Thermal properties and temperature
LO1: Describe, qualitatively, the thermal expansion of solids, liquids and gases at constant pressure.
When matter is heated, its particles gain kinetic energy and vibrate or move more vigorously. This increased motion causes the particles to push each other further apart, leading to an increase in volume known as thermal expansion.
- Solids: Particles are tightly packed in a fixed arrangement. When heated, they vibrate with larger amplitudes about their fixed positions, pushing neighbours away slightly. Expansion is small but measurable (e.g., railway tracks expanding in summer).
- Liquids: Particles are close but can slide past each other. Heating increases their kinetic energy, causing them to move further apart. Expansion is greater than in solids because the intermolecular forces are weaker.
- Gases: Particles are far apart and move randomly at high speeds. Heating significantly increases their speed and collision force with container walls. At constant pressure, gases expand much more than liquids or solids (e.g., hot air balloons rising).
LO3: Explain, in terms of the motion and arrangement of particles, the relative order of magnitudes of the expansion of solids, liquids and gases as their temperatures rise.
The order of expansion magnitude is: Gases > Liquids > Solids.
- Solids: Particles are held by strong intermolecular forces in a rigid lattice. They can only vibrate; they cannot move freely. Thus, the increase in separation is minimal.
- Liquids: Intermolecular forces are weaker than in solids. Particles can move past each other, allowing for greater separation upon heating.
- Gases: Intermolecular forces are negligible. Particles move independently at high speeds. A small increase in kinetic energy leads to a large increase in the volume occupied because there is no structure holding them together.
LO2: Describe some of the everyday applications and consequences of thermal expansion.
- Bimetallic Strip: Two different metals (e.g., brass and steel) are bonded together. Since they expand by different amounts when heated, the strip bends. This is used in thermostats to switch circuits on/off automatically.
- Loose Jar Lid: Running hot water over a tight metal lid on a glass jar causes the metal to expand more than the glass, loosening the fit.
- Gaps in Bridges/Railways: Expansion joints are left in bridges and railway tracks to allow for thermal expansion in hot weather without causing structural damage (buckling).
- Consequences: If expansion is constrained (e.g., a bolt fixed in a rigid hole), it can create immense stress, potentially cracking the material or deforming the bolt.
LO4: Know that a rise in the temperature of an object increases its internal energy.
Internal energy is the total random kinetic energy and potential energy of all particles in a substance.
When an object is heated, energy is transferred to its particles. This increases their kinetic energy (they move/vibrate faster) and often their potential energy (particles move further apart against intermolecular forces). Therefore, temperature rise correlates directly with an increase in internal energy.
LO5: Describe an increase in temperature of an object in terms of an increase in the average kinetic energies of all of the particles in the object.
Temperature is a measure of the average kinetic energy of the particles. It is not the total energy, but the mean energy per particle.
- As temperature rises, the distribution of particle speeds shifts towards higher speeds.
- The average kinetic energy increases linearly with absolute temperature.
Specific heat capacity (c) is defined as the amount of thermal energy required to raise the temperature of 1 kg of a substance by 1 °C (or 1 K).
The equation linking energy, mass, specific heat capacity, and temperature change is:
\Delta E = mc\Delta\theta
Where:
- \Delta E = thermal energy transferred (J)
- m = mass of the substance (kg)
- c = specific heat capacity (J/kg/°C or J/kg/K)
- \Delta\theta = change in temperature (°C or K)
Note: A change of 1 °C is equal to a change of 1 K. Therefore, the numerical value of c is the same for both units.
Solution:
Identify variables:
- m = 2.0 \text{ kg}
- c = 385 \text{ J/kg/°C}
- \Delta\theta = 60 - 20 = 40 \text{ °C}
Substitute into equation:
\Delta E = 2.0 \times 385 \times 40
\Delta E = 30,800 \text{ J}
Answer: The energy transferred is 30,800 J (or 30.8 kJ).
Students often use grams (g) instead of kilograms (kg) in the formula \Delta E = mc\Delta\theta. Since specific heat capacity is defined per kg, using grams will give an answer that is 1000 times too small.
Mistake 2: Kelvin vs. Celsius Conversion.
Students incorrectly add or subtract 273 to the temperature change (\Delta\theta). Remember, \Delta\theta in °C is numerically identical to \Delta\theta in K. You only convert absolute temperatures (e.g., 0 °C = 273 K) if you are dealing with absolute zero or gas laws, not for simple temperature differences in heat capacity calculations.
