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2.1.1 States of matter

A state of matter can be distinguished by whether it keeps its own shape, keeps its own volume and can be compressed easily.

State	Shape	Volume	Compression and flow
solid	fixed shape	fixed volume	difficult to compress; does not flow
liquid	takes the shape of the part of its container it occupies	fixed volume	difficult to compress; flows
gas	no fixed shape; spreads to fill its container	no fixed volume; expands to fill the container	easily compressed; flows

If a sample takes its container's shape, it could be a liquid or a gas. Decide between them by asking whether it keeps a fixed volume: a liquid does, while a gas fills all available space.

Shape alone is not enough to distinguish a liquid from a gas. This card describes observable bulk properties only; particle arrangement, separation and motion belong to the next Topic.

A change of state changes whether a substance is solid, liquid or gas. Name the process from its starting state and finishing state.

From	To	Name	Energy direction
solid	liquid	melting	energy is supplied
liquid	solid	freezing or solidification	energy is removed
liquid	gas	boiling or evaporation	energy is supplied
gas	liquid	condensation	energy is removed

Learn the reverse pairs: melting ↔ freezing/solidification, and boiling/evaporation ↔ condensation. Water droplets forming outside a cold glass are produced by condensation of water vapour in the surrounding air; they do not leak through the glass.

Do not swap the direction: melting is solid to liquid, not liquid to solid; condensation is gas to liquid, not liquid to gas. Direct solid-to-gas and gas-to-solid changes are explicitly not required by this syllabus objective.

2.1.2 Particle model

A simple particle model represents matter as tiny particles. To identify a state, compare three features: arrangement, average separation and motion.

State	Arrangement and separation	Motion
solid	closely packed in an ordered pattern	vibrate about fixed positions
liquid	closely packed but irregular, with no fixed pattern	move around and slide past one another
gas	widely separated and randomly arranged	move rapidly and randomly in all directions

In a simple diagram, use equal-sized circles. Draw a solid as close ordered rows, a liquid as close irregular circles, and a gas as circles spread far apart at random. The spaces are part of the model: a gas is easy to compress because its particles are far apart, whereas solids and liquids have little empty space between particles.

Do not draw larger particles when a substance changes state. The particle size and identity stay the same; it is their arrangement, separation and motion that change. The circles are schematic, so their exact number and drawn size are not measurements.

A higher temperature means that particles have a greater average kinetic energy and move faster on average; a lower temperature means less average kinetic energy and slower motion.

Heating transfers energy to the particles, so their average speed increases. Cooling removes energy, so their average speed decreases. In a sealed rigid container, heating changes speed but not the average separation, because the volume and number of particles stay fixed.

Absolute zero is the lowest possible temperature: −273 °C. At this temperature particles have their least kinetic energy. This is the syllabus statement to use when describing the lower limit of temperature.

Absolute zero is not 0 °C, and ‘least kinetic energy’ must not be replaced by ‘zero gravitational potential energy’. Do not claim that every particle has exactly the same speed: temperature describes average particle motion.

Gas pressure is produced because rapidly moving particles repeatedly collide with the surfaces of their container.

Each particle changes direction when it hits a surface. Across an enormous number of collisions, the surface experiences a continuous push. If a fixed amount of gas is heated in a rigid container, the particles move faster, hit the walls more often and make stronger impacts, so the pressure rises. Cooling reverses these changes and lowers the pressure.

The pressure does not rise because each particle expands or because new particles appear. Compare pressure changes only after stating what is held fixed; detailed force-per-area and momentum reasoning is developed in the Supplement card later in this Topic.

Microscopic particles suspended in a still liquid or gas can be seen moving continually in irregular, random directions. This observed motion is evidence for the kinetic particle model of matter.

