IGCSE Chemistry 0620: Particle Model, Changes of State and Sublimation

A workshop technician finds four sealed, unlabelled containers left over from a materials demonstration. According to the technician's notes, at room temperature: container A holds a block of solid candle wax, container B holds a small volume of liquid vegetable oil, container C holds helium gas trapped inside a small sealed balloon, and container D holds a small chunk of solid carbon dioxide (dry ice).

(a) Describe the particle separation and particle motion in container A (solid wax) and in container C (helium gas). [2]

(b) The candle wax in container A is heated gently until all of it turns into a liquid. State the name of this change of state. [1]

(c) The oil in container B is placed in a freezer and cools until all of it turns into a solid. State the name of this change of state. [1]

(d) The dry ice in container D is left standing, unsealed, at room temperature. Over several minutes, it disappears completely, without ever forming a puddle of liquid.

(i) Name this change of state. [1]

(ii) Describe how the particle separation and particle motion of the carbon dioxide change as this happens. [2]

Part (a): Particles in the solid and the gas

Container A (solid wax):

Particle separation: particles are close together, essentially touching.
Particle arrangement: particles are arranged in a regular, ordered pattern.
Particle motion: particles vibrate about fixed positions but do not move from place to place.

Container C (helium gas):

Particle separation: particles are far apart, with large gaps between them compared with their own size.
Particle arrangement: particles are arranged randomly, with no fixed pattern.
Particle motion: particles move quickly, in random directions, colliding with each other and with the walls of the balloon.

Part (b): Heating the wax

Container A starts as a solid and ends as a liquid. A solid turning into a liquid is called melting.

Part (c): Cooling the oil

Container B starts as a liquid and ends as a solid. A liquid turning into a solid is called freezing.

Part (d): The disappearing dry ice

(i) The dry ice goes directly from a solid to a gas, with no liquid stage in between. This change of state is called sublimation.

(ii) During sublimation:

Particle separation increases greatly: the particles start close together (touching, as in a solid) and end up far apart (as in a gas), with large gaps between them.
Particle motion changes from vibrating about fixed positions (solid) to moving quickly and randomly in all directions (gas), no longer held in place by the forces between neighbouring particles.

Final answers

(a) Solid wax: particles close together, regular arrangement, vibrating in fixed positions. Helium gas: particles far apart, random arrangement, moving quickly and randomly.
(b) Melting
(c) Freezing
(d)(i) Sublimation
(d)(ii) Particle separation increases from close together to far apart; particle motion changes from vibrating in fixed positions to moving quickly and randomly.