1. What Are Thermal Properties?
Thermal properties describe how a substance behaves when heat flows into it or out of it. Every object reacts differently to heating: some warm up quickly, some expand a lot, some melt easily, and others barely change. Understanding these behaviours helps in predicting how materials respond in everyday situations—like why metal spoons get hot in tea, why bridges need expansion gaps, or why water takes time to boil.
In simple words, thermal properties tell us how matter absorbs, stores, transfers, and responds to heat.
2. How Heat Affects Matter
Whenever heat is supplied to a substance, something about it changes. This change can be in temperature, in its physical state, or even in its size. These effects depend on how the particles inside the substance behave when they gain or lose energy.
2.1. Change in Temperature
When heat flows into an object, its particles start moving faster. This increase in particle motion shows up as a rise in temperature. Similarly, cooling slows the particles and lowers the temperature. This is the simplest effect of heating.
Example: A metal rod becomes warm when heated because its particles vibrate faster.
2.2. Change in State
Enough heating can break the bonds holding particles together, causing a solid to turn into a liquid or a liquid into a gas. Cooling reverses these changes. These changes of state happen at fixed temperatures for pure substances.
- Solid → Liquid (melting)
- Liquid → Gas (boiling/evaporation)
- Liquid → Solid (freezing)
- Gas → Liquid (condensation)
2.3. Change in Dimensions
Most materials expand when heated and contract when cooled. This happens because the particles need more space when they have more energy. The expansion may be along length, area, or volume depending on the object.
Example: Railway tracks require small gaps so they don’t bend when expanding on hot days.
3. Heat, Temperature & Internal Energy
These three ideas are related but not the same. Understanding the difference makes thermal phenomena much clearer.
3.1. Heat
Heat is the energy that flows from a hotter body to a colder one due to their temperature difference. It is not stored; it is transferred. Heat is measured in joules.
Symbol: Q
3.2. Temperature
Temperature tells how hot or cold an object is. On a deeper level, it measures the average kinetic energy of particles in the object.
Two objects with the same temperature might contain totally different amounts of heat.
3.3. Internal Energy
Internal energy is the total energy of all particles inside a substance. It includes their motion and their interaction energy.
When heat is supplied, internal energy increases. When heat is lost, internal energy decreases.
4. Thermal Equilibrium & the Zeroth Law
These ideas explain how temperature behaves and why thermometers work the way they do.
4.1. Thermal Equilibrium
Two bodies are in thermal equilibrium when there is no net flow of heat between them. This means both have the same temperature.
Example: A thermometer stops rising or falling once it reaches the temperature of the object being measured.
4.2. Zeroth Law of Thermodynamics
Zeroth Law: If body A is in thermal equilibrium with body B, and body B is in thermal equilibrium with body C, then A and C are also in thermal equilibrium.
This law lets us define temperature in a consistent way and forms the basis of all thermometers.
5. Modes of Heat Transfer
Heat can travel from one place to another in three main ways. The method depends on whether the material is solid, liquid, gas, or even empty space.
5.1. Conduction
In conduction, heat travels through a solid without the particles themselves moving from one place to another. Instead, they pass energy by vibrating and pushing neighbouring particles.
Example: A metal spoon becomes hot when its end is placed in boiling water.
5.2. Convection
Convection happens in liquids and gases. Here, warmer and lighter regions move upward, while cooler and heavier regions sink. This circulation carries heat along with the moving fluid.
Example: Warm air rising inside a room and cool air sinking.
5.3. Radiation
Radiation transfers heat through electromagnetic waves. It doesn’t need particles or any medium at all—radiation works even through a vacuum.
Example: The warmth felt from sunlight even though space is empty.
6. Thermal Expansion
Almost all substances expand when heated. The amount of expansion depends on the material and the temperature change. Expansion can be in length, area, or volume.
6.1. Linear Expansion
Linear expansion is the increase in length of a solid rod or wire when heated.
The relation is:
\( \Delta L = \alpha L \Delta T \)
Where \(\alpha\) is the coefficient of linear expansion.
6.2. Area Expansion
When a sheet or plate is heated, its surface expands.
The relation is:
\( \Delta A = \beta A \Delta T \)
6.3. Volume Expansion
Liquids and gases show volume expansion when heated. This happens because particles move faster and need more space.
\( \Delta V = \gamma V \Delta T \)
6.4. Anomalous Expansion
Water behaves differently: between 0°C and 4°C, instead of expanding on heating, it actually contracts. At 4°C, it has minimum volume and maximum density.
This unusual behaviour is called anomalous expansion of water.
7. Specific Heat Capacity
Different materials need different amounts of heat to raise their temperature. This is why metal heats up faster than water even under the same flame.
7.1. Concept of Heat Capacity
The heat required to raise the temperature of a substance is given by:
\( Q = mc\Delta T \)
Here, c is the specific heat capacity. A higher value means the material needs more heat to warm up.
7.2. Why Different Materials Heat Up Differently
The way particles are arranged and how strongly they interact affects how quickly a substance heats up. Water has a very high specific heat, so it warms up slowly—this is why coastal regions have steadier temperatures.
8. Change of State & Latent Heat
During melting or boiling, heat is absorbed but the temperature stays the same. This heat goes into changing the arrangement of particles rather than raising temperature.
8.1. Latent Heat of Fusion
This is the heat needed to convert a solid into a liquid at its melting point without changing its temperature.
Example: Ice melts into water at 0°C but stays at 0°C until all ice has melted.
8.2. Latent Heat of Vaporisation
This is the heat needed to convert a liquid into a gas at its boiling point without a temperature change.
Example: Water stays at 100°C while boiling.
9. Real-Life Importance of Thermal Properties
- Expansion gaps in bridges and railway tracks
- Thermometers based on thermal equilibrium
- Cooking utensils designed for quick heat flow
- Refrigerators working by absorbing and removing heat
- Hot-air balloons rising due to warm, lighter air