Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 1.1 — Solids, liquids and gases
• Topic 1.2 — Diffusion
• Topic 10.3 — Air quality and climate
(a)
Explanation: The kinetic particle theory models states of matter based on energy. Solids have low energy, keeping particles locked in a uniform lattice where they only vibrate. Liquids have enough energy to break the rigid lattice (moving randomly and sliding past each other) but stay close. Gases have high energy, spreading far apart and moving freely.
(b)
Solid to liquid: Melting
Gas to liquid: Condensing / Condensation
Solid to aqueous: Dissolving
(c)(i)
Answer: Gases diffuse due to the random motion/movement of particles.
Explanation: Particles in a gas collide and move randomly, naturally spreading from a region of higher concentration to lower concentration until evenly distributed.
(c)(ii)
Answer: Relative molecular mass ($M_{r}$).
Explanation: According to Graham’s Law of Effusion/Diffusion, at a constant temperature, gas molecules with a lower relative molecular mass will travel at a higher velocity than heavier molecules, thus diffusing faster.
(d)
1. Photochemical smog
2. Respiratory problems
(e)(i)
Answer: Carbon monoxide ($CO$).
Explanation: Incomplete combustion of carbon-containing fuels (like octane in petrol) produces carbon monoxide due to a lack of sufficient oxygen to form $CO_{2}$.
(e)(ii)
Answer: Nitrogen ($N_{2}$) and Carbon dioxide ($CO_{2}$).
Explanation: Inside a catalytic converter (which uses a platinum/rhodium catalyst), toxic $CO$ reduces oxides of nitrogen ($NO_{x}$) according to the general equation: $2CO + 2NO \rightarrow 2CO_{2} + N_{2}$.
