Thermal Properties and Temperature — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

Section 1 of 25Thermal expansion

Thermal expansion

What happens

• What happens when a material is heated at constant pressure its temperature rises, its volume increases and its density falls.

Particle explanation

• Particle explanation thermal energy is transferred, so the particles gain kinetic energy.
• The temperature rises.
• The particles vibrate or move faster.
• They move further apart.
• The separation increases and the material gets bigger.

Key point

• Key point the material expands, not the molecules, because only the separation changes.

Section 2 of 25Expansion in solids, liquids and gases

State	Expansion	Explanation
Solid	Slight	Strong forces hold particles in fixed positions so they only vibrate a little further apart
Liquid	More than solids	Weaker forces let particles move further apart
Gas	Greatest	Almost no forces between particles so they spread much further

Section 3 of 25Uses and consequences of expansion

1Liquid-in-glass thermometer

• Liquid-in-glass thermometer a liquid in a bulb expands along a thin capillary tube against a scale.
• A narrower tube makes it more sensitive because the same expansion moves further.
• Thinner glass around the bulb makes it respond faster.

2Bimetallic strip

• Bimetallic strip two metals joined together expand by different amounts, so the strip bends at a set temperature and can close a circuit.

3Buckling

• Buckling hot railway tracks, roads and bridges may buckle.
• Gaps are built in to allow room for expansion.

Section 4 of 25Internal energy and temperature

Internal energy

• Internal energy the total energy stored inside a system by its particles because of their motion and positions.
• Motion gives kinetic energy and relative positions give potential energy.

Temperature

• Temperature it is related to the average kinetic energy of the particles.
• A higher temperature means a higher average kinetic energy and a higher internal energy.

Section 5 of 25Specific heat capacity

Definition

• Definition the specific heat capacity c is the energy needed per unit mass per unit temperature rise.
• It is the energy to raise 1 kg of a substance by 1 °C.

Equation

• ΔE = mcΔθ,
  • or c = ΔE ÷ (mΔθ).
    What the symbols mean: E, Q or q = energy transferred (J) · m = mass · c = specific heat capacity (J kg⁻¹ °C⁻¹; water 4.18 J g⁻¹ °C⁻¹) · ΔT or Δθ = temperature change (°C or K)
  • (kg; g in chemistry calorimetry)
• ΔE in joules, m in kilograms, c in J/kg °C and Δθ in °C.

Meaning

• Meaning a low value heats and cools quickly, and a high value heats and cools slowly.
• A metal pan warms faster than the water in it because metal has the lower c.

Section 6 of 25More key facts: Thermal Expansion, Internal Energy and Specific Heat Capacity

1Water has a high c

• Water has a high c it takes a lot of energy to warm water, which is why it is used in central heating and cooling systems.

2Rearranging

• Δθ = ΔE ÷ (mc) and m = ΔE ÷ (cΔθ).

3Expansion order

• Expansion order gases expand most, then liquids, then solids.

4Bimetallic strip direction

• Bimetallic strip direction it bends towards the metal that expands less.

Section 7 of 25More from the specification

More from the specification

Distinction Between Boiling and Evaporation

• Evaporation occurs exclusively at the liquid surface at any temperature as energetic particles escape,
• whereas boiling occurs throughout the liquid at a fixed boiling point.

Melting Point and Latent Heat

• During melting, thermal energy is absorbed to weaken intermolecular bonds without raising temperature, keeping substance temperature steady at the melting point.

Specific Heat Capacity Experiment

• Using an electric immersion heater connected to a voltmeter and ammeter, an investigator can measure current and voltage over time to determine thermal energy input.

Calculating Absorbed Thermal Energy

• By recording the temperature change of a known liquid mass heated by an immersion heater, specific heat capacity is calculated using Q = m * c * Delta T.

Evaporation

• Escape of the most energetic molecules from a liquid surface at any temperature

Boiling

• Liquid becoming gas throughout the liquid at the boiling point

Melting

• Change from solid to liquid.

Current

• Flow of electrons in a circuit.

Heater

• A device that transfers electrical energy to the thermal store.

Immersion heater

• Electric heater placed in a liquid or block to supply energy

Intermolecular force

• An attractive force between particles.