Stage	What it says
observation	a visible speck follows a jagged path with sudden changes of direction and unequal step lengths
evidence	the motion continues even though the suspension as a whole is still
conclusion	matter contains particles that are in continual random motion

Record what is observed before explaining it: the path is random and jerky, not a smooth curve or motion in one common direction. The visible speck is a microscopic suspended particle; it is not an individual molecule of the liquid or gas.

Brownian motion is the continual random motion of microscopic particles in a suspension, caused by collisions with the much smaller particles of the surrounding liquid or gas.

The liquid or gas particles move rapidly and randomly. They strike every side of a suspended smoke particle or pollen grain, but the impacts are not perfectly balanced at each instant. The resulting unbalanced force changes the microscopic particle’s speed or direction. A new unequal set of impacts then changes its motion again, producing the observed irregular path.

The bright light makes the suspended particles visible; it does not drive their motion. Convection currents, collisions between the suspended particles, and the suspended particle’s own atoms are not the Brownian-motion mechanism described here.

The properties of a solid, liquid or gas result from the combined effects of particle separation, forces between particles and particle motion.

State	Separation and forces	Motion	Resulting properties
solid	particles are close; attractive forces are strong	particles vibrate about fixed positions	fixed shape and volume; difficult to compress
liquid	particles are close; forces keep them together but not in fixed positions	particles move and slide past one another	fixed volume but no fixed shape; flows; difficult to compress
gas	particles are far apart; forces are negligible except during collisions	particles move rapidly and randomly	fills its container; no fixed volume; easy to compress

‘Particle’ is a general model word. Depending on the substance, the relevant particles may be atoms, molecules, ions or electrons. Do not assume that every material is made of molecules.

No single feature explains every property. Close spacing helps explain low compressibility, while forces and freedom of motion distinguish a rigid solid from a flowing liquid. Strong forces alone do not mean that particles are motionless.

When a gas particle strikes and rebounds from a surface, its momentum changes. That change requires a force, and the particle exerts an equal force on the surface.

A huge number of impacts produces a total force on the wall. More collisions each second or a greater momentum change in each collision increases that total force. Gas pressure is the total normal force from these impacts divided by the area over which it acts.

$p = \frac{F}{A}$

Here $p$ is pressure, $F$ is the total force perpendicular to the surface, and $A$ is the surface area. For the same area, a larger collision force gives a larger pressure; for the same force, spreading it over a larger area gives a smaller pressure.

Pressure is not the same quantity as force. A correct particle explanation links collision and rebound → change of momentum → force on the surface → force per unit area.

In a Brownian-motion experiment, the visible microscopic particle and the invisible atoms or molecules of the surrounding fluid are different objects.

Object	Relative scale and motion	Correct description
atom or molecule of the liquid or gas	much lighter, fast-moving and not seen individually in this experiment	collides with the microscopic particle and may rebound
microscopic smoke particle or pollen grain	much larger and heavier; visible as a speck through the microscope	is moved by the changing, unequal total effect of many molecular collisions

For example, a fast nitrogen molecule can strike a stationary smoke particle: the smoke particle moves and the nitrogen molecule rebounds. Many such impacts from changing directions create the smoke particle’s irregular motion.

Say ‘smoke particle’ or ‘pollen grain’, not ‘smoke molecule’ or ‘pollen molecule’. Microscopic means small enough to require a microscope; it does not mean the same size as an atom or molecule.

2.1.3 Gases and the absolute scale of temperature

For a fixed mass of gas, pressure changes when particle speed changes or when the same particles collide with the walls more or less frequently.

Controlled change	Particle account	Pressure effect
temperature increases at constant volume	particles gain average kinetic energy, move faster, and make more frequent and harder collisions with the same walls	pressure increases
temperature decreases at constant volume	particles move more slowly, so collisions are less frequent and less forceful	pressure decreases
volume decreases at constant temperature	average speed stays the same, but particles cross a shorter distance and hit each unit area of wall more frequently	pressure increases
volume increases at constant temperature	average speed stays the same, but collisions with each unit area of wall become less frequent	pressure decreases

Name the control before predicting the pressure. Constant volume isolates the effect of temperature on particle speed. Constant temperature isolates the effect of volume on collision frequency; the particles do not speed up merely because the gas is compressed slowly.

