A Bridge Built With No Gaps Would Buckle in Summer
Learn how solids, liquids and gases expand by very different amounts, why engineers deliberately leave gaps and join two metals together, how density falls as temperature rises, and why water behaves backwards below four degrees.
Why do engineers leave gaps in a structure they want to be strong?
Because a solid that cannot expand will push, and the push is enormous.
A steel girder warming in the summer sun tries to grow longer. Give it room and it grows harmlessly. Bolt both ends rigidly in place and it cannot — so instead it bends sideways, cracks its supports, or lifts the roadway off its bearings.
So the gap is not sloppy workmanship. It is a designed allowance for something that will certainly happen. This page covers the second part of the ICSE Class 8 Physics chapter on heat transfer: how much each state expands, what engineers do about it, how density changes with temperature, and the one liquid that expands the wrong way.
A steel girder warming in the summer sun tries to grow longer. Give it room and it grows harmlessly. Bolt both ends rigidly in place and it cannot — so instead it bends sideways, cracks its supports, or lifts the roadway off its bearings.
So the gap is not sloppy workmanship. It is a designed allowance for something that will certainly happen. This page covers the second part of the ICSE Class 8 Physics chapter on heat transfer: how much each state expands, what engineers do about it, how density changes with temperature, and the one liquid that expands the wrong way.
Which expands most for the same rise in temperature?
Gases expand most, liquids less, and solids least — and the gaps between them are large, not marginal.
The reason is the strength of the intermolecular force.
- In a solid the force is strongest. Molecules only vibrate about fixed positions, and heating widens those vibrations slightly. The expansion is real but very small.
- In a liquid the force is weaker and the molecules already slide past one another, so they separate more easily. A liquid expands noticeably more than the solid form of the same substance.
- In a gas the force is negligible. The molecules are already far apart and fly freely, so extra energy pushes them much further out. A gas expands enormously — and this is why a gas has no fixed volume at all.
Everyday demonstrations of each:
- Solid. A tight metal bottle cap loosens after being held under hot water, because the metal cap expands more than the glass neck.
- Liquid. Mercury or coloured alcohol rising up the narrow tube of a thermometer — the whole instrument works on liquid expansion.
- Gas. A dented table-tennis ball popping back out in hot water, as the air inside expands; and a closed packet of snacks swelling in a warm vehicle.
The boundary case in a thermometer. When a mercury thermometer is put into hot water the reading briefly dips before climbing. The glass bulb receives the heat first and expands, enlarging the space available, so the mercury level falls for a moment. Only when the heat reaches the mercury — which then expands far more than the glass did — does the level rise.
That momentary dip is direct evidence that both the liquid and its container expand, and that the liquid expands more.
The reason is the strength of the intermolecular force.
- In a solid the force is strongest. Molecules only vibrate about fixed positions, and heating widens those vibrations slightly. The expansion is real but very small.
- In a liquid the force is weaker and the molecules already slide past one another, so they separate more easily. A liquid expands noticeably more than the solid form of the same substance.
- In a gas the force is negligible. The molecules are already far apart and fly freely, so extra energy pushes them much further out. A gas expands enormously — and this is why a gas has no fixed volume at all.
Everyday demonstrations of each:
- Solid. A tight metal bottle cap loosens after being held under hot water, because the metal cap expands more than the glass neck.
- Liquid. Mercury or coloured alcohol rising up the narrow tube of a thermometer — the whole instrument works on liquid expansion.
- Gas. A dented table-tennis ball popping back out in hot water, as the air inside expands; and a closed packet of snacks swelling in a warm vehicle.
The boundary case in a thermometer. When a mercury thermometer is put into hot water the reading briefly dips before climbing. The glass bulb receives the heat first and expands, enlarging the space available, so the mercury level falls for a moment. Only when the heat reaches the mercury — which then expands far more than the glass did — does the level rise.
That momentary dip is direct evidence that both the liquid and its container expand, and that the liquid expands more.
How is thermal expansion allowed for, and where is it useful?
Engineers either leave room for expansion or put it to work.
Leaving room — problems solved by design:
- Railway tracks are laid with small gaps between successive rails, so summer heat lengthens each rail into its gap instead of bending the track. Modern long-welded track solves the same problem by clamping the rails under tension rather than by leaving gaps.
