A Journey through States of Water
Evaporation and Condensation
Water is found in three states: solid (ice), liquid (water) and gas (water vapour). Water changes from one state to another when it is heated or cooled. Two of these changes — evaporation and condensation — are the engine of the water cycle.
Evaporation is the slow change of a liquid into its vapour at the surface, without boiling. When water is warmed by the Sun, the water at the top changes into invisible water vapour and rises into the air. Wet clothes dry, puddles disappear and a wet floor dries because of evaporation. Evaporation is faster when it is hotter, windier, and when the wet surface is larger and the air is dry.
Condensation is the opposite change — the change of vapour back into liquid water on cooling. When warm, moist air touches a cold surface, the vapour cools and turns into tiny water drops. This is why water droplets form on the outside of a cold glass of water, why dew appears on grass on cool mornings, and why mist forms on a cold mirror when you breathe on it.
An important point: when salty sea water evaporates, only the water leaves as vapour — the salt is left behind. This is how clean water keeps moving through nature even though the sea is salty.
Drying is caused by evaporation, which speeds up in heat and wind.
- Heat gives the water energy to turn into vapour faster.
- Wind carries the vapour away, so more water can evaporate.
The cold glass cools the moist air around it.
- Water vapour in the air touches the cold glass and cools.
- It condenses into tiny water drops on the outside of the glass.
Think about what happens during evaporation.
- When sea water evaporates, only the water rises as vapour.
- The salt stays behind in the sea, so the vapour (and the rain it forms) is free of salt.
Key Points
- Water has three states: ice (solid), water (liquid) and water vapour (gas).
- Evaporation is liquid → vapour at the surface; it is faster in heat, wind, large surface and dry air.
- Condensation is vapour → liquid on cooling (dew, drops on a cold glass).
- When sea water evaporates, the salt is left behind, so the vapour is pure water.
The Water Cycle
Water is always on the move, travelling between the oceans, the air and the land in a never-ending journey called the water cycle. The Sun’s heat is the engine that drives it. The cycle has a few main steps:
- Evaporation: The Sun heats the water in oceans, rivers and lakes, turning it into water vapour that rises into the air. Plants also release water vapour from their leaves — this is called transpiration.
- Condensation: High up, the air is cold. The vapour cools and condenses into countless tiny water droplets, which gather to form clouds.
- Precipitation: When the droplets in a cloud join and grow heavy, they fall back to the ground as rain, or as snow and hail in cold places.
- Collection: The fallen water flows over land into rivers, lakes and oceans, or soaks into the ground as groundwater. From there the Sun warms it again and the cycle repeats.
The water cycle is vital: it gives us fresh rain water for drinking and farming, it keeps rivers and wells filled, and it uses the same water again and again — the water you drink today may once have been part of an ocean long ago.
Find the source of energy for evaporation.
- The Sun’s heat evaporates water and powers the cycle.
Recall how water moves up and comes back down.
- Water first evaporates, then condenses into clouds, then falls as precipitation.
Plants add water vapour to the air.
- This release of vapour from leaves is called transpiration.
Key Points
- The water cycle is the constant movement of water between sea, air and land, driven by the Sun.
- Steps: evaporation (and transpiration) → condensation (clouds) → precipitation (rain/snow) → collection.
- It gives fresh water for drinking and farming and refills rivers and wells.
- The cycle uses the same water again and again.
Water Conservation
Although water covers most of the Earth, only a very small part of it is fresh water that we can use for drinking, cooking and farming — most is salty sea water or frozen in ice. As the number of people grows and some areas face droughts (long periods without rain), clean water is becoming scarce. Water conservation means using water carefully and not wasting it, so that there is enough for everyone, now and in the future.
We can all save water with simple habits:
- Turn off the tap while brushing teeth or soaping hands.
- Repair leaking taps and pipes quickly — a dripping tap can waste a lot of water.
- Use a bucket instead of a running shower or hose.
- Reuse water where you can — for example, water used to wash vegetables can water plants.
Two important large-scale methods are:
- Rainwater harvesting — collecting and storing rainwater (from rooftops, for example) so it can be used later or allowed to soak into the ground to refill the groundwater.
- Drip irrigation — watering crops drop by drop directly at their roots, which uses far less water than flooding a field.
We should also keep our water clean by not throwing waste into rivers and lakes, because polluted water harms people, animals and plants.
Choose everyday water-saving habits.
- Turn off the tap while brushing teeth.
- Use a bucket for bathing instead of a running shower; also repair leaking taps.
It is a method of saving rainwater.
- It means collecting and storing rainwater, often from rooftops.
- The stored water can be used later, or it soaks into the ground to refill groundwater.
Think about who depends on clean water.
- Throwing waste pollutes the water.
- Polluted water harms people, animals and plants that use it.
Key Points
- Only a small part of Earth’s water is usable fresh water, so it must be saved.
- Water conservation = using water carefully and not wasting it.
- Simple habits: close taps, fix leaks, use a bucket, reuse water.
- Large-scale methods: rainwater harvesting and drip irrigation; also keep water clean.