Change of State 🌡️ – Quick Notes
1. States of Matter & Their Transitions
- Solid → Liquid ❄️ to 💧: called melting / fusion.
- Liquid → Solid 💧 to ❄️: called freezing.
- Liquid → Gas 💧 to 💨: called vaporization.
- Solid ⇄ Gas ❄️⇄💨 (no liquid step): called sublimation (e.g., dry ice, iodine).
During each change the temperature stays flat—even while you keep heating or cooling—because the energy goes into breaking or making molecular bonds, not raising temperature. :contentReference[oaicite:0]{index=0}
2. Melting Point 🧊
The melting point is the temperature where solid and liquid coexist in thermal equilibrium. It depends on pressure; at standard atmospheric pressure we call it the normal melting point. :contentReference[oaicite:1]{index=1}
Cool demo: Press a weighted wire over ice. It slices through because pressure lowers the melting point just beneath the wire; water refreezes above it. This regelation lets ice skates glide smoothly. ⛸️ :contentReference[oaicite:2]{index=2}
3. Boiling Point 🔥
The boiling point is the temperature where liquid and its vapor coexist. Increase external pressure 🔼 and the boiling point rises (think pressure-cooker). Decrease pressure 🔽 (on a mountain) and the boiling point falls, so cooking takes longer. :contentReference[oaicite:3]{index=3}
4. Triple Point ⭐
At one special combination of pressure and temperature, all three phases show up together. For water that sweet spot is \(273.16\;\text{K}\) and \(6.11\times10^{-3}\;\text{Pa}\). :contentReference[oaicite:4]{index=4}
5. Latent Heat 💡
When a substance changes state, the latent heat \(L\) measures energy per kilogram needed for the jump:
\( Q = m\,L \) (no temperature change)
- Latent heat of fusion \(L_{f}\): for solid ⇄ liquid.
- Latent heat of vaporization \(L_{v}\): for liquid ⇄ gas.
For water at \(1\;\text{atm}\): \(L_{f}=3.33\times10^{5}\;\text{J kg}^{-1}\), \(L_{v}=22.6\times10^{5}\;\text{J kg}^{-1}\). Steam at \(100^\circ\text{C}\) packs much more energy than boiling water, so steam burns hurt more! 🥵 :contentReference[oaicite:5]{index=5}
5.1 Latent-Heat Values for Common Substances
| Substance | Melting Point (°C) | \(L_{f}\) (\(10^{5}\) J kg-1) | Boiling Point (°C) | \(L_{v}\) (\(10^{5}\) J kg-1) |
|---|---|---|---|---|
| Ethanol | –114 | 1.0 | 78 | 8.5 |
| Gold | 1063 | 0.645 | 2660 | 15.8 |
| Lead | 328 | 0.25 | 1744 | 8.67 |
| Mercury | –39 | 0.12 | 357 | 2.7 |
| Nitrogen | –210 | 0.26 | –196 | 2.0 |
| Oxygen | –219 | 0.14 | –183 | 2.1 |
| Water | 0 | 3.33 | 100 | 22.6 |
Use these numbers whenever you crunch heat-transfer questions. 💪 :contentReference[oaicite:6]{index=6}
6. Sample Heat-Change Equation
To warm ice from \(-12^\circ\text{C}\) to steam at \(100^\circ\text{C}\) you add four chunks of energy:
\(Q = m s_{\text{ice}}\Delta T_1 + m L_{f} + m s_{\text{water}}\Delta T_2 + m L_{v}\)
Just plug in each piece and add them up. 📈 :contentReference[oaicite:7]{index=7}
High-Yield Ideas for NEET 🔥
- Equation \(Q = m\,L\) and how to apply it in calorimetry problems.
- Why boiling point changes with pressure (pressure cooker vs. high altitude).
- Values of \(L_{f}\) and \(L_{v}\) for water and their role in everyday phenomena (steam burns, ice-skating).
- The concept and significance of the triple point—especially for water.
