Singapore MOE · Primary 6 · Science

Water and Changes of State

Revision notes · 3 parts · 2026-07-16

Understanding Changes of State

Water is essential for life and exists in three states: solid, liquid, and gas. This section helps you understand how water changes between these states through processes like melting, freezing, evaporation, boiling, and condensation. You will also learn about the factors that affect how fast water evaporates and how these changes explain everyday observations around you.

❓ Before you read — have a guess: What are the three states in which water can exist?

Solid (ice), liquid (water), and gas (water vapour).

⚡ Key points — the 6 things to remember

Water exists as solid (ice), liquid (water), and gas (water vapour).
Water changes state by gaining or losing heat.
Evaporation occurs at the surface of a liquid at any temperature, turning liquid into invisible water vapour.
Boiling occurs throughout a liquid at a fixed boiling point, turning liquid into invisible water vapour.
Condensation is when invisible water vapour loses heat and changes back into liquid water.
The rate of evaporation increases with higher temperature, faster wind speed, and larger exposed surface area.

What you must be able to do

Describe the processes of evaporation, condensation, melting, freezing, and boiling as changes of state of water.
Identify that water exists in three states: solid (ice), liquid (water), and gas (water vapour).
Describe the conditions (heating/cooling) that cause water to change between states.
Explain that evaporation occurs at the surface of a liquid at any temperature, while boiling occurs throughout the liquid at a fixed boiling point.
Describe how temperature, wind speed, and surface area affect the rate of evaporation.
Explain everyday phenomena using changes of state (e.g., dew formation, drying of clothes, sweating, formation of water droplets on a cold glass).

States of Water and Basic Changes

Water can exist in three forms or states: solid (like ice), liquid (like drinking water), and gas (like invisible water vapour in the air). These states can change when water gains or loses heat.

Melting is when a solid changes to a liquid by gaining heat.
Freezing is when a liquid changes to a solid by losing heat.
The melting point is the fixed temperature at which a solid changes to a liquid.

⚠️ Common Mistake Thinking sugar 'melts' in water. — Correct: Melting is a change of state from solid to liquid. Dissolving is when a substance mixes evenly into a liquid to form a solution. Sugar dissolves in water, it does not melt.

💡 Exam Tip When describing changes of state, always mention whether heat is gained or lost. For example, 'ice melts by gaining heat'.

📍 Singapore Focus When you put ice cubes into your drink or enjoy an ice kacang, the ice (solid water) melts into liquid water as it gains heat from the warmer surroundings.

Evaporation and Boiling

Evaporation is the process where liquid water changes into invisible water vapour (a gas) from the surface of the liquid. Boiling is another process where liquid water changes into water vapour, but it happens throughout the liquid.

Evaporation can happen at any temperature, even below the boiling point.
Boiling happens only at a specific temperature called the boiling point (100°C for pure water).
Both processes require water to gain heat to change from liquid to gas.

⚠️ Common Mistake Confusing evaporation with boiling, thinking water must be heated to 100°C to evaporate. — Correct: Evaporation happens at any temperature, even below 100°C. Boiling only happens at the boiling point (100°C for pure water).

⚠️ Common Mistake Thinking water vapour is visible. — Correct: Water vapour is an invisible gas. What you see as 'steam' or 'mist' is actually tiny liquid water droplets formed when water vapour cools and condenses.

⚠️ Common Mistake Thinking that drying of wet clothes means the water disappears rather than evaporates into the air. — Correct: The water from wet clothes changes into invisible water vapour and mixes with the air. It does not disappear.

⚠️ Common Mistake Students may think that boiling and evaporation are the same process simply occurring at different speeds. — Correct: Evaporation occurs only at the surface of the liquid at any temperature. Boiling occurs throughout the liquid at a fixed boiling point. They are distinct processes.

💡 Exam Tip When asked to compare evaporation and boiling, clearly state two differences: where they occur (surface vs. throughout) and at what temperature (any temp vs. boiling point).

💡 Exam Tip Avoid using the word 'steam' when referring to water vapour; use 'water vapour' for the invisible gas and 'tiny water droplets' for visible steam/mist.

📍 Singapore Focus When you hang wet laundry outdoors in Singapore's sunny weather, the water in the clothes evaporates into the air, making them dry.