- Bridges rest on expansion joints — those interlocking metal teeth in the road surface — and often on rollers at one end, so the deck can lengthen without straining the piers.
- Overhead wires between electric poles are strung slightly slack in summer. They contract and tighten in winter, and a wire strung taut in summer would snap when the cold came.
- Concrete roads and floors are cast in slabs with filled joints between them, for the same reason.
- Pipes carrying hot water are fitted with a loop or bend, which can flex as the pipe lengthens.
Putting it to work — expansion used deliberately:
- A bimetallic strip is two different metals, usually brass and iron, riveted together along their length. Brass expands more than iron for the same temperature rise, so on heating the brass side becomes longer and the strip bends towards the iron side. Cooling bends it the other way. The strip is therefore a device that converts a temperature change into a movement.
This is used in a thermostat for an iron, a heater or a refrigerator: the bending strip breaks an electrical contact when the set temperature is reached and remakes it when things cool. It is also the working part of a fire alarm and of many flashing indicator circuits.
- Fitting an iron rim to a wooden cartwheel. The rim is heated so it expands, slipped over the wheel, then cooled — it contracts and grips immovably. The same trick fits steel tyres to railway wheels.
- Hot rivets in steelwork are hammered in while hot and grip tightly as they cool and contract.
And one everyday hazard. Pour boiling water into a thick glass tumbler and it may crack. The inner surface heats and expands while the outer surface is still cold and has not, and the resulting strain splits the glass. Thin glass heats through quickly and is safer — which is why laboratory glassware is thin.
Leaving room — problems solved by design:
- Railway tracks are laid with small gaps between successive rails, so summer heat lengthens each rail into its gap instead of bending the track. Modern long-welded track solves the same problem by clamping the rails under tension rather than by leaving gaps.
- Bridges rest on expansion joints — those interlocking metal teeth in the road surface — and often on rollers at one end, so the deck can lengthen without straining the piers.
- Overhead wires between electric poles are strung slightly slack in summer. They contract and tighten in winter, and a wire strung taut in summer would snap when the cold came.
- Concrete roads and floors are cast in slabs with filled joints between them, for the same reason.
- Pipes carrying hot water are fitted with a loop or bend, which can flex as the pipe lengthens.
Putting it to work — expansion used deliberately:
- A bimetallic strip is two different metals, usually brass and iron, riveted together along their length. Brass expands more than iron for the same temperature rise, so on heating the brass side becomes longer and the strip bends towards the iron side. Cooling bends it the other way. The strip is therefore a device that converts a temperature change into a movement.
This is used in a thermostat for an iron, a heater or a refrigerator: the bending strip breaks an electrical contact when the set temperature is reached and remakes it when things cool. It is also the working part of a fire alarm and of many flashing indicator circuits.
- Fitting an iron rim to a wooden cartwheel. The rim is heated so it expands, slipped over the wheel, then cooled — it contracts and grips immovably. The same trick fits steel tyres to railway wheels.
- Hot rivets in steelwork are hammered in while hot and grip tightly as they cool and contract.
And one everyday hazard. Pour boiling water into a thick glass tumbler and it may crack. The inner surface heats and expands while the outer surface is still cold and has not, and the resulting strain splits the glass. Thin glass heats through quickly and is safer — which is why laboratory glassware is thin.
Formula
How does density change when a substance is heated?
Density falls as temperature rises, because
and heating increases while leaving unchanged. Nothing is added or removed — the same matter simply occupies more room.
Worked example. Water has a density of about at and about at . Take of it and find the volume at each temperature:
So the same kilogram swells by about per litre — roughly four per cent — while its density drops by
which is a fall of about .
Why this matters more than it sounds. A less dense fluid rises through a denser one, exactly as the law of floatation predicts. So heating part of a fluid makes that part rise and cooler fluid sink to take its place.
That single consequence drives:
- Hot air rising above a flame or a hot road
- Smoke going up a chimney
- A hot-air balloon lifting, because the heated air inside is less dense than the air outside
- Land and sea breezes, and the whole business of convection in the next part of this chapter
A useful boundary check. Cooling reverses everything: volume shrinks, density rises, and the cooled portion sinks. Which makes water's behaviour in the next section genuinely strange.
and heating increases while leaving unchanged. Nothing is added or removed — the same matter simply occupies more room.
