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GCSE level Physics exam revision notes on density
Density and particle theory: Part 5.7
Comparing the relative density of the liquid and
solid state and the curious case of water and its biological significance and graphs of density versus temperature
explained
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INDEX physics notes: Density,
particle models, factors affecting density
5.7 The relative density of the liquid and
solid state
and the curious case of water
- the anomalous density of solid water (ice)
Before discussing the structure of
ice we need to look at the anomalous density behaviour of water.
Left graph -
density versus temperature
for a typical liquid:
(1)
Increase in temperature of solid, increase in thermal vibration,
molecules move increasing a little more apart, density falls.
(2)
Melting occurs when intermolecular forces weakened and increased freedom of movement moves the molecules
a little bit apart decreasing the density.
(3)
Increasing the temperature increases the KE of the liquid molecules,
more energetic collisions, increasing with increase in temperature,
steadily lowers the density of the liquid as the molecules bash each
other a bit further apart as the intermolecular forces weaken more.
All of this is normal
expected behaviour.
Right graph -density
versus temperature for water:
You also need to
refer to the diagrams of ice structure below, as well as the graph above.
GCSE physics students do NOT have to
know the details of hydrogen bonding mentioned on the ice diagrams.
BUT know its the strongest type of
intermolecular force, which you do have to know about in chemistry.
Its the same intermolecular forces
that holds the double helix together in DNA and RNA, and partly
responsible for holding together the specific 3D protein shape of
enzymes - you know more about hydrogen bonding than you think!
(1)
Increase in temperature of solid, increase in thermal vibration,
molecules move increasing a little more apart, density of ice falls
- normal behaviour.
(2)
Ice melts, but instead of an expected decrease in density, you get an increase
in density of liquid water compared to ice - anomalous behaviour.
This is due to the partial breakdown of the open crystal
structure of ice and the liquid molecules, despite their greater KE
of movement,
they actually have the freedom to get closer together (about 10%
less volume) and so the density increases - you need to use a little imagination
when looking at the ice diagrams - imagine when the open crystal
structure breaks down, the water molecules can actually get closer
together.
(3a-b)
From 0oC to 100oC there is a continuous
breakdown of the 'ice-like' structures in liquid water.
YES! the ice structure does not completely break down at 0oC
on melting.
Clumps of water molecules persist and gradually get
broken down with increase in temperature - increase in KE of
molecules.
At the same time normal thermal expansion is going on!
From 0oC to 4oC the effect of ice structure
breakdown outweighs the normal thermal expansion, so you get a 2nd anomaly of the maximum density at 4oC
(just over 1.0 g/cm3 or 1000 kg/m3).
(3b) The maximum density at 4oC
is because the breakdown of ice-like structures in liquid water is
exactly balanced by the effect of normal thermal expansion.
(3c)
From 4oC the increasing KE of the molecules and more
energetic collisions outweighs the break down of the
clumps of water molecules and normal thermal expansion takes place.
ice diagram (i) |
ice diagram (ii) |
An important 'biological'
consequence of
the
curious density behaviour of water
The anomalous density behaviour
of ice has really important implications for aquatic life.
Because ice forms and floats on
the surface of water, life can go on as normal in the liquid water below the ice.
The ice actually provides some
insulation from the cold atmosphere, and in deeper ponds, rivers and
lakes, most aquatic life can go on as normal.
Burst pipes!
Because water expands
on freezing, water carrying pipes that are not insulated with some
kind of lagging (plastic or cloth) may burst when the ambient
temperature drops below 0oC and the liquid water turns to
solid ice - less dense with a larger volume, quite a pressure is
created.
Key points Density and particle theory: The anomalous
density of liquid water
Information
sources for Doc Brown's key points: IGCSE-GCSE physics are based on
textbooks and syllabus-specifications for students taking the UK AQA, Edexcel,
OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE
physics 9-1 level science examinations
A structured set of summary
revision notes on density and the anomalous behaviour of
ice, including its domestic and biological consequences,
tailored to the core requirements of WJEC, CCEA, CIE, AQA, Edexcel,
and OCR GCSE/IGCSE Physics specifications.