Condensation and Factors Affecting Evaporation

Condensation is the process where invisible water vapour (a gas) changes back into liquid water. This happens when water vapour loses heat and cools down. The rate of evaporation refers to how fast water changes from liquid to gas.

Condensation occurs when moist air touches a surface that is cooler than the air, causing the water vapour to lose heat.
Dew is liquid water droplets formed by condensation on cool surfaces, like grass, in the morning.
The rate of evaporation is affected by:
1. Temperature: Higher temperature leads to faster evaporation.
2. Wind speed: Faster wind leads to faster evaporation.
3. Exposed surface area: Larger surface area leads to faster evaporation.

⚠️ Common Mistake Believing that condensation on a cold glass means the glass is leaking or that water passes through the glass. — Correct: The water droplets on the outside of a cold glass come from the water vapour in the surrounding air, not from inside the glass.

⚠️ Common Mistake Students often confuse condensation with the cold surface itself producing water or sweating. — Correct: The cold surface causes water vapour in the air to lose heat and change into liquid water droplets. The surface itself does not produce water.

⚠️ Common Mistake Believing that condensation only occurs when it is 'very cold'. — Correct: Condensation occurs when moist air touches any surface that is cooler than the air, causing the water vapour to cool and turn into liquid. This can happen even on warm days in humid places like Singapore.

💡 Exam Tip When explaining condensation, always mention 'water vapour in the air' and 'loses heat' or 'cools down' to form 'liquid water droplets'.

💡 Exam Tip For factors affecting evaporation, remember the three key factors: temperature, wind speed, and exposed surface area. Explain how each factor increases or decreases the rate.

📍 Singapore Focus In Singapore's humid mornings, dew forms on grass or car windscreens as water vapour in the air cools and condenses on these cooler surfaces. When you have a cold drink, water droplets form on the outside of the glass due to condensation of water vapour from the surrounding air. Also, your spectacle lenses may fog up when moving from an air-conditioned room to the outdoors because the cooler lenses cause water vapour in the warmer outdoor air to condense on them. To make clothes dry faster, you can spread them out (increase surface area), hang them in the sun (increase temperature), or in a windy spot (increase wind speed).

🪤 Watch for these traps

The wrong answers examiners plant to catch you — and how to dodge them.

Tempting answerSteam is water vapour.

Looks right because Steam is visible and is produced when water is heated, which is related to water changing to a gas.

But actually Water vapour is an invisible gas. What we see as 'steam' or 'mist' is actually tiny liquid water droplets that have condensed from water vapour.

✓ How to avoid it Remember that gases are generally invisible. If you can see it, it's likely tiny liquid droplets, not the gas itself.

Tempting answerThe water droplets on a cold glass come from the drink inside the glass.

Looks right because The glass is cold and contains liquid, so it seems like the water is 'leaking' or 'sweating' from within.

But actually The water droplets form from the invisible water vapour in the surrounding air. This water vapour loses heat and condenses into liquid water when it touches the cold surface of the glass.

✓ How to avoid it Think about the source of the water. If the glass is sealed, the water must come from the outside air, not the inside.

Tempting answerBoiling is just a faster version of evaporation.

Looks right because Both processes involve liquid water changing into water vapour.

But actually Evaporation occurs only at the surface of the liquid at any temperature, while boiling occurs throughout the liquid at a fixed boiling point. They are distinct processes with different conditions.

✓ How to avoid it Always recall the two key differences: where the change happens (surface vs. throughout) and the temperature requirement (any temp vs. fixed boiling point).

🔍 Question Decoder

What exam phrasings are really asking for.

When the question says…	It is testing…	Your move
Explain why clothes dry faster on a sunny, windy day.	Factors affecting the rate of evaporation.	Relate high temperature and high wind speed to a faster rate of evaporation.
What causes water droplets to form on the outside of a cold can?	The process of condensation.	Explain that water vapour in the surrounding air loses heat and condenses on the cold surface.
Describe two differences between evaporation and boiling.	Comparison of evaporation and boiling processes.	State where each process occurs (surface vs. throughout) and the temperature conditions (any temperature vs. fixed boiling point).

🧭 Answer → Because → Link

How to answer: Open-ended explanation questions

1Answer the question directly (the claim). 1 mark
2Provide scientific evidence or reason (because...). 1 mark
3Link back to the question or concept using keywords (therefore...). 1 mark

Example: Explain why a wet towel spread out dries faster than one left bunched up.

