Preview
IB DP Physics — Unit B: The Particulate Nature of Matter | SL & HL | B.1 Thermal Energy Transfers
📋 What You Will Learn
- Observe and describe what happens to matter at the particle level during heating, cooling, and phase changes
- Define specific heat capacity and explain why different materials require different amounts of energy to change temperature
- Define specific latent heat and distinguish between latent heat of fusion and latent heat of vaporisation
- Interpret heating and cooling curves, identifying regions of temperature change and phase change
- Apply the equations \( Q = mc\Delta T \) and \( Q = mL \) to solve quantitative problems
- Analyse graph-based and figure-based questions in the style of IB Paper 1B and Paper 2 examinations
🌍 Real-World Context On a hot summer day, you might notice that a glass of iced water stays cold for a surprisingly long time — far longer than you would expect if the ice simply "absorbed heat" the way a warm object does. Even more striking, the temperature of the water in the glass barely rises while ice is still present, no matter how warm the room becomes. Meanwhile, a pot of water on a stove takes several minutes to reach boiling point, and then — even with the burner still on at full power — the temperature of the boiling water refuses to rise above 100 °C. Something is absorbing all that energy, yet the thermometer shows no change. Understanding why requires us to look inside matter itself, at the bonds between particles and the energy needed to break them.
🔍 Guiding Question How does thermal energy transfer to or from a substance determine whether its temperature changes or its physical state changes — and what does this tell us about the structure of matter at the particle level?
Physical Setup: Heating a Substance from Solid to Gas
Imagine placing a block of ice in a beaker and applying a constant, steady source of heat — perhaps an electric heater delivering energy at a fixed rate. You record the temperature of the substance every 30 seconds and plot the results. What you observe is not a smooth, continuous rise in temperature. Instead, you see a graph with sloping sections separated by flat plateaus. During the sloping sections, temperature rises steadily. During the flat plateaus, the temperature stays completely constant even though energy is still being supplied.
This behaviour is the central puzzle of this topic. The flat regions are not measurement errors — they represent something physically real happening inside the material. To understand them, we need to think about what it means for a substance to change its state of matter.
Notice three key features of this heating curve. First, the slope of each rising section is different — this reflects the fact that solid ice, liquid water, and steam each require a different amount of energy per degree of temperature rise per kilogram. Second, the two flat plateaus occur at fixed, characteristic temperatures (0 °C and 100 °C for water at standard pressure) — these are the melting point and boiling point. Third, the length of each plateau tells us how much energy is needed to complete the phase change — the longer the plateau, the more energy is required.
Core Concepts and Definitions
When we supply thermal energy to a substance, that energy can do one of two things: it can increase the average kinetic energy of the particles