These comparisons require the same fixed mass of gas. If gas leaks in or out, or if temperature and volume both change, one simple comparison is not enough to predict the result without further information.

The kelvin and Celsius scales have equal-sized intervals but different zero points: 0 K corresponds to −273 °C.

$T\,(\mathrm{K}) = \theta\,(^{\circ}\mathrm{C}) + 273$

To convert °C to K, add 273. To convert K to °C, rearrange to $\theta = T - 273$ .

Examples: $25\,^{\circ}\mathrm{C} = 25 + 273 = 298\,\mathrm{K}$ . For nitrogen at $77\,\mathrm{K}$ , $\theta = 77 - 273 = -196\,^{\circ}\mathrm{C}$ .

Write K, not °K: kelvin has no degree sign. A temperature can be negative on the Celsius scale while remaining positive in kelvin; for example, −196 °C is 77 K.

For a fixed mass of gas at constant temperature, pressure and volume are inversely proportional: increasing one decreases the other so that their product stays constant.

$pV = \text{constant} \qquad \text{so} \qquad p_1V_1=p_2V_2$

A gas at $120\,\mathrm{kPa}$ occupies $50\,\mathrm{cm^3}$ and is compressed at constant temperature to $30\,\mathrm{cm^3}$ . Then $p_2=(120\times50)/30=200\,\mathrm{kPa}$ . The pressure rises because the volume falls.

On a graph of $p$ against $V$ , the relationship is a decreasing curved line: doubling $V$ halves $p$ , and halving $V$ doubles $p$ . The curve becomes less steep as volume increases and does not meet either axis. A graph of $p$ against $1/V$ is a straight line through the origin.

Use one consistent volume unit on both sides and one consistent pressure unit on both sides; matching units cancel in the ratio. Do not use $pV=\text{constant}$ if the gas mass or temperature changes.

2.2.1 Thermal expansion of solids, liquids and gases

At constant pressure, most solids, liquids and gases expand when heated and contract when cooled: their dimensions or volume change with temperature.

State	When heated at constant pressure	When cooled
solid	length, area and volume increase slightly; any hole in the solid also becomes larger	length, area, volume and holes become smaller
liquid	volume increases, usually more than the containing solid	volume decreases
gas	volume increases substantially if its boundary can move	volume decreases

Expansion does not add matter: the mass stays constant. Because the same mass occupies a larger volume, density decreases on heating; contraction at constant mass increases density.

The particles themselves do not get larger. ‘Gas expands when heated’ here assumes constant pressure and space for the gas boundary to move. In a sealed rigid container the volume cannot expand, so heating raises the pressure instead.

Thermal expansion is useful when a changing size produces a measurement or movement, but unwanted expansion must be allowed for so structures do not bend, buckle or crack.

Situation	Expansion or contraction	Design response or useful effect
railway tracks and bridges	long sections expand in hot weather	leave expansion gaps or joints so the sections can lengthen without buckling
overhead cables	metal contracts in cold weather	install the cable with slack so contraction does not create a dangerously large tension
tight metal lid on a glass jar	for the same warming, the metal lid expands more than the glass neck	the lid opening becomes relatively larger, making the lid easier to remove
liquid-in-glass thermometer	the liquid expands more than the glass bulb	liquid rises along a narrow uniform capillary; a narrower tube gives a larger rise per degree
thermostat with a bimetallic strip	its two bonded metals expand by different amounts	the strip bends and can open or close an electrical contact

For each example, identify what changes temperature, which part expands or contracts, and what would happen without the design feature. A gap is useful because the material expands into it; the gap itself is not the expanding object.

Do not explain every heating effect as expansion: melting and simply becoming hotter are different processes. The application must depend on a change in dimensions or volume.

