Water and Changes of State; Water Cycle
Water and Changes of State; Water Cycle
Water cycle is the continuous movement of water from the earth, to the sky and back to earth. It can occur because water is able to change into the solid, liquid and gaseous states.
Evaporation and condensation enable water cycle to take place
Heat energy from the Sun causes water on the earth to evaporate, forming water vapour. Water evaporates from ground and water bodies. Plans and animals also release water vapour into the air.
Warm air cools as it rises to the sky. The water vapour in it condenses into tiny water droplets, which gather to form clouds.
When the water droplets become big and heavy, they fall from the clouds as rain, snow or hail. Clouds are made up of tiny water droplets. Thus they are in the liquid state. Water droplets are visible but water vapour is not.
Rain falls onto the earth as fresh water. Some of it falls onto the ground and into water bodies.
Water can exist in 3 interchangeable states: solid (ice), liquid (water in reservoir, from tap) and gaseous (water vapour and steam)
Water is able to change from one state to another by gaining or losing heat
Heat loss - freezing (liquid to solid) and condensation (gas to liquid)
Freezing point of water is the temperature at which the liquid (water) changes from a liquid state to a solid state due to heat loss
Condensation is the change in the state of water from a gas (steam or water vapour) to a liquid (water) due to heat loss.
Heat gain - melting (solid to liquid) and boiling (liquid to gas)
Melting is the change in the state of water from a solid (ice) to a liquid (water) due to heat gain. Melting is the process by which a solid
from the surroundings and becomes a liquid.
Boiling is the change in state of water from a liquid (water) to a gas (steam) due to heat gain. Boiling and evaporation is the process by which a liquid
from the surroundings and becomes a gas.
Evaporation is the change in the state of water from a liquid (water) to a gas (water vapour) due to heat gain. Unlike boiling, evaporation occurs all the time and at any temperature below boiling point.
Evaporation produces a cooling effect. When water evaporates, it gains heat from its surroundings. Thus, the surroundings lose heat and cooling is experienced.
Factors affecting the rate of evaporation are temperature, presence of wind, area of exposed surface area and humidity of air
The higher the temperature, the higher the rate of evaporation
When water evaporates, the water vapour it forms usually collects just above the water surface. When wind blows, it takes away the water vapour and creates space for more water vapour to form. The stronger the wind, the higher the rate of evaporation.
Area of exposed surface - is the area of the water that is exposed to air. The greater the area of exposed surface, the higher the rate of evaporation
Humidity is the measure of the amount of water vapour in the air. The lower the humidity, the higher the rate of evaporation. On a dry day, humidity is low and there is little water vapour in the air. Thus it is easier for water to evaporate. On a humid day, humidity is high and there is a lot of water vapour in the air. Thus it is more difficult for water to evaporate.
Why is water cycle important? All living things need water to stay alive. The water cycle is the only natural way to refresh water. It ensures a constant supply of fresh water for living things on Earth. Evaporation helps to separate water from the dirt and salt on the ground and in water bodies.
Water is an important, yet limited resource on Earth. Water conservation is the careful use of water in order to avoid its wastage. It can be promoted through campaigns, posters and water rationing exercises.
Reduce: take a shower instead of a bath, turn off tap when brushing teeth
Reuse - use rainwater to wash floors, use water from washing machine to wash toilets
Recycle - treat waste water and make it usable
Need for water in humans - in the digestive system, water is needed to digest food, to absorb digested food and to pass out waste. In the respiratory system, water is needed for gaseous exchange between the lungs and the air we inhale. In the circulatory system, water is needed in the blood to carry oxygen and digested food to different parts of the body.
Water is needed in plants: for germination to occur, a seed needs sufficient water to activate the life processes within the seed itself. During photosynthesis, plants need water and carbon dioxide in the presence of sunlight to make food. In the plant transport system, water helps in the movement of substances within the plant.