Worked example. Water has a density of about at and about at . Take of it and find the volume at each temperature:
So the same kilogram swells by about per litre — roughly four per cent — while its density drops by
which is a fall of about .
Why this matters more than it sounds. A less dense fluid rises through a denser one, exactly as the law of floatation predicts. So heating part of a fluid makes that part rise and cooler fluid sink to take its place.
That single consequence drives:
- Hot air rising above a flame or a hot road
- Smoke going up a chimney
- A hot-air balloon lifting, because the heated air inside is less dense than the air outside
- Land and sea breezes, and the whole business of convection in the next part of this chapter
A useful boundary check. Cooling reverses everything: volume shrinks, density rises, and the cooled portion sinks. Which makes water's behaviour in the next section genuinely strange.
Why does water expand when it is cooled below four degrees?
Water is the exception to everything above. Between and it contracts on heating and expands on cooling — the reverse of normal behaviour. This is the anomalous expansion of water.
Tracing it carefully. Start with water at and warm it:
- From to it contracts, so its density rises.
- At its volume is least and its density is greatest, about .
- Above it behaves normally: it expands and its density falls.
So water is at its densest at — not at its freezing point, as every other common liquid is.
What this does to a pond in winter. As the air cools, the surface water cools, grows denser and sinks, with warmer water rising to take its place. This churning continues until the whole pond reaches .
Now the rule flips. Cooling the surface below makes it less dense, so it stops sinking and stays on top. It cools further in place, reaches and freezes into a sheet of ice at the surface.
Beneath that sheet the water is still at about — the densest water, sitting at the bottom where it sank. Ice is a poor conductor of heat, so the sheet insulates what is below it, and the deep water stays liquid all winter.
Fish and other aquatic life survive there. Had water behaved normally, the coldest water would have kept sinking and the pond would have frozen solid from the bottom up.
Two separate oddities, often confused. Water expanding by about nine per cent when it actually freezes is why ice floats. The anomaly described here is different and is about the liquid between and — it is what decides where in the pond the coldest water sits, and therefore that the freezing starts at the top.
Tracing it carefully. Start with water at and warm it:
- From to it contracts, so its density rises.
- At its volume is least and its density is greatest, about .
- Above it behaves normally: it expands and its density falls.
So water is at its densest at — not at its freezing point, as every other common liquid is.
What this does to a pond in winter. As the air cools, the surface water cools, grows denser and sinks, with warmer water rising to take its place. This churning continues until the whole pond reaches .
Now the rule flips. Cooling the surface below makes it less dense, so it stops sinking and stays on top. It cools further in place, reaches and freezes into a sheet of ice at the surface.
Beneath that sheet the water is still at about — the densest water, sitting at the bottom where it sank. Ice is a poor conductor of heat, so the sheet insulates what is below it, and the deep water stays liquid all winter.
Fish and other aquatic life survive there. Had water behaved normally, the coldest water would have kept sinking and the pond would have frozen solid from the bottom up.
Two separate oddities, often confused. Water expanding by about nine per cent when it actually freezes is why ice floats. The anomaly described here is different and is about the liquid between and — it is what decides where in the pond the coldest water sits, and therefore that the freezing starts at the top.
Exam tip
Exam tip: say which way the bimetallic strip bends and why
For a bimetallic strip, name the metal that expands more and state that the strip bends towards the metal that expands less. Brass expands more than iron, so on heating it bends towards the iron side. Saying merely it bends earns nothing.
Give the order of expansion as gases > liquids > solids, and justify it with the intermolecular force — weakest in gases, strongest in solids.
When a question asks for a problem of expansion, name the structure and the remedy: railway tracks with gaps, bridges with expansion joints, wires strung slack.
For density, quote and say the mass is unchanged while the volume grows. That is the whole reason the density falls.
Be precise about water: it is **densest at , and it behaves anomalously only between and **. Above it is entirely normal.
And for the frozen-pond question, give the full chain: surface water cools and sinks until the pond is at , then cooler surface water stops sinking, freezes on top, and the ice insulates the water below.
Carry units on every density line — , not a bare number.
Give the order of expansion as gases > liquids > solids, and justify it with the intermolecular force — weakest in gases, strongest in solids.
When a question asks for a problem of expansion, name the structure and the remedy: railway tracks with gaps, bridges with expansion joints, wires strung slack.