For the same temperature rise at constant pressure, the usual order of volume expansion is gas > liquid > solid.

State	Particle arrangement and forces	Effect of heating	Relative expansion
solid	particles are close and held strongly in fixed positions	vibrations become more energetic and average separation increases only slightly	least
liquid	particles are close but can move past one another; attractions are weaker than in a solid	faster motion produces a larger increase in average separation	intermediate
gas	particles are already far apart and attractions are negligible	faster particles spread much farther apart while the gas volume grows to keep pressure constant	greatest

Heating increases particle kinetic energy. The size change comes from an increase in average separation, not from particles swelling. Stronger constraints in a solid resist separation most; the weakly constrained gas changes volume most.

Compare equal temperature rises under the same constant-pressure condition. The statement is an order of magnitudes, not a claim that every solid, liquid or gas has one identical expansion value.

2.2.2 Specific heat capacity

When an object's temperature rises, its internal energy increases. Internal energy is energy stored by all the particles in the object; temperature is a measurement, not an energy store.

The statement describes a change in one object: compare it before and after heating. Energy transferred to the object raises its internal energy, and a temperature rise is evidence of that increase.

Temperature and internal energy are not the same quantity. A large amount of cooler material can have more internal energy than a small hot object, so temperature alone does not give an object's total internal energy.

An increase in temperature means an increase in the average kinetic energy of all the particles in the object.

State	What the greater average kinetic energy looks like
solid	particles vibrate more energetically about their fixed positions
liquid	particles move around one another faster on average
gas	particles travel faster on average between collisions

‘Average’ matters: particles do not all have identical kinetic energy or move at one identical speed. The distribution changes so that the mean kinetic energy of the whole collection is greater.

Do not say that heating makes particles themselves larger. The temperature change is explained by their motion and kinetic energy, not by a change in particle size.

Specific heat capacity, c, is the energy required per unit mass of a substance for a unit increase in temperature. Its unit is J/(kg °C), equivalently J kg⁻¹ °C⁻¹.

c=\frac{\Delta E}{m\Delta T}\qquad\text{and therefore}\qquad\Delta E=mc\Delta T

Here ΔE is the increase in internal energy in joules, m is mass in kilograms, and ΔT is the temperature rise in °C. A temperature interval has the same numerical size in °C and K.

Example: a 0.80 kg aluminium block receives 18 000 J and warms by 25 °C.

$c=\frac{18\,000}{0.80\times25}=900\text{ J/(kg °C)}$

The substitution uses the temperature change, not the final temperature.

A larger c means more energy is needed to produce the same temperature rise in the same mass. It does not mean the substance automatically heats faster; with the same energy input, a larger c gives a smaller temperature rise.

Measure the sample's mass m, the electrical energy supplied ΔE, and its temperature rise ΔT, then calculate $c=\Delta E/(m\Delta T)$ . If heater power P is known, $\Delta E=Pt$ ; otherwise use $\Delta E=VIt$ .

Feature	Solid block	Liquid
sample mass	measure the block on a balance	measure filled container minus empty container
heater and thermometer	place them in snug holes with good thermal contact	immerse both in the liquid; keep them clear of the container wall
limiting heat transfer	wrap the block in insulation	use an insulated container and lid
uniform temperature	allow the block to conduct energy through it	stir the liquid gently while heating

Measure m and the initial temperature.
Switch on the heater and start the timer together. Record P and t, or record V, I and t.
Record the final temperature and calculate $\Delta T=T_{final}-T_{initial}$ .
Calculate the supplied energy and then $c=\Delta E/(m\Delta T)$ .
Repeat and compare values; use a moderate temperature rise so the change is clear without making heat loss excessive.

Insulate the sample, use a lid for a liquid, ensure good heater contact in a solid, and stir a liquid. Some electrical energy warms the heater, thermometer and container or escapes to the surroundings. Treating all supplied energy as energy gained by the sample makes the calculated c too large.

