P5 Science - Water Cycle | Notes

The water cycle is the continuous movement of water from Earth to the sky and back to Earth.

• The heat from the Sun is the driving force for the water cycle
• Water evaporates from the ground and water bodies such as seas and rivers
• Living things also release water vapour through breathing, perspiration and urination
• The water vapour condenses to form tiny water droplets
• The water droplets gather to form clouds
• When the water droplets in the clouds get too heavy, they fall as rain
• Some rain seeps into the ground and some rain is collected in the water bodies such as rivers and lakes

Evaporation and condensation are two important processes that enable the water cycle to occur.

• Water changes its state during these two processes
• During evaporation, water (liquid state) gains heat from the Sun and evaporates into water vapour (gaseous state)
• During condensation, water vapour (gaseous state) loses heat to the surrounding air and condenses back into water droplets (liquid state)
• Other than rain (liquid state), water can also fall back to Earth as snow (solid state) and hailstones (solid state)

• Water is essential for the survival of living things
• The water cycle ensures the continuous supply of fresh water on Earth
• During evaporation, salt and dirt are left behind in the water bodies and the ground
• When it rains, some dirt in the air such as chemicals and dust particles are removed by the rainwater — this cleans the air

Water is needed to carry out life processes in the human body.

Digestive System:

• Water is needed to digest food in the digestive system
• Water in saliva softens the food so that it is easier to swallow
• Digestive juices produced in the digestive system need water to work
• Water helps to dissolve the nutrients in digested food so that they can be absorbed into the bloodstream
• Water helps to soften solid waste so that it can be passed out of the body easily

Respiratory System:

• Water is needed for the exchange of gases to take place in the respiratory system
• The exchange of carbon dioxide for oxygen happens in the layer of water which lines the air sacs in our lungs

Circulatory System:

• Water is needed for the transport of substances in the circulatory system
• Digested food, oxygen and waste materials are transported by blood, which is made up mainly of water

• Seeds require water to germinate and grow
• Plants need water and carbon dioxide to make food in the presence of light during photosynthesis
• Water is needed for the transport of substances such as mineral salts and food in the water-carrying tubes and food-carrying tubes of plants respectively

• Only a very little amount of water on Earth is fresh water suitable for human consumption
• Clean water is a limited natural resource which is being threatened by some human activities
• Water pollution is caused by the release of harmful substances into our water resources
• Untreated wastes from homes and factories may release deadly chemicals and foul smell affecting living things living in and near the water
• Oil spills from ships cause the living things in the sea to die from a lack of oxygen
• Oil spills also cause the feathers of birds to clump together and this reduces their ability to keep the birds warm
• When water is polluted, we have fewer clean water sources to use
• If rivers and seas are polluted, we will have lesser food to eat

Conservation of water is using water wisely and not wasting water.

We can conserve water by reducing, reusing and recycling water.

• The water cycle is driven by heat from the Sun — a continuous loop of evaporation and condensation
• Water changes state during the cycle: liquid (water), gas (water vapour), solid (snow, hail)
• Evaporation = gains heat; Condensation = loses heat
• The water cycle ensures a continuous supply of fresh water on Earth
• Water is essential for human life processes: digestion, respiration, and circulation
• Water is essential for plant life processes: germination, photosynthesis, and transport
• Water pollution threatens this limited natural resource — conserve by reducing, reusing, and recycling