✅ Earns the mark

“A wet towel spread out dries faster (Answer) because it has a larger exposed surface area, allowing more water to evaporate into the air (Because). Therefore, the rate of evaporation is faster, and the towel dries more quickly (Link).”

This answer directly addresses the question, provides the scientific reason (larger exposed surface area, more water evaporates), and links it to the outcome (faster rate of evaporation, dries more quickly) using precise keywords.

❌ Loses the mark

“The spread out towel dries faster because it is open.”

This answer is too vague. 'Open' is not a scientific term. It does not mention 'exposed surface area', 'evaporation', or 'rate of evaporation', which are key scientific concepts for earning marks.

🗝️ Words that earn marks

gains heat Describing melting or evaporation/boiling.

loses heat Describing freezing or condensation.

changes state Referring to water transforming between solid, liquid, or gas.

water vapour Referring to the invisible gaseous form of water.

rate of evaporation Discussing how quickly water turns into gas.

exposed surface area Explaining how spreading out a liquid affects evaporation.

condenses Describing water vapour turning into liquid water droplets.

evaporates Describing liquid water turning into water vapour.

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The Water Cycle

The water cycle describes the continuous journey of water on, above, and below the Earth's surface. It involves water changing between its solid, liquid, and gaseous states through processes like evaporation, condensation, and precipitation. Plants also play a role by releasing water vapour into the air.

❓ Before you read — have a guess: What are the three main processes that move water through the water cycle?

Evaporation, condensation, and precipitation.

⚡ Key points — the 6 things to remember

The water cycle is the continuous movement of water on Earth.
Water changes state (liquid to gas, gas to liquid) as it moves through the cycle.
Evaporation turns liquid water into invisible water vapour.
Condensation turns water vapour into tiny liquid droplets, forming clouds.
Precipitation is water falling from clouds, usually as rain.
Transpiration is when plants release water vapour into the atmosphere.

What you must be able to do

Describe how water changes state in the water cycle, including evaporation, condensation, and precipitation.
Identify that plants release water vapour through transpiration, which is part of the water cycle.

Understanding the Water Cycle

The Water cycle is the continuous movement of water on, above, and below the Earth's surface. It is a natural process that constantly recycles water.

Water exists in three states: solid (ice), liquid (water), and gas (water vapour).
The water cycle involves water changing between these states.
This cycle ensures that water is continuously available on Earth.

The main stages of the water cycle.

The water cycle describes how water travels from oceans, lakes, and land into the atmosphere and then back to Earth. This movement is driven by the Sun's energy, which causes water to change its state.

💡 Exam Tip When asked to describe the water cycle, always mention the key processes and how water changes its state at each stage.

📍 Singapore Focus The formation of clouds and rain, which we often see in Singapore's tropical climate, is a key part of the water cycle.

Key Processes: Evaporation, Condensation, Precipitation, and Transpiration

These are the main processes that move water through the water cycle, involving changes of state and movement of water vapour.

Evaporation turns liquid water into invisible water vapour.
Condensation turns invisible water vapour into tiny liquid water droplets, forming clouds.
Precipitation is water falling from clouds to the Earth's surface, usually as rain.
Transpiration is the process where plants release water vapour from their leaves into the atmosphere.

The Sun's heat causes water from oceans, rivers, and even wet land to turn into an invisible gas called water vapour. This process is called Evaporation. Plants also release water vapour into the air through their leaves; this is called Transpiration. As this water vapour rises, it cools down. When it cools enough, the water vapour changes back into tiny liquid water droplets, which group together to form clouds. This process is Condensation. When these clouds become heavy with water droplets, the water falls back to Earth as rain, snow, or hail. This is called Precipitation. The water then collects in oceans, rivers, and on land, ready to evaporate again.

Summary of Water Cycle Processes
Process	Change of State	What Happens
Evaporation	Liquid to Gas	Liquid water turns into invisible water vapour.
Transpiration	Liquid to Gas	Plants release water vapour from their leaves.
Condensation	Gas to Liquid	Water vapour turns into tiny liquid water droplets (clouds).
Precipitation	No change of state	Water (e.g., rain) falls from clouds to Earth.