2.2.3 Melting, boiling and evaporation

During melting and boiling, a substance absorbs energy while its temperature remains constant until the change of state is complete.

Change	Energy transfer	What changes while temperature is constant
melting: solid → liquid	energy enters the substance	particles become less strongly held and can move past one another
boiling: liquid → gas	energy enters the substance	particles overcome attractions and separate widely as gas

Temperature depends on average particle kinetic energy. During the change of state, the input increases particle separation and potential energy rather than average kinetic energy, so the temperature does not rise. After the change is complete, further energy input can raise the temperature again.

A flat section on a heating curve does not mean that no energy is entering. It means energy is changing the state instead of increasing the temperature. This course objective is qualitative; no specific-latent-heat calculation is required.

At standard atmospheric pressure, pure water melts and freezes at 0 °C, and boils and condenses at 100 °C.

Temperature	Processes in opposite directions	Coexisting states during the change
0 °C	melting ↔ freezing	ice and liquid water
100 °C	boiling ↔ condensation	liquid water and water vapour

These values assume standard atmospheric pressure and pure water. The numbers are phase-change temperatures, not the lowest and highest temperatures a thermometer can read.

Condensation changes a gas to a liquid; solidification changes a liquid to a solid. In both changes, the substance releases energy as attractions constrain the particles more strongly.

Change	Particle account
condensation: gas → liquid	particles come much closer together; attractions keep them close while they still move randomly past one another
solidification: liquid → solid	particles become held in fixed positions in an ordered, closely packed arrangement; they continue to vibrate

At the phase-change temperature, average kinetic energy and temperature remain constant while potential energy decreases and energy is transferred out. Particles do not disappear, shrink or stop moving.

Evaporation occurs when more-energetic particles at the surface of a liquid have enough energy to overcome attractions and escape into the gas above the liquid.

Liquid particles have a range of kinetic energies. At any moment, some surface particles are energetic enough to leave, even when the liquid is below its boiling point. Particles deeper in the liquid cannot escape directly through the surface.

Evaporation is not a stream of bubbles through the liquid. It is a surface process and can occur at many temperatures, not only at the boiling point.

Evaporation causes the temperature of the remaining liquid to decrease.

More-energetic particles are more likely to escape → they carry away more than the average kinetic energy → the particles left behind have a lower average kinetic energy → the liquid's temperature falls.

Cooling is about the average energy of the particles that remain. It is not because moving air directly slows every particle in the liquid.

Boiling and evaporation both change liquid to gas, but they occur in different ways.

Feature	Boiling	Evaporation
where	throughout the liquid	only at the surface
temperature	at the boiling point for the stated pressure	can occur at many temperatures
visible behaviour	vapour bubbles form within the liquid and rise	no vapour bubbles form throughout the liquid
effect during the process	with continued energy input, temperature stays constant while boiling	preferential escape often cools the remaining liquid

Steam-like mist above a liquid is not evidence that evaporation occurs throughout it. The key test is whether vapour bubbles form within the body of the liquid.

Evaporation is faster when the liquid is warmer, when its exposed surface area is larger, and when air moves more quickly over the surface.

Change	Effect on rate	Particle reason
raise temperature	increases	a larger fraction of particles has enough energy to escape
increase exposed surface area	increases	more particles are at the surface and able to escape at once
increase air movement	increases	vapour particles are carried away, so fewer return to the liquid

A wide dish evaporates faster than a narrow container holding the same volume at the same temperature. Wet clothes dry fastest when spread out in warm, moving air.

Moving air does not speed evaporation by adding kinetic energy directly to the liquid. Its main role is to remove vapour from above the surface.

An object in contact with an evaporating liquid cools because energy transfers from the object to the liquid and is carried away as particles escape.

Evaporation removes energetic liquid particles → the liquid needs energy to continue evaporating → energy transfers from the warmer object into the liquid → the object's internal energy and temperature decrease.