Section 8 of 25Aim and variables

Aim

• Aim to determine the specific heat capacity of a substance by linking the energy supplied electrically to the rise in temperature

Variables

• Variables time is the independent variable and temperature is the dependent variable
• Control variables are the current supplied and the potential difference supplied.

Section 9 of 25Equipment and resolution

Equipment	Purpose
Thermometer (resolution 0.1 °C)	Measures the temperature change of the solid or water
Solid aluminium block or beaker of water (about 400 cm³)	The substance heated
Immersion heater	Heats the solid or water
Power supply (d.c., about 10 V)	Supplies steady power to the heater
Ammeter (0.1 A) in series	Measures the current through the heater
Voltmeter (0.1 V) in parallel	Measures the potential difference across the heater
Digital balance (0.1 g)	Measures the mass of the solid or water
Stopwatch (0.01 s)	Times the heating

Section 10 of 25Method for water

1

Place the beaker on the digital balance, press zero, add about 250 cm³ of water and record its mass.

2

Place the immersion heater and thermometer in the water.

3

Connect the circuit with the ammeter in series with the power supply and heater and the voltmeter in parallel with the heater.

4

Use a direct current supply so the current and potential difference readings stay steady.

5

Record the initial temperature of the water at time 0 s.

6

Switch on the supply at about 10 V and start the stopwatch.

7

Record the temperature, voltage and current every 60 s for 10 minutes.

Section 11 of 25Method for the aluminium block

1

Record the mass of the block on the digital balance.

2

Put the thermometer in one hole of the block and the immersion heater in the other.

3

Connect the same circuit with the ammeter in series and the voltmeter in parallel with the heater.

4

Record the initial temperature of the block at time 0 s.

5

Switch on at about 10 V, start the stopwatch and record the voltage and current.

6

Continue recording temperature, voltage and current every 60 s for 10 minutes.

Section 12 of 25Results and analysis

Results table

• Results table columns for time, voltage, current, temperature, energy supplied and temperature change

Energy supplied

• E = I V t
  What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
• I = average current in A, V = average potential difference in V and t = heating time in s.
• E is the change in thermal energy in J.

Temperature change

• Δθ = final temperature − initial temperature

Specific heat capacity

• Specific heat capacity from ΔE = mcΔθ, c = ΔE ÷ (m Δθ)
• Mass must be in kg and c is in J/kg °C.

Graph option

• Graph option plotting temperature against energy gives a straight line with gradient 1 ÷ (mc)

Section 13 of 25Systematic and random errors

Problem	Effect	Improvement
Balance not zeroed (systematic)	Mass wrong	Zero the balance before weighing
Thermometer touches the glass (systematic)	Reads the glass, not the liquid	Keep it in the liquid, away from the sides and base
Thermal energy lost to surroundings (systematic)	Measured c is too high	Wrap or lid the beaker and heat for a shorter time
Water lost by evaporation (systematic)	Mass changes	Use the average of the initial and final mass
Temperature still rising after switch-off (systematic)	Final value too low	Wait until the temperature stops rising
Uneven temperature in the water	Unrepresentative reading	Stir constantly while heating
Meter readings fluctuate between two values (random)	Inconsistent readings	Be consistent, for example always choose the higher value

Section 14 of 25Safety

Hot heater

• Hot heater never touch the immersion heater and have a heatproof mat ready to place it on

Stability

• Stability use a clamp stand to hold the heater and thermometer if the beaker could tip

Eyes and spills

• Eyes and spills wear goggles and stand up throughout so you can react quickly to spills

Section 15 of 25More from the specification

1

Thermal expansion energy in joules

• Thermal expansion energy in joules absorbing thermal heat measured in joules increases atomic kinetic vibrations,
  • increasing interatomic separation within solids.

2

Kinetic theory of thermal expansion

• As temperature rises, atoms or molecules vibrate more vigorously, pushing neighbouring particles slightly further apart and causing macroscopic volume expansion.

3

Evaporation vs boiling

• Evaporation occurs at any temperature at the liquid surface as high-energy molecules escape,
• whereas boiling occurs throughout the liquid at a fixed boiling point.