⚠️ Common Mistake Only evaporation from water bodies like oceans and rivers adds water to the air in the water cycle. — Correct: Besides evaporation from water bodies and land, plants also release water vapour into the air through transpiration, which is an important part of the water cycle.

💡 Exam Tip Remember that water vapour is an invisible gas. Visible clouds or steam are actually tiny liquid water droplets that have already condensed.

💡 Exam Tip When explaining transpiration, clearly state that it is plants releasing water vapour.

📍 Singapore Focus Deforestation, which is the cutting down of many trees in neighbouring regions, can reduce the amount of transpiration. This can lead to less water vapour in the atmosphere and potentially affect rainfall patterns in those areas.

🪤 Watch for these traps

The wrong answers examiners plant to catch you — and how to dodge them.

Tempting answerClouds are made of water vapour.

Looks right because Clouds are formed from water in the air.

But actually Water vapour is an invisible gas. Clouds are visible because they are made of tiny liquid water droplets or ice crystals that have condensed from water vapour.

✓ How to avoid it Remember that water vapour is invisible. If you can see it (like clouds or steam), it's already condensed into liquid droplets.

Tempting answerWater disappears when it dries.

Looks right because We cannot see the water after a wet surface dries.

But actually The water does not disappear; it changes state from liquid to an invisible gas called water vapour through evaporation.

✓ How to avoid it Always think about changes of state. Water does not vanish; it transforms into an invisible gas.

🔍 Question Decoder

What exam phrasings are really asking for.

When the question says…	It is testing…	Your move
Describe the water cycle.	Your understanding of the main processes (evaporation, condensation, precipitation) and the changes of state involved.	List the processes in order, explaining what happens at each stage and mentioning the state changes (liquid to gas, gas to liquid).
How do plants contribute to the water cycle?	Your knowledge of the process of transpiration.	Explain that plants release water vapour into the atmosphere through transpiration.

🧭 Answer → Because → Link

How to answer: Open-ended questions requiring an explanation of a scientific phenomenon.

1Directly answer the question (make your claim). 1 mark
2Provide scientific evidence or reasons (start with 'Because...'). 1 mark
3Connect your evidence back to the scientific concept or the question (Link). 1 mark

Example: Explain how deforestation can affect the water cycle in a region.

✅ Earns the mark

“Deforestation can reduce the amount of rainfall in a region. Because trees release water vapour into the atmosphere through transpiration, cutting them down means less water vapour is added to the air. This leads to less water vapour available to condense and form clouds, resulting in less precipitation.”

This answer directly states the effect (less rainfall), provides the scientific reason (less transpiration, less water vapour), and links it to the water cycle processes (less condensation, less precipitation), using precise keywords.

❌ Loses the mark

“Cutting down trees makes the place hotter and drier, so there is less rain.”

This answer states a general effect but lacks scientific keywords like 'transpiration', 'water vapour', 'condensation', and 'precipitation'. It does not explain the scientific mechanism clearly.

🗝️ Words that earn marks

Evaporation describing liquid water turning into invisible water vapour.

Condensation describing water vapour turning into tiny liquid water droplets (forming clouds).

Precipitation describing water (e.g., rain) falling from clouds to the Earth's surface.

Transpiration describing plants releasing water vapour into the atmosphere.

Water vapour referring to the invisible gaseous state of water in the air.

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Water as a Precious Resource: Scarcity and Management

Water is essential for all life, but fresh water, which we need for drinking and daily use, is a very limited resource on Earth. This section explains why fresh water is scarce, how human actions can affect its availability, and how Singapore manages its water supply through strategies like the Four National Taps. You will also learn about the importance of conserving water for a sustainable future.

❓ Before you read — have a guess: Is most of Earth's water readily available for us to drink and use?

No, most of Earth's water is salt water or trapped in ice, making fresh water a very limited resource.

⚡ Key points — the 6 things to remember

Only a small fraction of Earth's total water is accessible fresh water, making it a precious resource.
Human activities like deforestation and urbanisation can negatively impact the natural water cycle and reduce water availability.
Singapore uses a diversified approach called the Four National Taps to ensure a secure and sustainable water supply.
NEWater is treated used water purified to a high standard, making it safe for various uses and even indirect drinking.
Desalination converts salt water into fresh water but is an energy-intensive and costly process.
Water conservation is crucial for sustainable living and involves reducing water use in homes, schools, and industries.

