Density
4 marks · HardExplain, using ideas about mass and volume, why the density of liquids generally changes less with temperature than the density of gases.
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Must show: Liquids do not expand as much as gases when heated., The change in mass is negligible because the substance stays the same., Gases expand a lot on heating so their volume increases greatly and density decreases more., Liquids change volume much less with temperature, so their density changes only slightly.
Also accept: In gases, heating causes particles to move further apart, so density drops more.
Common mistakes: Confusing mass loss with density change.; Not referring to the difference in volume change.
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Marking points
B1 (a) · 1 mark · States that mass remains (almost) the same for both liquids and gases.
M1 (a) · 1 mark · Explains that volume increases when heated.
M2 (a) · 1 mark · Describes that gases expand much more than liquids with temperature.
A1 (a) · 1 mark · States that therefore, density of gases changes more than liquids because density depends on mass and volume.
Worked solution
Density is calculated as mass divided by volume. When a substance is heated, its mass stays almost the same, but gases expand much more than liquids—their particles move further apart, so their volume increases more. This means the density of gases decreases more when heated. Liquids expand only a little when heated, so their density changes only slightly.