Primary Science

Primary Science: Water Cycle

For Primary School students, learning about the water cycle can be a fascinating introduction to the natural world. The understanding of how water moves through the Earth’s system is not only essential for scientific knowledge but also for developing an appreciation for the environment.

In this article, we will delve deeper into the water cycle, focusing on the key stages that explain how water evaporates, condenses, and precipitates. So, join us on this journey as we provide everything that a primary school student needs to know about the water cycle. Let’s explore the fascinating world of water together!

Introducing: Water Cycle Process every Primary kid should know!

primary-science-water-cycle-process-1

The water cycle is so important because it is how water reaches plants, animals and us! It is believed that Earth’s water came from water-rich meteoroids that collided with the planet. The water cycle began approximately 3.8 billion years ago when rain fell and formed the oceans.

Presently, our planet is fortunate to have an abundance of water, with 71% of Earth’s surface covered in water. However, it is worth noting that not all of Earth’s water is drinkable.

The water cycle is powered by the sun, which provides the necessary heat for water to evaporate and rise into the atmosphere. The Earth’s gravity then ensures that the water does not escape the planet. This mechanism is a testament to the perfect balance of our ecosystem.

Recalling the primary four topics of Heat Energy and Matter would greatly aid in understanding the water cycle. These topics include conduction, convection, radiation, and states of matter. By understanding these concepts, one can appreciate the role that heat energy plays in the water cycle, as well as how matter changes states throughout the process.

Let’s recall about Heat Energy and Matter, shall we?

Firstly, matter can exist in three states: solid, liquid and gas. In the water cycle, it is important  for us to distinguish between the gaseous form of water, known as “water vapor,” and the liquid form of water, known as “water droplets.”

Secondly, heat is a form of energy that travels from a hotter region to a cooler region. This is important as we will need to identify what gains heat and from where, and what loses heat to something. When matter gains or loses heat, it changes state.

In the water cycle, our story begins with the sun, a giant ball of gas that provides energy to Earth. Water bodies like oceans, lakes and rivers will gain heat (absorb heat) from the sun. This causes water to evaporate, and transform into water vapour.

The change in state is from liquid to gas and it is an essential part of the water cycle. Understanding the concepts of heat energy and matter is crucial to comprehending the water cycle fully.

primary-science-harry-potter-pig-potter-pot--water-cycle

How can you remember the process in the water cycle?

A method to remember this sequence is “Harry Potter Potter” or “Harry Potter’s Pot/ Pig” etc.

Harry – Heat gain/ loss

Potter- Process

Potter- Product

Let’s see how it works for evaporation:

The water gained heat (H- heat gain) from the sun, evaporating (P- Process) into water vapour (P- Product).

Got it?

Let’s resume our story.

Warm water vapour rises, coming into contact with the cooler atmosphere, losing heat to the cooler atmosphere and condenses into water droplets. The change in state is from gas to liquid.

(See if you can spot Harry Potter Potter!)

(Heat- Heat loss, Process- Condenses, Product- Water droplets)

Some schools teach WCLCW for condensation.

W- Water vapour

C- Comes into contact with (the cooler atmosphere)

L- Losing heat

C- Condenses into

W- Water droplets

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Water droplets gather to form clouds.

Yes, clouds are actually liquid water droplets!

As more water droplets gather, the cloud gains mass and eventually, the water droplets fall as rain, also known as precipitation.

Snow or hail is also considered as precipitation.

In 2014, Singapore experienced hail! Leading to the creation of the iconic music remix (News report: Hailstorm in western Singapore – 25Jun2013) (Remix: Singapore Woman Hail Reaction REMIX)

As rain falls back onto Earth, the water cycle starts again.

Let’s put our knowledge of the water cycle to the test by applying it to commonly asked questions in Primary Science!

Q1) An ice cube was heated with a Bunsen burner for twenty minutes. The table below records the change in temperature of the ice cube.