Sweat cools skin, a volatile liquid used on skin feels cold, and a wet cloth can cool a container. Faster evaporation usually increases the cooling rate because energy is removed from the object more quickly.

Do not stop at ‘the liquid cools’. To explain cooling of the object, state the energy-transfer direction: from the object into the evaporating liquid and then away with the vapour.

2.3.1 Conduction

A good thermal conductor transfers energy quickly through itself; a poor conductor, or thermal insulator, transfers energy slowly.

Method	Set-up and observation	Conclusion
wax-pin comparison	use rods of equal length and diameter; attach identical pins with equal wax blobs at the same distance from one end; heat the ends equally and record when each pin falls	the rod whose wax melts first is the better conductor
temperature comparison	place equal lengths of different rods equally deep in hot water; measure temperature at points the same distance from the water at equal times	the rod whose measured point warms faster is the better conductor

Keep rod dimensions, distance from the heated end, initial temperature, heating time or water temperature, wax amount and thermometer position the same. Repeat readings so the comparison depends on material rather than geometry or heating.

Feeling metal and plastic at the same room temperature shows how quickly they transfer energy to or from the hand, not that the metal began colder.

Thermal conduction in every solid occurs through lattice vibrations. Metals have an additional, faster route: mobile delocalised electrons.

Solid	How energy moves from hotter to cooler regions
all solids	particles at the hot end vibrate more energetically and transfer energy to neighbouring lattice particles, so the vibration spreads through the solid
metals	delocalised electrons gain energy in the hot region, move through the lattice and transfer energy in collisions with ions; lattice vibration also occurs

The atoms, molecules or ions remain around fixed lattice positions; they do not travel from the hot end to the cold end. Energy is passed through the solid down the temperature difference.

Positive ions do not move freely through a metal. Metals usually conduct better than non-metals because mobile electrons provide an extra energy-transfer route, not because metal atoms migrate along the object.

Gases and most liquids are poor thermal conductors because their particles are not held in a closely connected, fixed lattice.

Material	Particle arrangement	Consequence for conduction
non-metal solid	particles are close in a lattice	vibrations pass energy repeatedly to neighbouring particles
liquid	particles are close but not fixed in a lattice	collisions can transfer energy, but there is no continuous lattice-vibration pathway
gas	particles are far apart	collisions are much less frequent, so conduction is especially slow

Poor conduction does not mean no energy transfer. Liquids and gases can still transfer energy slowly by particle collisions, and bulk movement may also transfer energy by convection; this card is comparing conduction only.

Solids do not fall into only two absolute groups. Many solids conduct thermal energy better than thermal insulators but less well than good thermal conductors.

Position on the conduction scale	Behaviour for the same shape and temperature difference
good thermal conductor	transfers energy rapidly
intermediate solid	transfers energy at a moderate rate
thermal insulator	transfers energy slowly

‘Conductor’ and ‘insulator’ are comparative descriptions. Choose a material by comparing transfer rates under the same conditions; a solid called an insulator still conducts some energy.

Do not infer that every non-metal is equally insulating or that every metal has one identical conductivity. Material, thickness, area and temperature difference all matter in a real comparison.

2.3.2 Convection

Convection is thermal energy transfer by the bulk movement of a fluid. It is an important transfer method in liquids and gases because their particles can move from place to place.

Medium	Can convection occur?	Reason
liquid	yes	the liquid can flow and carry internal energy
gas	yes	the gas can flow and carry internal energy
solid	no	its particles do not move through the solid as a bulk flow
vacuum	no	there is no matter to move

Convection circulates water heated in a pan and air heated in a room. The fluid itself moves, carrying energy from one region to another.

Do not say ‘heat rises’. Warmer, less-dense fluid may rise, while cooler, denser fluid moves down to replace it; thermal energy is carried with the moving fluid.

A convection current is a continuous circulation caused by density differences within a liquid or gas.

