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🌑️ Topic 2.2 β€” Thermal Properties and Temperature

Thermal physics deals with how materials behave when heated or cooled. This topic explains how substances expand, how their internal energy changes, and how changes of state happen.

2.2.1 Thermal Expansion of Solids, Liquids, and Gases

πŸ”₯ 1. What is Thermal Expansion?​

When a substance is heated, its particles gain kinetic energy and vibrate or move faster. This increased movement causes most substances to expand (increase in size).

➑ Key Rule: Most materials expand when heated and contract when cooled.

βœ” Expansion at constant pressure:​

  • Solids: Expand a little.
  • Liquids: Expand more than solids.
  • Gases: Expand the most.

This difference comes from the arrangement and movement of particles.

πŸ”¬ 2. Everyday Applications & Consequences of Thermal Expansion​

βœ” Useful Applications​

ObjectPurpose of Expansion
Railway linesGaps are left between rails so they don’t buckle in hot weather.
Bridges (expansion joints)Prevent cracking or bending when temperature changes.
ThermostatsUse a bimetallic strip (two metals that expand differently) to switch circuits on/off.
Lids on jarsHeating the metal lid expands it, making it easier to open.

❗ Consequences (Problems caused by expansion)​

  • Electric cables sag in hot weather and become tight in cold weather.

  • Cracks form in roads when tar expands in high temperatures.

  • Glass breaks when heated unevenly (thermal stress).

🧬 3. Why do different states of matter expand differently?​

StateParticle ArrangementAmount of ExpansionReason
SolidsPacked tightly, vibrate in fixed positionsSmallestLimited space to move
LiquidsClose but can flowMoreParticles already have space to move
GasesFar apart and move freelyLargestLots of room to spread out

➑ Exam Tip:
A gas expands the most because the increase in kinetic energy leads to much more movement of particles.

2.2.2 Specific Heat Capacity (SHC)

Specific heat capacity tells us how hard it is to heat a substance.

πŸ”₯ 1. Heating increases internal energy​

When you heat an object:

  • Its particles move faster.

  • The total internal energy increases.

Internal energy = kinetic energy of moving particles + potential energy due to forces between them.

⚑ 2. What does a rise in temperature mean?​

Temperature rises because:

  • The average kinetic energy of particles increases.

  • Particles vibrate faster (in solids) or move faster (in liquids/gases).

➑ So, a hotter object has particles with more average kinetic energy.

πŸ“˜ 3. Specific Heat Capacity β€” Definition & Formula​

βœ” Definition:​

Specific heat capacity (c) is the energy required to raise the temperature of 1 kg of a substance by 1 Β°C.

βœ” Formula:​

Where:

  • Ξ”E = Energy supplied (J)
  • m = Mass (kg)
  • Δθ = Temperature change (Β°C or K)

Rearranged formula often used in exams:

πŸ§ͺ 4. Experiments to measure SHC​

A. Measuring the SHC of a Solid​

Equipment:

  • Block of metal with hole
  • Heater (50 W)
  • Ammeter, voltmeter
  • Thermometer
  • Stopwatch

Steps:

  1. Insert heater into the metal block.

  2. Measure the mass of the block.

  3. Record initial temperature.

  4. Switch on the heater and record voltage (V) and current (I).

  5. Heat for a known time (t), record final temperature.

  6. Calculate energy supplied:

  7. Substitute into:

Sources of error:
– Heat lost to surroundings β†’ cover block with insulation.

B. Measuring SHC of a Liquid​

Equipment:

  • Beaker with liquid
  • Immersion heater
  • Thermometer
  • Balance
  • Stopwatch

Method:

  1. Measure mass of beaker and liquid.
  2. Record starting temperature.
  3. Heat using known power for a set time.
  4. Record final temperature.
  5. Use the same formula

Liquids lose more heat than solids β†’ use insulation.

2.2.3 Melting, Boiling & Evaporation

❄️ 1. What happens during melting and boiling?​

Even though energy is supplied, temperature stays constant.

Why?
β†’ Energy is used to break bonds between particles (potential energy increases), not to increase kinetic energy.

Effects:​

  • Melting: Solid β†’ Liquid

  • Boiling: Liquid β†’ Gas

🌑️ 2. Melting & Boiling point of water​

At standard atmospheric pressure (1 atm):

  • Melting point: 0 Β°C
  • Boiling point: 100 Β°C

πŸ’¨ 3. Condensation & Solidification (Freezing)​

βœ” Condensation (Gas β†’ Liquid):​

  • Particles lose energy.
  • They move slower and come closer together.

βœ” Solidification (Liquid β†’ Solid):​

  • Particles lose kinetic energy and form fixed positions.

🌬️ 4. Evaporation β€” what happens?​

Evaporation is when more energetic particles escape from the surface of a liquid.

Features:

  • Happens below boiling point.
  • Only particles at the surface escape.
  • Remaining liquid has lower average kinetic energy β†’ it cools down.

❄️ 5. Evaporation causes cooling​

When high-energy particles escape:

  • Average kinetic energy decreases.
  • Temperature drops.

Example:

  • Sweat evaporates β†’ takes heat away β†’ body cools.

βœ” 6. Difference between Boiling and Evaporation​

FeatureEvaporationBoiling
TemperatureAny temperatureAt fixed boiling point
Where it happensSurface onlyThroughout the liquid
BubblesNoYes
SpeedSlowFast
Cooling effectStrongWeak

βœ” 7. Factors affecting evaporation​

  1. Temperature:
    Higher temperature β†’ particles have more energy β†’ faster evaporation.

  2. Surface Area:
    Larger area β†’ more particles exposed β†’ faster evaporation.

  3. Air Movement (Wind):
    Wind removes saturated air β†’ increases evaporation rate.

βœ” 8. Cooling of an object in contact with an evaporating liquid​

If an object touches a liquid that is evaporating:

  • The liquid takes energy from the object.
  • Evaporation absorbs heat (endothermic process).
  • Object becomes cooler.

Example:

  • Alcohol rubbed on skin feels cold because it evaporates quickly, absorbing heat from your skin.

🎯 Exam Tips (Very Important!)

⭐ Thermal Expansion​

  • Always relate expansion to particle spacing.

  • When asked why bridges have gaps β†’ mention expansion joints.

⭐ Specific Heat Capacity​

  • Ensure temperature is in Β°C consistently.
  • Use correct mass units in kg, not grams.
  • For experiments, mention insulation and heat loss.

⭐ Melting/Boiling​

  • Emphasize temperature remains constant during change of state.
  • Mention energy is used to break intermolecular bonds, not heat increase.

⭐ Evaporation​

  • Always include:
    β€œMore energetic particles escape first, lowering average kinetic energy.”