Describe, qualitatively, the thermal expansion of solids, liquids and gases at constant pressure

2.2.1 Thermal Expansion of Solids, Liquids and Gases

Solids – Think of a metal rail on a train track

When a solid is heated, its particles vibrate more vigorously and push against each other, so the solid expands. The change in length is usually small but measurable. The relationship is given by the linear expansion formula: $$\Delta L = \alpha L_0 \Delta T$$ where $\alpha$ is the linear expansion coefficient, $L_0$ the original length, and $\Delta T$ the temperature change. 🔧 Example: A 2‑m steel rail expands by about 0.8 mm when the temperature rises from 20 °C to 80 °C.

Liquids – Imagine a glass of water in a hot bath

Liquids expand more than solids because their molecules are less tightly packed. The volume change is described by: $$\Delta V = \beta V_0 \Delta T$$ where $\beta$ is the volumetric expansion coefficient. 💧 Example: Water expands by about 0.23 % per °C. A 1‑L bottle of water will increase its volume by roughly 2.3 mL when heated from 20 °C to 80 °C.

Gases – Picture a balloon in a warm room

Gases are the most responsive to temperature changes. At constant pressure, the ideal gas law shows that volume is directly proportional to absolute temperature: $$V = \frac{nRT}{P} \;\;\;\Rightarrow\;\;\; \frac{\Delta V}{V_0} = \frac{\Delta T}{T_0}$$ So a 1‑L balloon at 20 °C (293 K) will expand to about 1.15 L when the temperature rises to 80 °C (353 K). 🌬️ Example: The air inside a car tyre expands by about 10 % when the temperature rises from 15 °C to 35 °C.

Quick Comparison Table

Key Take‑aways

1. All materials expand when heated, but gases expand the most, followed by liquids, then solids.
2. Expansion coefficients ($\alpha$, $\beta$, $\gamma$) quantify how much a material changes per degree of temperature change.
3. In engineering, expansion joints and clearances are designed to accommodate this behaviour.
4. Remember: Temperature ↑ → Volume ↑ (at constant pressure).