4

Specific heat capacity

• Specific heat capacity (c) is the thermal energy required to raise the temperature of 1 kg of a substance by 1 °C.
  • (delta Q = m * c * delta T)

5

Thermal internal energy

• Thermal internal energy heating increases translational and vibrational kinetic energy of constituent particles, raising substance temperature.

6

Thermal expansion

• Thermal expansion vibrating particles push slightly further apart as kinetic energy increases, causing bulk expansion of solids and liquids.

7

Latent heat during phase change

• Latent heat during phase change thermal energy transferred during melting or boiling overcomes intermolecular forces without increasing particle kinetic energy.

8

Kinetic energy

• K = ½mv², energy of motion.

9

Internal energy

• Total kinetic and potential energy of the particles in a system

10

Intermolecular force

• An attractive force between particles.

11

Boiling

• Liquid becoming gas throughout the liquid at the boiling point

12

Molecule

• Tiny particle of the fluid, too small to be seen.

13

Expansion

• Increase in size on heating as particles move further apart.

14

Melting

• Change from solid to liquid.

15

Force

• A push or pull measured in newtons.

16

Joule

• Unit of energy, 1 kg·m²/s².

Section 16 of 25Fixed points and constant temperature

Fixed points

• Fixed points pure water melts at 0 °C and boils at 100 °C at standard atmospheric pressure

During a change of state

• During a change of state the temperature of the substance does not change
• This shows as the horizontal sections of a heating curve.
• Energy is still being transferred to or away from the thermal store of the substance.

Heating

• Heating energy is transferred to the system and increases its internal energy

Cooling

• Cooling energy is transferred away from the system and decreases its internal energy

Section 17 of 25Melting

Before melting

• Before melting heating a solid raises its temperature until the melting point is reached

At the melting point

• At the melting point added energy goes into the potential energy of the molecules, not their kinetic energy
• It overcomes the intermolecular forces holding the lattice.
• The internal energy rises even though the temperature stays constant.

Change of arrangement

• Change of arrangement the regular lattice where molecules only vibrate becomes an irregular, random arrangement
• The molecules can then move around and slide over one another.

Section 18 of 25Boiling

Boiling point

• Boiling point boiling occurs at a higher temperature than melting

Energy use

• Energy use the added energy completely overcomes the intermolecular forces and becomes potential energy of the molecules

Constant temperature

• Constant temperature the temperature does not rise at the boiling point even though energy is still added

Result

• Result the liquid becomes vapour (a gas) and the internal energy continues to rise

Section 19 of 25Condensation and solidification

Condensation

• Condensation when a gas cools, kinetic energy decreases until the boiling point is reached
• At the boiling point, energy transferred away reduces the potential energy of the particles.
• They can then only flow over one another, so the gas becomes a liquid.

Solidification

• Solidification when a liquid cools, kinetic energy decreases until the melting point is reached
• At the melting point the particles lose more potential energy and are bound to fixed positions, vibrating about them.
• The liquid becomes a solid.

Constant temperature

• Constant temperature only potential energy decreases, so the kinetic energy and temperature stay constant during each change

Section 20 of 25Evaporation

Definition

Definition the change of a liquid to a gas at any temperature, from the surface only

Mechanism

• Mechanism molecules in a liquid have a range of energies, and the most energetic molecules near the surface escape
• The average energy of the liquid relates to its temperature.

Cooling effect

• Cooling effect the escaping molecules have the most energy, so the average kinetic energy of the remaining molecules falls and the liquid cools
• An object in contact with the liquid is also cooled, which is used in refrigerators and air conditioners.

Section 21 of 25Factors affecting the rate of evaporation

Factor	Effect on rate	Reason
Higher temperature	Increases	More molecules have enough energy to escape the intermolecular forces
Larger surface area	Increases	Molecules escape only at the surface
More air movement	Increases	Vapour is carried away and replaced by drier air that accepts vapour more easily

• Not volume
  • the rate depends on surface area, not on the volume of liquid

Section 22 of 25Boiling compared with evaporation

Feature	Boiling	Evaporation
Change of state	Liquid to gas	Liquid to gas
Temperature	Only at the boiling point	Any temperature between melting and boiling point
Where	Throughout the liquid, seen as bubbles	From the surface only, with no bubbles

Section 23 of 25More from the specification

1Specific Heat Capacity Definition

• Specific heat capacity c is defined as the thermal energy required to raise the temperature of 1 kg of a substance by 1 degree Celsius (or 1 Kelvin), measured in joules per kilogram per degree Celsius (J kg^-1 °C^-1.