What you must be able to do

Explain why fresh water is limited, even though Earth has a lot of water.
Describe how human activities like cutting down forests and building cities can change the water cycle and affect how much water is available.
Explain Singapore's water management plans, including the Four National Taps: local catchment water, imported water, NEWater, and desalination.
Explain why saving water is important and suggest ways to conserve water in different situations.

Earth's Limited Fresh Water

Fresh water is water that has very little salt, making it suitable for drinking, agriculture, and most human uses. Salt water, found in oceans, contains high levels of salt and cannot be used directly.

About 71% of Earth's surface is covered by water.
However, most of this water (about 97%) is salt water in oceans.
Only about 3% of Earth's water is fresh water.
A large portion of this fresh water is trapped in glaciers (large sheets of ice) and ice caps (permanent ice covering land at the poles).
This means less than 1% of Earth's total water is easily accessible fresh water for human use.

Most of Earth's water is salt water, with only a small fraction being fresh water.

Despite Earth appearing to have abundant water, the amount of fresh water we can actually use is very small. This makes fresh water a precious and limited resource.

⚠️ Common Mistake Many people think water is unlimited because 71% of Earth is covered by water. — Correct: While Earth has a lot of water, most of it is salt water. Only a tiny fraction (less than 1%) is accessible fresh water for our daily needs.

💡 Exam Tip When explaining why fresh water is limited, always mention both the high percentage of salt water AND the fresh water trapped in glaciers/ice caps. Do not just say 'most is salt water'.

📍 Singapore Focus Countries in arid regions, like some in the Middle East, face severe fresh water shortages and rely heavily on expensive methods like desalination to meet their water needs. This highlights why fresh water is so precious globally.

How Human Activities Affect Water

Human activities can change the natural water cycle, affecting how much water is available in an area. Two major activities are deforestation and urbanisation.

Deforestation is the clearing of forests for other land uses, such as farming or building.
When forests are cut down, there are fewer plants to carry out transpiration (release water vapour into the air). This can reduce the amount of water vapour in the atmosphere, potentially leading to less rainfall.
Trees also help to absorb rainwater and slow down surface runoff (water flowing over the land surface). Without trees, more rainwater flows away quickly, increasing soil erosion and reducing the amount of water that soaks into the ground.
Urbanisation is the process of building cities and towns, replacing natural land with concrete and buildings.
In urban areas, concrete and paved surfaces prevent rainwater from soaking into the ground. This increases surface runoff, which can lead to flash floods and less water replenishing underground sources.
Increased runoff also means less water evaporates from the land, affecting the local water cycle.

Human actions can disrupt the delicate balance of the water cycle. By removing forests or covering land with concrete, we change how water moves between the land, air, and water bodies, which can lead to less available fresh water or increased flooding.

Impact of Human Activities on Water Cycle
Activity	Effect on Water Cycle	Impact on Water Availability
Deforestation	Less transpiration, more surface runoff, less water soaking into ground.	Reduced rainfall, less groundwater, increased soil erosion.
Urbanisation	More impermeable surfaces (concrete), increased surface runoff, less water soaking into ground.	Increased risk of flash floods, less groundwater recharge.

💡 Exam Tip When asked about human impact, always link the activity (e.g., deforestation) to a specific change in the water cycle (e.g., less transpiration) and then to the consequence (e.g., less rainfall).

📍 Singapore Focus In Singapore, urbanisation with many HDB estates and concrete surfaces means that rainwater quickly becomes surface runoff. Our drainage systems are designed to manage this, but it also means less water naturally soaks into the ground to become groundwater.

Singapore's Four National Taps

Singapore, being a small island nation with limited natural water resources, has developed a comprehensive strategy called the Four National Taps to ensure a sustainable and diversified water supply.

Local catchment water: Rainwater collected in reservoirs. Singapore has expanded its network of drains, canals, and reservoirs to collect as much rainwater as possible.
Imported water: Water bought from Malaysia, as agreed upon by long-term water agreements.
NEWater: High-grade reclaimed water produced from treated used water. It undergoes advanced purification processes, including membrane technologies like reverse osmosis, to remove impurities and make it ultra-clean and safe for use.
Desalination: The process of removing salt from seawater to produce fresh water. This is done at desalination plants.
NEWater is mainly used for industrial purposes, such as in wafer fabrication plants and air-conditioning cooling towers. It is also introduced into reservoirs to blend with raw water before being treated for drinking, known as indirect potable use.
The Four National Taps ensure Singapore has a resilient water supply, reducing reliance on any single source.