Time Temperature (°C)
1 minute 0
3 minutes 0
5 minutes 1
7 minutes 12
9 minutes 30
11 minutes 50
17 minutes 100
20 minutes 100

a) From 1 minute to 3 minutes, was there heat gain? Explain your answer.

Ans: Yes. The ice cube is still gaining heat from the bunsen burner and will remain at 0°C until all ice has melted into water. The reason for this is that the melting point of ice is fixed at 0°C.

b) From 17 minutes to 20 minutes, was there heat gain? Explain your answer.

Ans: Yes. The water is still gaining heat from the bunsen burner and will remain at 100°C until all the water has evaporated into water vapour. The reason for this is that the boiling point of water is fixed at 100°C.

A common misconception students have is that a constant temperature means that there is no heat gain. This is incorrect. Heat energy is not the same as temperature! Temperature is a measurement of the degree of hotness / how hot or cold something is. But heat is a form of energy, measured in Joules (J).

primary-science-boiling-melting-graph

Q2) Set-ups P and Q were placed on a table. They contained equal amounts of water at 25°C and equal amounts of ice cubes. The beaker in set up P was wider.

(Adapted from PSLE 2020)

psle-2020-science-beaker-ice-cube-melting

a) After a while, more water droplets formed in set-up P. Explain why.

Ans: More water evaporated as the exposed surface area of the water in set up P was larger, thus gaining heat faster and water evaporates into water vapour faster. More water vapour comes into contact with the cooler underside of the lid, losing heat and condenses into more water droplets.

Take note that the comparison words “faster” and “more” are important!

There are three factors affecting the rate (speed) of evaporation, remember them using the acronymn WET.

  • Wind
  • Exposed surface area
  • Temperature

b) If we add blue food colouring to the water, will the water droplets be blue?

Ans: No. Only water evaporates, leaving the blue food colouring behind.

When we spill a milo drink on the floor, the water will evaporate and eventually leave a milo stain behind. If the milo powder was able to evaporate as well, we may have milo as rain!

Q3) Watch this video on how Robinson Crusoe made drinking water.Evaporation and Condensation- Robinson Crusoe Makes Drinking Water

Sally recreated the experiment at home. Her set up is as shown below.

a) Name two ways she can increase the amount of drinking water collected in the container.

Ans: Firstly, she can place a heat source like a bunsen burner below the beaker. Secondly, she can replace the stone with ice.

b) Explain your answer in part (a)

Ans: Placing a heat source below the beaker heats up the water faster. When water gains heat faster, it evaporates faster to become more water vapour, which will condense to become more water droplets to be collected in the container.

Replacing the stone with ice makes the plastic sheet lose heat to the ice, so the plastic sheet becomes colder. When the warm water vapour comes into contact with the colder plastic sheet, the water vapour loses heat faster and condenses faster, forming more water droplets to be collected in the container.

Q4) Fook Choy wore a face mask in an air-conditioned room. His spectacles became fogged when he breathed. (PSLE 2021 Q35)

Explain why his spectacles became fogged.

Ans: The spectacles are cold due to losing heat to the cooler surrounding air in the air-conditioned room. When he breathed out, warm water vapour from the exhaled air escaped the face mask, coming into contact with the cooler inner surface of his spectacles. The warm water vapour lost heat to the cooler spectacle lens, condensing into water droplets, fogging up his glasses.

Firstly, we have to explain why the glasses are cold. The key words are “lost heat”, and identify what is the cooler region- the cooler surrounding air in the air-conditioned room.

This is a concept under Heat, a P4 topic.

Secondly, students have to recall that when humans exhale, we give off water vapour. This is a concept under Air and the Respiratory system, a P5 topic.

We then explain what happens to the warm water vapour- that it escapes the face mask, and comes into contact with the cooler inner surface of the lens.

Lastly, explain how the invisible water vapour becomes visible water droplets on the glasses through condensation.

Hopefully this helps explain any misconceptions you may have about the topic of water cycles! It is a tedious topic, but remember the following acronyms: HPP (Heat, process and product), and WET (wind exposed surface area and temperature).

Read also: Real-Life Applications of Primary Math Every Kid Should Know!