2Thermal Energy Equation

• The heat energy Q transferred when a mass m is heated through temperature change Delta T is calculated using Q = m * c * Delta T,
• where liquid water has an exceptionally high capacity of 4200 J kg^-1 °C^-1.

3Electrical Method for Heat Capacity

• In a calorimetry experiment, an electrical immersion heater supplied with a steady voltage V and current I heats a metal block,
• allowing energy input to be determined via Q = V * I * t and monitored using a sensitive thermometer.

4Thermal Expansion of Matter

• When a solid, liquid,
• or gas is heated, particle kinetic energy increases and the substance expands as average particle separations increase,
• causing noticeable volumetric expansion.

5Specific heat capacity definition

• Specific heat capacity definition specific heat capacity measures thermal energy in joules required to raise one kilogram of material by one degree.

6Immersion heater electrical calibration

• Immersion heater electrical calibration an electric immersion heater supplied with known direct current transfers thermal energy to heated liquid inside insulated calorimeters.

7Thermal expansion of materials

• Thermal expansion of materials when matter is heated, particles vibrate more vigorously, causing bulk material to expand in volume across dimensions.

8Immersion heater

• Electric heater placed in a liquid or block to supply energy

9Potential difference

• Work done by a unit charge passing through a component.

10Capacity

• Charge a cell can supply, in A h or C.

11Expansion

• Increase in size on heating as particles move further apart.

12Current

• Flow of electrons in a circuit.

13Heater

• A device that transfers electrical energy to the thermal store.

14Thermometer

• A device that reaches equilibrium with an object to measure its temperature

15Joule

• Unit of energy, 1 kg·m²/s².

16ΔE

ΔE mcΔθ [1 mark]

What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg⁻¹ °C⁻¹; water 4.18 J g⁻¹ °C⁻¹) · ΔT or Δθ = temperature change (°C or K)

Section 24 of 25Key terms

Term	Meaning
Thermal expansion	The increase in size of a material when its temperature rises.
Internal energy	Sum of the kinetic and potential energies of the particles in a system.
Specific heat capacity	Energy needed per kilogram per degree Celsius of temperature rise.
Bimetallic strip	Two metals joined that bend when heated because they expand differently.
Capillary tube	A very narrow tube, as in a thermometer.

Term	Meaning
Specific heat capacity	Energy needed per unit mass per unit temperature rise, in J/kg °C.
Immersion heater	Electric heater placed directly in the liquid or block.
Systematic error	An error that shifts all readings the same way.
Random error	An unpredictable variation between readings.
Resolution	The smallest change an instrument can show.

Term	Meaning
Fixed point	Melting or boiling point of pure water used as a reference temperature (0 °C and 100 °C).
Internal energy	Total kinetic and potential energy of the particles of a system.
Evaporation	Change of a liquid to a gas at the surface at any temperature.
Boiling	Change of a liquid to a gas throughout the liquid at the boiling point.
Condensation	Change of a gas to a liquid.
Solidification	Change of a liquid to a solid.

Section 25 of 25Exam technique

1

Saying molecules get bigger when heated — the separation increases.

2

Using grams in ΔE = mcΔθ.

3

Confusing temperature with internal energy — temperature relates to average kinetic energy only.

4

Writing the unit of c as J/kg without the degree Celsius.

1

Placing the voltmeter in series — it must be in parallel with the heater.

2

Using the mass in grams to find c — convert to kilograms.

3

Giving human error or poor practice as a source of error — name a specific cause.

4

Forgetting to convert minutes to seconds in E = I V t.

1

Saying the temperature rises during melting — it stays constant.

2

Saying energy added during melting becomes kinetic energy — it becomes potential energy.

3

Counting surface only and no bubbles as two differences between boiling and evaporation — they are one difference.

4

Saying evaporation depends on the volume of liquid — it depends on surface area.