Singapore's approach to water management is a model for other countries. By diversifying its water sources, Singapore has transformed from a water-scarce nation to one with a secure and sustainable water supply.

⚠️ Common Mistake Some people think desalination is a simple and cost-free way to get fresh water. — Correct: Desalination is a complex process that requires a lot of energy and is therefore expensive. It is not a simple or cheap solution for water scarcity.

💡 Exam Tip When describing NEWater, remember to mention it is 'treated used water' that undergoes 'advanced purification' to make it 'ultra-clean and safe'. Do not just say 'recycled water'.

🧠 Memory Aid Remember the Four National Taps as L.I.N.D. (Local, Imported, NEWater, Desalination).

📍 Singapore Focus NEWater is a key part of Singapore's water sustainability. It is so pure that it is used in high-tech industries. While it is safe to drink, it is mostly used for non-potable purposes and to top up reservoirs, showing how Singapore maximises every drop of water.

Conserving Our Water

Water conservation means using water wisely and reducing waste. Sustainable water management involves planning and managing water resources to meet current needs without compromising the ability of future generations to meet their own needs.

Given that fresh water is a limited resource, water conservation is extremely important for everyone.
Every drop of water saved contributes to a more secure water future.
Practices at home include taking shorter showers, turning off the tap while brushing teeth or washing dishes, and using water-efficient appliances (e.g., washing machines, toilets).
In schools, students can report leaky taps, use water sparingly during activities, and learn about water-saving habits.
Industries can implement water recycling systems, use water-efficient processes, and monitor their water consumption closely.
Sustainable water management also involves protecting water sources from pollution and managing land use to support the water cycle.

Water conservation is a shared responsibility. By adopting water-saving habits and supporting sustainable practices, we can help ensure there is enough water for everyone, now and in the future.

⚠️ Common Mistake Some students think water conservation only means saving hot water. — Correct: Water conservation means saving any type of water – hot or cold – because all fresh water is a limited resource and requires energy and resources to treat and deliver.

💡 Exam Tip When asked to suggest water conservation practices, provide specific actions (e.g., 'take shorter showers', 'turn off tap while brushing teeth'), not just general statements like 'save water'.

📍 Singapore Focus Singapore actively promotes water conservation through campaigns and water-efficient labelling for appliances. In homes, using a thimble-sized cup for rinsing after brushing teeth can save a surprising amount of water over time. These small actions collectively make a big difference for our national water supply.

🪤 Watch for these traps

The wrong answers examiners plant to catch you — and how to dodge them.

Tempting answerDesalination is the best solution for all countries facing water scarcity because oceans are vast.

Looks right because It seems logical that if there's a lot of ocean water, converting it to fresh water would solve everything.

But actually Desalination is very energy-intensive and expensive. It is not a practical or affordable solution for all countries, especially those with limited resources.

✓ How to avoid it Remember that 'best' or 'easiest' solutions often have hidden costs or challenges. Consider the practical limitations like energy and cost.

Tempting answerBuilding more reservoirs is the only way Singapore can increase its local water supply.

Looks right because Reservoirs collect rainwater, so more reservoirs mean more water.

But actually While reservoirs are important, Singapore also maximises local catchment by improving drainage systems and has limited land for new large reservoirs. NEWater and desalination are also crucial for increasing supply.

✓ How to avoid it Think broadly about Singapore's multi-pronged approach (Four National Taps) rather than focusing on just one method.

🔍 Question Decoder

What exam phrasings are really asking for.

When the question says…	It is testing…	Your move
Explain why fresh water is a limited resource.	Understanding of water distribution on Earth.	State that most water is salt water and much of the fresh water is inaccessible (glaciers/ice caps).
Describe how urbanisation affects the water cycle.	Knowledge of human impact on water cycle.	Explain how concrete surfaces increase surface runoff and reduce water soaking into the ground.
Name and explain two of Singapore's Four National Taps.	Recall and description of Singapore's water management strategies.	List two taps (e.g., NEWater, Desalination) and briefly describe what each involves.
Suggest ways to conserve water at home.	Application of water conservation practices.	Provide specific, actionable examples like 'take shorter showers' or 'turn off the tap while brushing teeth'.

