. Examiner commentary: Correct overall idea and a valid human cause, but the mechanism lacks detail — it should mention the Earth re-emitting infrared that the gases absorb and re-radiate. That detail would secure the top band.

Grade 3 answer "The greenhouse gases make a layer that stops heat escaping like a greenhouse. Pollution makes it worse." Mark: 2/6. Examiner commentary: Shows a basic idea of heat being trapped, but "pollution" is vague and there is no mention of infrared, specific gases or specific human activities. Naming CO₂/methane and the infrared mechanism would gain marks.

Question 6(b) — Sample Answers

Grade 9 answer "Increasing the concentration means there are more acid particles in the same volume. This makes collisions between the reacting particles more frequent, so there are more successful collisions per second and the rate increases." Mark: 3/3. Examiner commentary: All three collision-theory points — more particles per volume, more frequent collisions, more successful collisions per second.

Grade 6 answer "There are more particles so they collide more, which speeds up the reaction." Mark: 2/3. Examiner commentary: Credit for more particles and more frequent collisions, but it does not make clear this means more successful collisions per second, the precise point examiners look for.