Examiners look for a clear method that isolates the variables. For a solid:
- Apparatus: Mention an electric heater inserted into a hole in the metal block and a thermometer in another hole.
- Measurements: State that you measure the mass (m) of the block, the initial temperature, and the time the heater is on (to calculate energy E = VIt).
- Procedure: Record the final temperature after a set time.
- Calculation: State that you use c = \Delta E / m\Delta\theta.
Why this works: This directly addresses the definition of specific heat capacity by measuring energy input (\Delta E), mass (m), and temperature rise (\Delta\theta) explicitly. Mentioning insulation or a lid is a bonus point as it minimizes energy loss to surroundings, improving accuracy.
When a substance changes state (melts or boils), the temperature remains constant even though heat is still being supplied. This is known as a temperature plateau on a heating curve.
- Why? The energy supplied is used to break the intermolecular bonds holding the particles in their current structure, rather than increasing their kinetic energy (speed).
- Melting: Solid → Liquid. Bonds between fixed lattice positions are broken.
- Boiling: Liquid → Gas. All remaining intermolecular bonds are broken, allowing particles to escape completely.
LO9: Know the melting and boiling temperatures for water at standard atmospheric pressure.
- Melting point of water: 0 °C
- Boiling point of water: 100 °C
Note: These values are only true at standard atmospheric pressure. Lower pressure lowers the boiling point.
LO10: Describe condensation and solidification in terms of particles.
These are the reverse processes of boiling and melting. They involve a loss of internal energy (exothermic).
- Condensation (Gas → Liquid): Gas particles lose kinetic energy, slow down, and come close enough for intermolecular forces to pull them together into a liquid state. Energy is released to the surroundings.
- Solidification (Liquid → Solid): Liquid particles lose kinetic energy, vibrate less, and settle into fixed positions in a lattice structure. Energy is released to the surroundings.
Evaporation is the change of state from liquid to gas that occurs only at the surface of a liquid, at any temperature below the boiling point.
- Particles in a liquid have a range of kinetic energies (some fast, some slow).
- The most energetic particles at the surface may have enough energy to overcome intermolecular forces and escape into the air as vapour.
LO12: Know that evaporation causes cooling of a liquid.
Because the most energetic particles leave the liquid, the average kinetic energy of the remaining particles decreases. Since temperature is proportional to average kinetic energy, the temperature of the remaining liquid drops. This is why sweating cools the body.
LO15: Explain the cooling of an object in contact with an evaporating liquid.
If a liquid evaporates from the surface of an object (e.g., water on skin), the liquid absorbs thermal energy from the object to provide the energy needed for particles to escape. This transfer of heat from the object to the liquid causes the object to cool down.
LO13: Describe the differences between boiling and evaporation.
| Feature | Evaporation | Boiling |
|---|---|---|
| Location | Occurs only at the surface of the liquid. | Occurs throughout the bulk of the liquid (bubbles form inside). |
| Temperature | Can occur at any temperature below the boiling point. | Occurs only at a specific temperature (the boiling point). |
| Bubble Formation | No bubbles are formed. | Bubbles of vapour form within the liquid and rise to the surface. |
| Rate | Generally slower. | Rapid and vigorous. |
| Energy Source | Can occur using ambient thermal energy. | Requires continuous external heat supply to maintain temperature. |
LO14: Describe how temperature, surface area and air movement over a surface affect evaporation.
- Temperature: Higher temperature means particles have higher average kinetic energy. More particles have enough energy to escape, so the rate of evaporation increases.
- Surface Area: A larger surface area exposes more liquid particles to the air. This allows more particles to escape simultaneously, so the rate of evaporation increases.
- Air Movement (Wind): Moving air blows away the vapour molecules that have just escaped from the surface. This prevents them from returning to the liquid, maintaining a concentration gradient that encourages further evaporation. Thus, air movement increases the rate.
Do not just say 'energy is lost.' You must link the particle energy to temperature.
Correct Phrasing: "The particles with the highest kinetic energy escape from the surface. This leaves behind particles with a lower average kinetic energy. Since temperature is a measure of average kinetic energy, the temperature of the remaining liquid decreases.
Why this works: Examiners award marks for identifying that it is specifically the high-energy particles that leave (not just any particle) and explicitly linking the reduction in average kinetic energy to the drop in temperature.