Step	What happens
1	a region of fluid is heated, expands and becomes less dense
2	the warmer, less-dense fluid rises
3	cooler, denser fluid moves down to replace it
4	continued heating and cooling maintain a circulating current that transfers energy

Fluid	Demonstration	Observation and conclusion
liquid	place a small coloured tracer near the bottom at one side of a beaker of water and heat gently below it	the coloured water rises above the heater, moves across the top and descends on the cooler side, showing a convection current
gas	use a convection box with two chimneys; warm the air below one chimney and introduce smoke at the other	smoke is drawn down the cool chimney, crosses the box and rises out above the heater, tracing the air current

Cooling can drive the same cycle in reverse locally: water beside ice cools, becomes denser and sinks; warmer water rises to replace it. The result is still a convection current.

Rising and sinking are consequences of density differences, not separate transfer methods. A complete explanation links temperature change to density change, fluid motion, replacement and circulation.

2.3.3 Radiation

Thermal radiation is infrared radiation, a region of the electromagnetic spectrum. Every object emits infrared radiation.

Claim	Meaning
type of radiation	infrared electromagnetic radiation
emitters	all objects, not only visibly glowing objects
transfer	an object can emit infrared that another object absorbs

Visible light may also be emitted by a very hot object, but the thermal-radiation region named in this syllabus is infrared.

Infrared radiation does not require a material medium. As electromagnetic radiation, it can transfer thermal energy through a vacuum.

Process	Needs matter between source and receiver?	Can cross a vacuum?
conduction	yes	no
convection	yes, a moving fluid	no
radiation	no	yes

Energy from the Sun and stars reaches Earth across space by radiation. The receiving surface warms when it absorbs that radiation.

A vacuum does not block radiation. It prevents conduction and convection between separated objects because those processes require matter.

Surface colour and texture affect how strongly infrared radiation is emitted, absorbed and reflected.

Surface feature	Absorption	Emission	Reflection
black	better	better	poorer
white	poorer	poorer	better
dull	better	better	poorer
shiny	poorer	poorer	better

For the strongest contrast, dull black surfaces are good absorbers and emitters, while shiny white or polished silver surfaces are good reflectors and poor absorbers and emitters. Compare otherwise identical surfaces at the same temperature and area.

Do not describe a surface as a ‘conductor of radiation’. Conduction is a different transfer process; surfaces absorb, emit or reflect infrared radiation.

For an object in a steady state with no change of state, temperature remains constant when the rate at which it receives energy equals the rate at which it transfers energy away.

Energy transfer in	Energy transfer out	Temperature result
equal rates	equal rates	constant

Energy can continue to enter and leave the object. Constant temperature means no net change in its internal energy because the two rates balance.

Thermal balance is not the same as zero energy transfer. A heater may supply energy continuously while the object transfers energy away at the same rate. During melting or boiling, temperature can also remain constant while internal energy changes; that phase-change case is treated separately.

When no change of state occurs, temperature changes if the rate of energy transfer into an object differs from the rate of transfer out.

Comparison of rates	Internal energy	Temperature
input rate greater than output rate	increases	rises
input rate equal to output rate	constant	remains constant
input rate less than output rate	decreases	falls

As a hot object cools, its temperature difference from the surroundings becomes smaller, so its cooling rate usually decreases. It approaches the surroundings' temperature as the rates move towards balance.

Compare rates, not just total amounts already transferred. With no phase change, the sign of input rate minus output rate determines whether temperature is rising or falling at that moment.

Earth's average surface temperature is controlled by the balance between incoming radiation absorbed from the Sun and outgoing infrared radiation emitted towards space.

Planetary balance	Temperature tendency
absorbed incoming rate greater than outgoing rate	average temperature rises
absorbed incoming rate equal to outgoing rate	average temperature is steady
absorbed incoming rate less than outgoing rate	average temperature falls

An increase in greenhouse gases can reduce the rate at which outgoing infrared escapes to space. While absorbed incoming radiation exceeds outgoing radiation, the surface warms until a new balance may be reached.

