Melting and Boiling — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

Section 1 of 9Fixed points of water

Melting point

• Melting point ice melts at 0 °C

Boiling point

• Boiling point pure water boils at 100 °C

Condition

• Condition accepted values for pure water at standard atmospheric pressure

During a change of state

• During a change of state the temperature does not change although energy is still transferred to or from the substance

Section 2 of 9Melting

Energy input

• Energy input at the melting point extra energy does not raise the temperature

Potential energy

• Potential energy the extra energy goes into overcoming the intermolecular forces, so internal energy rises

Kinetic energy

• Kinetic energy unchanged, which is why temperature is constant

Arrangement

• Arrangement the regular lattice with vibrating particles becomes an irregular arrangement in which particles slide over one another

Section 3 of 9Boiling

Boiling

• Boiling also called vaporisation, occurs at a higher temperature than melting

Energy

• Energy enough energy is transferred for the intermolecular forces to be completely overcome

Temperature

• Temperature stays constant at the boiling point while the extra energy goes into potential energy

Result

• Result liquid water becomes water vapour, a gas

Section 4 of 9Condensation and solidification

Cooling

• Cooling energy is transferred away and internal energy decreases

Condensation

• Condensation at the boiling point the loss of potential energy lets particles only flow over one another

Solidification

• Solidification at the melting point particles become bound and only vibrate about fixed points

Kinetic energy

• Kinetic energy stays unchanged during both, so temperature is constant

Section 5 of 9Heating curves

Heating curves

Graph

• Graph temperature on the y-axis against time on the x-axis for steady heating of ice

Sloping parts

• Sloping parts the solid, liquid and gas warming up
• The liquid slope is less steep than the solid because c of water is higher than c of ice

Flat parts

• Flat parts at the melting point and boiling point during changes of state

Flat boiling section

• Flat boiling section much longer than melting because far more energy is needed to boil a mass of liquid

Cooling curve

• Cooling curve the same shape in reverse

Section 6 of 9Specific latent heat

Specific latent heat

Specific latent heat of fusion Lf

• Specific latent heat of fusion Lf energy to melt 1 kg of solid at its melting point with no temperature change

Specific latent heat of vaporisation Lv

• Specific latent heat of vaporisation Lv energy to turn 1 kg of liquid into gas at its boiling point

Equation

• E = mL with E in joules, m in kilograms and L in J/kg

Water values

• Lf is about 3.3 × 10⁵ J/kg and Lv about 2.3 × 10⁶ J/kg, so boiling needs much more energy

Applications

• Ice cools drinks well because melting absorbs a lot of energy
• Steam burns are severe because condensing steam releases a lot of energy

Section 7 of 9More from the specification

More from the specification

Specific Heat Capacity Determination

• Specific heat capacity is the thermal energy required per kilogram to raise the temperature of a substance by 1°C;
  • it is measured using an electric immersion heater system where electrical energy (calculated from current, voltage,
• and time, E = I·V·t) increases internal energy (Q = m·c·ΔT).

Evaporation Mechanism and Cooling

• Evaporation occurs from the surface of a liquid at any temperature when molecules with above-average kinetic energy overcome intermolecular attractions and escape,
• which lowers the average kinetic energy of the remaining liquid.

Thermal Expansion of Matter

• As thermal energy increases, particle kinetic energy increases,
• leading to greater molecular vibration and spacing,
• which causes the macroscopic thermal expansion of solids, liquids,
• and gases.

Specific Heat Capacity Determination

• Specific heat capacity is the thermal energy required per kilogram to raise the temperature of a substance by 1°C;
  • it is measured using an electric immersion heater system where electrical energy (calculated from current, voltage,
• and time, E = I·V·t) increases internal energy (Q = m·c·ΔT).
  What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)

Evaporation Mechanism and Cooling

• Evaporation occurs from the surface of a liquid at any temperature when molecules with above-average kinetic energy overcome intermolecular attractions and escape,
• which lowers the average kinetic energy of the remaining liquid.

Thermal Expansion of Matter

• As thermal energy increases, particle kinetic energy increases,
• leading to greater molecular vibration and spacing,
• which causes the macroscopic thermal expansion of solids, liquids,
• and gases.

Immersion heater

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

Potential difference

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

Capacity

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

Thermal

• A convection current in the air.

Evaporation

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

Expansion

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

Current

• Flow of electrons in a circuit.

Heater

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

Thermometer

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

Thermal energy

• Energy of random molecular motion, increased by friction.

ΔE

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 8 of 9Key terms

Term	Meaning
Melting point	Temperature at which a solid becomes a liquid
Boiling point	Temperature at which a liquid becomes a gas throughout its volume
Condensation	Gas changing to liquid, giving out energy
Specific latent heat	Energy to change the state of 1 kg without a temperature change
Heating curve	Graph of temperature against time during heating

Section 9 of 9Exam technique

1

Saying the temperature rises during melting or boiling — it stays constant while particle arrangement changes.

2

Saying the energy disappears — it is stored as potential energy of the particles.

3

Believing boiling occurs only at the surface — bubbles form throughout the liquid.

4

Mixing up Lf and Lv — use the one matching the change of state.