🧭 Answer → Because → Link

How to answer: Open-ended questions requiring explanation or justification.

1Answer the question directly (make your claim). 1 mark
2Provide evidence or reasons from the context or your knowledge (start with 'because...'). 1 mark
3Link your reasons back to the scientific concept or principle using keywords. 1 mark

Example: Explain why deforestation can lead to less rainfall in an area.

✅ Earns the mark

“Deforestation can lead to less rainfall in an area. This is because when trees are cut down, there are fewer plants to carry out transpiration. Transpiration releases water vapour into the atmosphere, which is needed for cloud formation and rainfall. Therefore, less transpiration means less water vapour, leading to less rainfall.”

This answer directly addresses the question, provides a clear reason (fewer plants for transpiration), and links it to the scientific concept of water vapour's role in rainfall, using key terms.

❌ Loses the mark

“Cutting down trees makes it rain less. Trees make rain.”

This answer is too vague. It does not explain the scientific process (transpiration, water vapour) that links deforestation to reduced rainfall, missing key scientific terms and a clear explanation.

🗝️ Words that earn marks

limited resource Explaining why fresh water is precious or scarce.

salt water / glaciers and ice caps Explaining why most of Earth's water is not accessible fresh water.

transpiration / surface runoff Describing how deforestation or urbanisation affects the water cycle.

Four National Taps Referring to Singapore's overall water management strategy.

treated used water / advanced purification Describing NEWater production.

energy-intensive and expensive Explaining the challenges or drawbacks of desalination.

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Glossary

Key terms across this topic, in alphabetical order.

Boiling — The process where liquid water changes into invisible water vapour (a gas) throughout the liquid at a fixed temperature.

Boiling point — The fixed temperature at which a liquid changes to a gas throughout the liquid (e.g., 100°C for pure water).

Condensation — The process where invisible water vapour (a gas) loses heat and changes back into liquid water.

Deforestation — The clearing of forests for other land uses, such as agriculture or urban development.

Desalination — The process of removing salt from seawater to produce fresh water.

Dew — Liquid water droplets formed by condensation on cool surfaces, typically in the morning.

Evaporation — The process where liquid water changes into invisible water vapour (a gas) from the surface of the liquid.

Four National Taps — Singapore's diversified water supply strategy, comprising local catchment water, imported water, NEWater, and desalination.

Freezing — The process where a liquid changes to a solid by losing heat.

Fresh water — Water that contains very little dissolved salt, suitable for drinking and agriculture.

Gas — A state of matter that has no fixed shape or volume and spreads out to fill its container (e.g., water vapour).

Glaciers — Large, slow-moving masses of ice formed over many years from compacted snow.

Ice caps — Thick layers of ice covering large areas, especially near the Earth's poles.

Imported water — Water purchased from other countries, such as Malaysia, as part of water agreements.

Liquid — A state of matter that has a fixed volume but takes the shape of its container (e.g., water).

Local catchment water — Rainwater collected from local land areas and stored in reservoirs.

Melting — The process where a solid changes to a liquid by gaining heat.

Melting point — The fixed temperature at which a solid changes to a liquid (e.g., 0°C for pure ice).

NEWater — High-grade reclaimed water produced from treated used water through advanced purification technologies.

Precipitation — Any form of water (like rain, snow, or hail) that falls from the atmosphere to the Earth's surface.

Rate of evaporation — How fast liquid water changes into water vapour.

Reverse osmosis — An advanced membrane technology used in NEWater production and desalination to remove impurities and salts from water.

Salt water — Water that contains a high concentration of dissolved salts, found in oceans and seas.

Solid — A state of matter that has a fixed shape and volume (e.g., ice).

Surface runoff — Water that flows over the land surface rather than soaking into the ground.

Sustainable water management — Planning and managing water resources to meet current needs without harming the ability of future generations to meet their own needs.

Transpiration — The process by which plants release water vapour into the atmosphere through their leaves.

Urbanisation — The process of building cities and towns, replacing natural landscapes with concrete and buildings.

Water conservation — The careful use and protection of water resources to prevent waste and ensure availability.

Water cycle — The continuous movement of water on, above, and below the surface of the Earth.

Water vapour — The invisible gaseous state of water.