Incoming sunlight and outgoing infrared are different parts of Earth's energy budget. A temperature rise indicates a rate imbalance, not that Earth has stopped emitting radiation.

A good emitter produces a larger infrared detector response than a poor emitter when surface temperature, area and distance are the same.

Stage	What to do
set up	use equal-area dull black and shiny or white surfaces on the same hot container, or identical containers with equal hot-water volumes and temperatures
measure	place the same infrared detector or matched black-bulb thermometers at equal perpendicular distances; record readings at equal times
result	the dull black surface gives the larger detector response or faster thermometer rise
conclusion	dull black is the better emitter; shiny or white is the poorer emitter

Control surface area, surface temperature, detector distance and angle, water volume and initial temperature, container dimensions and observation time.

A faster fall in a hot container's temperature can also indicate stronger emission, but only when other energy-transfer paths and all comparison variables are controlled.

A good absorber produces a faster temperature rise than a poor absorber when both receive the same infrared radiation.

Stage	What to do
set up	use identical dull black and shiny or white cans containing equal volumes of water at the same initial temperature
expose	place both cans at equal distance and orientation from the same radiant heater; switch it on for the same time
measure	record both water temperatures at equal time intervals
result and conclusion	the water in the dull black can warms faster, so dull black is the better absorber

Control can material and dimensions, exposed area, water mass, initial temperature, heater power, distance, angle and heating time.

This compares absorption of incoming radiation. Do not infer the result from thermal conductivity of the can, and do not change both surface and geometry at once.

For otherwise comparable objects, infrared emission increases with surface temperature and with emitting surface area.

Change while other factors stay fixed	Rate of infrared emission
higher surface temperature	greater
lower surface temperature	smaller
larger emitting surface area	greater total rate
smaller emitting surface area	smaller total rate

Compare one factor at a time: use the same surface finish when testing temperature or area. Surface colour and texture also affect emission, as covered separately.

A larger area increases the total emitted power; it does not mean each square centimetre emits more when temperature and surface finish are unchanged.

2.3.4 Consequences of thermal energy transfer

Everyday heating devices use material choice and fluid circulation to direct thermal energy where it is useful and reduce transfer where it is unsafe.

Pan feature or process	Thermal explanation
metal base	a good conductor transfers energy rapidly from the heater into the pan and contents
plastic or wooden handle	a poor conductor reduces transfer to the hand
water in the pan	warmer, less-dense water rises and cooler, denser water sinks, so convection circulates energy through the liquid

Step in room heating	Explanation
air near the heater warms	its density decreases
warm air rises	cooler, denser air moves down to replace it
circulation continues	the convection current transfers energy around the room

Name the process at each stage. Conduction transfers energy through the solid pan; convection is the bulk circulation of water or air. Saying only ‘heat rises’ does not explain room heating.

Complex thermal systems can involve conduction through solids, convection by moving fluids and infrared radiation across space at the same time.

Process	Role in a wood or coal fire
conduction	energy spreads through touching parts and within solid fuel, helping nearby material heat up
convection	hot gases rise and cooler oxygen-containing air is drawn towards the fire; rising gases carry energy upward
radiation	flames and hot fuel emit infrared that warms surroundings and can preheat nearby fuel

Process	Role in cooling the engine
conduction	energy passes from engine metal to coolant and from hot coolant through radiator tubes into metal fins
convection	pumped coolant carries energy to the radiator; moving air carries energy away from the tubes and fins
radiation	hot radiator surfaces also emit infrared to the surroundings

A radiator uses metal tubes and many fins: good conduction moves energy to the surface, while the large area and airflow increase transfer to the air. Radiation occurs, but forced convection is usually the main external cooling route.

The name ‘radiator’ does not mean a car radiator cools mainly by radiation. Identify each path from the material that carries energy: solid metal, moving coolant or air, or infrared radiation.

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