How does water move through the environment, and how is it made safe to use?
The water cycle and its processes; sources of fresh water; water as a resource; and the treatment of water to make it safe to drink and to deal with waste water.
An SQA National 5 Environmental Science answer on the water cycle and water resources, covering evaporation, condensation, precipitation and other processes, sources of fresh water, the treatment of drinking water and waste water, and threats to water quality.
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What this dot point is asking
The SQA wants you to describe the water cycle and name its processes, identify sources of fresh water, explain why water is a valuable resource, and describe how water is treated to make it safe to drink and how waste water is dealt with, as well as the threats to water quality.
The water cycle
The water cycle (the hydrological cycle) is the continuous movement of water between the hydrosphere, atmosphere and land. Its processes are:
- Evaporation. Heat from the Sun turns liquid water from the sea, rivers and lakes into water vapour, which rises into the air.
- Transpiration. Plants release water vapour from their leaves, adding to the vapour in the air.
- Condensation. As vapour rises and cools, it condenses into tiny droplets, forming clouds.
- Precipitation. Droplets join and fall as rain, snow, sleet or hail.
- Run-off (surface flow). Water on land flows over the surface into rivers and back to the sea.
- Infiltration and groundwater flow. Some water soaks into the ground, becoming groundwater, which moves slowly and feeds rivers and springs.
Water as a resource and its sources
Water is essential for life, for farming, for industry and for homes. But most of the Earth's water is salt water in the oceans, which cannot be used directly. Only a small fraction is fresh water, and much of that is locked in ice, so the accessible supply in rivers, lakes and groundwater is limited. This is why fresh water is a precious resource that must be managed and not wasted or polluted.
Treating drinking water
Water taken from rivers, lakes or reservoirs is not safe to drink as it is, so it is treated in stages:
- Filtration. The water passes through filters (such as layers of sand and gravel) to remove larger solid particles and debris.
- Sedimentation (settling). The water stands so suspended solids sink to the bottom; chemicals can be added to make fine particles clump together and settle faster.
- Disinfection (chlorination). A disinfectant, usually chlorine, is added to kill bacteria and other harmful microorganisms, making the water safe to drink.
Treating waste water
Water used in homes and industry becomes waste water (sewage). If released untreated it pollutes rivers and seas, so it is treated first: solids are screened and settled out, microorganisms break down the dissolved organic matter, and the cleaned water is returned to the environment. Treating waste water protects water quality and aquatic life downstream.
Threats to water quality
Water quality is reduced by pollution:
- Sewage adds microorganisms and organic matter; if untreated it lowers oxygen in water and spreads disease.
- Fertiliser run-off from farmland adds nitrates and phosphates, which can cause eutrophication: rapid algal growth, then decay that uses up oxygen and kills aquatic life.
- Chemicals and oil from industry, and litter such as plastics, poison organisms and damage habitats.
Examples in context
Example 1. A reservoir supplying a city. Rain falling on hills collects in a reservoir (the store), is piped to a treatment works where it is filtered, settled and chlorinated, and is then supplied to homes. After use it returns as waste water to be treated before release. This traces water from the cycle through human use and back.
Example 2. Eutrophication in a farm pond. After heavy rain washes fertiliser from a field into a pond, algae grow rapidly and cover the surface. When the algae die, decomposers use up the oxygen, fish suffocate, and biodiversity falls. This shows how fertiliser run-off damages water quality.
Try this
Q1. Name the process in the water cycle by which water vapour cools to form clouds. [1 mark]
- Cue. Condensation.
Q2. State the purpose of adding chlorine during drinking-water treatment. [2 marks]
- Cue. Disinfection: chlorine kills bacteria and other harmful microorganisms, making the water safe to drink.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksDescribe the water cycle, naming four processes by which water moves between the sea, the air and the land.Show worked answer →
A 4-mark describe answer needs four named processes in a sensible order, so plan one mark per correctly described process.
Evaporation: heat from the Sun turns liquid water from the sea, rivers and lakes into water vapour, which rises into the air.
Condensation: as the vapour rises and cools, it condenses into tiny droplets, forming clouds.
Precipitation: the droplets join and fall as rain, snow, sleet or hail onto the land and sea.
Run-off (or surface flow): water that falls on land flows over the surface into rivers, which carry it back to the sea, completing the cycle. Transpiration from plants and infiltration into the ground would also score.
Markers reward each correctly named and described process. A bare list of words without descriptions may not gain full marks.
SQA N5 style3 marksDescribe the main stages used to treat water to make it safe to drink.Show worked answer →
This asks for the drinking-water treatment stages, so describe each step and what it removes.
Filtration: the water is passed through filters (such as sand and gravel) to remove larger solid particles and debris.
Sedimentation or settling: the water is allowed to stand, sometimes with chemicals added to clump particles together, so suspended solids sink and can be removed.
Disinfection (chlorination): a disinfectant such as chlorine is added to kill bacteria and other harmful microorganisms, making the water safe to drink.
Markers reward the correct stages described in a sensible order. Naming only one step, or describing waste-water treatment instead of drinking-water treatment, loses marks.
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