Ch8: A Journey Through States of Water

Water (chemical formula H₂O) exists in three states depending on temperature and pressure:

• Solid: Ice, snow, hail, frost, glaciers, permafrost. Water molecules locked in a rigid crystal lattice.
• Liquid: Water, dew, rain, clouds (liquid droplets), fog. Molecules move freely but stay close.
• Gas: Water vapour, steam. Molecules move rapidly and spread out. Water vapour is invisible — "steam" you see is actually tiny liquid droplets.

All three states are chemically identical (H₂O). Changes between states are physical changes — no new substance is formed.

Boiling point changes with altitude: At higher altitudes, atmospheric pressure is lower, so water boils at a lower temperature.

• Sea level: 100°C
• Darjeeling (~2,000 m): ~93°C
• Leh (~3,500 m): ~88°C
• Siachen Base Camp (~6,400 m): ~84°C

Consequence: Food takes longer to cook at high altitude because water boils at a lower temperature, reducing the cooking temperature. This is why pressure cookers are essential in the hills — they increase pressure and raise the boiling point.

Sublimation applications:

• Freeze-drying (lyophilisation): Food and pharmaceuticals are frozen, then water is sublimed out under vacuum. Preserves flavour, nutrition, and extends shelf life (space food, military rations, coffee powder).
• Dry ice: Solid CO₂ sublimates; used in food transport and special effects.
• Removal of frost from aircraft wings using de-icing (prevents sublimation from solid state).

Anomalous expansion: Most substances contract (get denser) as they cool. Water does the opposite between 4°C and 0°C — it expands on freezing.

• Water is densest at 4°C (density ~1.0 g/cm³).
• Ice has density ~0.92 g/cm³ — less dense than liquid water.
• Therefore, ice floats on water.

Ecological significance: In winter, surface water of lakes cools to 4°C and sinks (it's densest). Below 4°C, water becomes less dense and stays at the top. The surface freezes into ice, which acts as an insulating layer. The liquid water below stays at ~4°C, allowing aquatic life (fish, microorganisms) to survive. Without this unique property, entire water bodies would freeze solid and aquatic ecosystems would collapse.

Engineering significance: Burst water pipes in winter — water expands when it freezes, cracking pipes. This is a major infrastructure problem in cold regions (Himalayan towns, North America, Europe).

Steps of the water cycle:

1. Evaporation: Solar energy heats surface water (oceans cover 71% of Earth's surface; oceans contribute ~86% of total evaporation). Water vapour rises.
2. Transpiration: Plants release water vapour through stomata in leaves. Evaporation + Transpiration = Evapotranspiration (ET). Forests are critical for ET — deforestation disrupts local water cycles.
3. Condensation: Rising water vapour cools at altitude and condenses into tiny droplets around dust/pollen particles (condensation nuclei), forming clouds.
4. Precipitation: Water falls as rain, snow, sleet, or hail depending on temperature.
5. Surface runoff: Water flows over land into rivers and streams.
6. Infiltration: Water seeps into soil and recharges groundwater (aquifers).
7. Return to oceans: Rivers carry water back to the sea, completing the cycle.

Solar energy drives the entire cycle. The water cycle is also a heat redistribution mechanism — latent heat absorbed during evaporation is released during condensation, moderating global temperatures.