Solids, Liquids and Gases
5 marks · NormalA student observes the boiling of water and describes the random motion of bubbles inside it.
Explain how the random motion of water molecules results in boiling and formation of bubbles.
Describe, in terms of pressure, what happens to the bubbles as they rise towards the surface.
State the property of pressure in liquids that explains why bubbles are spherical.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) As water is heated, molecules move randomly and more rapidly. Some have enough energy to form vapour, creating bubbles.
(b) Pressure outside the bubble decreases as it rises. The gas inside expands, causing the bubble to get bigger as it approaches the surface.
(c) Pressure acts equally in all directions in a liquid at rest.
Common mistakes: Omitting mention of equal pressure in all directions
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Mentions random/rapid motion of molecules causes vapour formation
A1 (a) · 1 mark · Links formation of bubbles to molecules breaking away to form gas
M2 (b) · 1 mark · Describes decrease in external pressure as bubbles rise
A2 (b) · 1 mark · Explains bubble expands due to lower external pressure
A3 (c) · 1 mark · States pressure in liquids acts equally in all directions
Worked solution
a) At boiling, water molecules have enough energy (random, rapid motion) to overcome attractions and form bubbles of vapour inside the liquid.
b) As the bubble rises, water pressure decreases, so the vapour inside expands, making the bubble larger.
c) Pressure acts equally in all directions in a liquid at rest.