Density and Anomalous Behaviour of Ice
What Is Density?
- Density
= Mass ÷ Volume, ρ = m / V
where:
- ρ = density (kg/m³ or g/cm³)
- m = mass (kg or g)
- V = volume (m³ or cm³)
Why Is Ice Anomalous?
- Most substances become denser
when cooled.
- Water behaves unusually:
- As it cools below 4°C,
it expands instead of contracting.
- Ice has a lower density
than liquid water due to its open hexagonal structure
formed by hydrogen bonds.
- This structure creates more
space between molecules, increasing volume and decreasing
density.
Domestic Consequences about the anomalous
density of ice compared to other substances
| Example |
Impact |
| Frozen pipes |
Water expands when freezing, causing
pipes to burst. |
| Ice cubes float |
Useful in drinks; shows ice is less
dense than water. |
| Fridge/freezer design |
Must accommodate expansion of water
when frozen. |
Biological Consequences of the anomalous
density of ice compared to other substances
| Example |
Impact |
| Ice floats on lakes |
Forms an insulating layer, protecting
aquatic life in winter. |
| Seasonal mixing |
Melting ice contributes to nutrient
cycling in lakes. |
| Survival of ecosystems |
Prevents complete freezing of water
bodies, allowing life to persist. |
Typical Exam Board Requirements about the
anomalous density of ice compared to other substances
|
Key Focus Areas |
| Particle model, density,
floating/sinking, ice behaviour. |
| Density calculations, anomalous
expansion of water. |
| Particle spacing and density; water’s
unusual behaviour. |
| Density and changes of state;
biological relevance. |
| Density, floating ice, and
environmental consequences. |
| Density differences, ice structure,
and real-world effects. |
Common Misconceptions about the anomalous
density of ice compared to other substances
- “Ice is heavier than water”
– Ice is less dense, so it floats.
- “All substances contract when
cooled” – Water
expands below 4°C.
- “Density only affects solids”
– Density applies to all states of matter.
Student Tips about the anomalous density of
ice compared to other substances
- Draw particle diagrams
showing ice’s open structure versus liquid water.
- Use real-world examples
to explain floating and insulation.
- Practice density
calculations and unit conversions.
- Link to energy stores:
cooling reduces kinetic energy, affecting particle arrangement.
- Revise required practicals:
measuring density and observing floating/sinking.
Keywords, phrases and learning objectives for density
Comparing relative density of liquid and solid states.
Know the about the anomalous density of
ice.
Be able to explain the graphs of density versus
temperature for all substances apart from water.
Be able to interpret the graph of density versus
temperature for water.
Know about the biological
significance of the curious case of water
graphs of density versus
temperature explained
WHAT NEXT?
TOP of page
INDEX of physics notes on density and
particle models
Revision notes on graph explaining the
change in density of water from solid to liquid compared to other
substance based on the syllabus-specifications
for students taking IGCSE/GCSE level physics examinations, summary
revision notes and key points on graph explaining the change in density
of water from solid to liquid compared to other substance for students taking the AQA
igcse/gcse physics notes on graph explaining the change in density of water from
solid to liquid compared to other substance, Edexcel gcse
physics notes on graph explaining the change in density of water
from solid to liquid compared to other substance, OCR 21st century GCSE
physics notes on graph explaining the change in density of water
from solid to liquid compared to other substance, OCR gateway
GCSE physics notes on graph explaining the change in density of
water from solid to liquid compared to other substance, WJEC gcse physics notes on
graph explaining the change in density of water from solid to liquid
compared to other substance, CCEA
gcse physics notes on graph explaining the change in density of water
from solid to liquid compared to other substance for students taking CIE Cambridge igcse
physics, exam revision notes on
graph explaining the change in density of water from solid to liquid
compared to other substance, useful for US grade 9-10 physics courses,
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