Kinetic Particle Model of Matter: Question 7
Syllabus 2.1.1
A small block of solid wax is heated slowly in a test tube. It first melts to form a liquid, and heating is then continued until the liquid wax boils to form a gas.
(a) Describe how the arrangement of the wax particles changes as the solid wax melts. [2]
(b) Describe how the motion of the wax particles changes as the liquid wax is heated further, up to the point where it boils. [2]
(c) State how the average spacing between the wax particles changes when the liquid boils to form a gas. [1]
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Worked solution
Part (a): Change of arrangement during melting
Before melting, the wax particles are packed closely together in a regular, ordered arrangement, held in fixed positions by strong forces of attraction, and can only vibrate about those positions.
As heating continues, the particles absorb energy and vibrate more and more. At the melting point, they have gained enough energy to partly overcome the forces holding them in place. The rigid, regular arrangement collapses into a disordered one in which particles are still close together (touching their neighbours) but are now free to move around and slide past each other. This is the liquid.
Part (b): Change of motion up to boiling
Once the wax is liquid, continued heating keeps transferring energy to the particles, so their average kinetic energy increases and they move faster. Faster-moving particles collide with one another, and with the walls of the test tube, more frequently and more energetically.
As the temperature approaches the boiling point, more and more particles gain enough energy to completely break free of the (now much weaker) attractive forces holding the liquid together. At boiling point, particles throughout the liquid have enough energy to escape as a gas.
Part (c): Spacing change on boiling
In the liquid, particles are still close together, essentially touching their neighbours. Once the liquid boils and becomes a gas, the particles move apart to occupy a much larger volume, so the average spacing between them increases enormously. Gas particles are, on average, many times further apart than liquid particles.
Final answers
- (a) The regular, fixed arrangement of vibrating particles in the solid breaks down; particles gain enough energy to move around each other while staying close together, forming a liquid.
- (b) Particles gain kinetic energy and move faster, colliding more often and more energetically, until enough of them can overcome the remaining attractive forces and escape as a gas at boiling point.
- (c) The average particle spacing increases enormously on boiling (liquid particles close together → gas particles far apart).