Introduction
Water is the most important substance on Earth. Uniquely, water exists naturally in all three states of matter — solid (ice), liquid (water), and gas (water vapour) — and it continuously changes between these states. This endless cycle is vital to life and weather on our planet.
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Key Concepts and Definitions
- States of water:
- Solid (Ice): Water below 0°C. Ice has a definite shape and volume.
- Liquid (Water): Water between 0°C and 100°C. Takes the shape of its container but has a fixed volume.
- Gas (Water vapour / Steam): Water above 100°C or water that has evaporated at lower temperatures.
Melting: Change from solid to liquid. Ice melts at 0°C (melting point of water).
Freezing: Change from liquid to solid. Water freezes at 0°C (freezing point of water).
Evaporation: Change from liquid to gas at any temperature below boiling point. Faster when temperature is high, humidity is low, surface area is large, or wind is present.
Boiling: Rapid change from liquid to gas throughout the liquid, at 100°C (boiling point of water at sea level).
Condensation: Change from gas (water vapour) to liquid. Seen as dew, fog, clouds, and water droplets on a cold glass.
Sublimation: Direct change from solid to gas without passing through liquid (e.g., dry ice, camphor).
The Water Cycle (Hydrological Cycle): The continuous movement of water on, above, and below Earth's surface through evaporation, condensation, and precipitation.
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Factors Affecting Evaporation
- 1.Evaporation increases when:
- 2.Temperature increases — more molecules gain enough energy to escape.
- 3.Surface area increases — more molecules are exposed to air.
- 4.Humidity decreases — drier air can hold more water vapour.
- 5.Wind speed increases — wind carries water vapour away, making room for more evaporation.
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Worked Examples
Melting of ice in a glass
- Ice cubes at -5°C are placed in a glass at room temperature.
- Heat from the surroundings warms the ice to 0°C.
- At 0°C, ice melts to liquid water.
- Change: solid → liquid (melting).
Wet clothes drying on a line
- Water in wet clothes evaporates even at room temperature (25–35°C).
- Clothes dry faster on a sunny, windy day because both temperature and wind speed are high.
- Change: liquid → gas (evaporation).
Formation of dew on grass
- At night, the ground cools quickly.
- Water vapour in air near the ground cools below its dew point.
- Water vapour condenses into tiny liquid droplets on grass blades.
- Change: gas → liquid (condensation).
Boiling water for cooking
- Water is heated on a stove.
- At 100°C, bubbles form throughout the liquid and rise to the surface.
- Steam (water vapour) is produced rapidly.
- Change: liquid → gas (boiling at boiling point).
Making ice in a freezer
- Liquid water is placed in trays in a freezer (temperature around -18°C).
- Water loses heat and its temperature drops to 0°C.
- At 0°C, water freezes into ice.
- Change: liquid → solid (freezing).
Cloud and rain formation
- The Sun heats oceans and lakes → water evaporates → rises as vapour.
- At high altitude, vapour cools and condenses around tiny dust particles → forms clouds (tiny water droplets).
- Droplets join → grow heavier → fall as rain (precipitation).
Why a cold bottle gets wet on the outside
- The glass bottle is much colder than the surrounding air.
- Water vapour in air near the bottle condenses on the cold surface.
- This is condensation — gas → liquid.
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Key Points to Remember
- Melting point and freezing point of water = 0°C
- Boiling point of water = 100°C (at sea level)
- Evaporation occurs at any temperature; boiling only at 100°C
- The water cycle: evaporation → condensation → precipitation → collection
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Common mistakes
- Confusing evaporation (slow, at any temperature, from surface only) with boiling (fast, at 100°C, throughout the liquid).
- Thinking water vapour is visible — it is not. Steam (tiny water droplets) is visible; pure water vapour is invisible.
- Saying ice "melts" in a freezer — it actually freezes. Direction matters.
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Summary
Water exists as solid, liquid, and gas. Changes of state — melting, freezing, evaporation, boiling, condensation — are driven by temperature and energy changes. The water cycle continuously moves water through these states, sustaining all life on Earth.