For density, quote and say the mass is unchanged while the volume grows. That is the whole reason the density falls.
Be precise about water: it is **densest at , and it behaves anomalously only between and **. Above it is entirely normal.
And for the frozen-pond question, give the full chain: surface water cools and sinks until the pond is at , then cooler surface water stops sinking, freezes on top, and the ice insulates the water below.
Carry units on every density line — , not a bare number.
Did you know
Why does a household iron switch itself off and on again?
Inside the iron sits a bimetallic strip forming part of the electrical circuit.
When the iron is cold the strip is straight and the contact is closed, so current flows and the element heats. As the temperature climbs, the brass side lengthens more than the iron side and the strip curls — until it curls far enough to pull the contact apart. The current stops.
With no current the iron cools, the strip straightens, the contact remakes, and heating resumes. The cycle repeats indefinitely, which is why the indicator lamp on an iron or a room heater blinks slowly on and off rather than staying lit.
What makes this elegant is that nothing measures the temperature and nothing decides anything. A strip of two metals riveted together simply cannot help bending when it warms, and that involuntary bending is doing the whole job of a thermostat.
When the iron is cold the strip is straight and the contact is closed, so current flows and the element heats. As the temperature climbs, the brass side lengthens more than the iron side and the strip curls — until it curls far enough to pull the contact apart. The current stops.
With no current the iron cools, the strip straightens, the contact remakes, and heating resumes. The cycle repeats indefinitely, which is why the indicator lamp on an iron or a room heater blinks slowly on and off rather than staying lit.
What makes this elegant is that nothing measures the temperature and nothing decides anything. A strip of two metals riveted together simply cannot help bending when it warms, and that involuntary bending is doing the whole job of a thermostat.
Key takeaways
Thermal expansion and the water anomaly: quick revision
- Substances expand on heating and contract on cooling, because molecules vibrate or move further apart.
- Order of expansion for the same temperature rise: gases > liquids > solids, following the strength of the intermolecular force.
- Evidence: a metal cap loosening in hot water (solid), a thermometer's liquid rising (liquid), a dented ball popping out in hot water (gas).
- A mercury thermometer's level dips first in hot water, because the glass expands before the mercury does.
- Allowed for by gaps in railway tracks, expansion joints and rollers on bridges, slack overhead wires, jointed concrete slabs and looped hot-water pipes.
- Used deliberately in a bimetallic strip — brass expands more than iron, so heating bends it towards the iron — driving thermostats, fire alarms and blinking indicators; also in fitting iron rims and hot rivets.
- Thick glass cracks with boiling water because the inside expands while the outside has not.
- Density falls on heating since with fixed: of water fills at and at , as the density drops from to .
- Less dense fluid rises — hot air, smoke up a chimney, hot-air balloons, and all convection.
- Anomalous expansion of water: between and it contracts on heating and expands on cooling, so it is **densest at **.
- A pond therefore churns until it is all at , then freezes from the top, and the insulating ice leaves liquid water below for aquatic life.
Try a few questions on this now — being able to state which side a bimetallic strip bends towards, and why, is worth more marks in this chapter than any other single fact.
- Order of expansion for the same temperature rise: gases > liquids > solids, following the strength of the intermolecular force.
- Evidence: a metal cap loosening in hot water (solid), a thermometer's liquid rising (liquid), a dented ball popping out in hot water (gas).
- A mercury thermometer's level dips first in hot water, because the glass expands before the mercury does.
- Allowed for by gaps in railway tracks, expansion joints and rollers on bridges, slack overhead wires, jointed concrete slabs and looped hot-water pipes.
- Used deliberately in a bimetallic strip — brass expands more than iron, so heating bends it towards the iron — driving thermostats, fire alarms and blinking indicators; also in fitting iron rims and hot rivets.
- Thick glass cracks with boiling water because the inside expands while the outside has not.
- Density falls on heating since with fixed: of water fills at and at , as the density drops from to .
- Less dense fluid rises — hot air, smoke up a chimney, hot-air balloons, and all convection.
- Anomalous expansion of water: between and it contracts on heating and expands on cooling, so it is **densest at **.
- A pond therefore churns until it is all at , then freezes from the top, and the insulating ice leaves liquid water below for aquatic life.
Try a few questions on this now — being able to state which side a bimetallic strip bends towards, and why, is worth more marks in this chapter than any other single fact.