Introduction
When I was a child, I would spend hours by the window during rainy days, just watching the rain fall from the clouds. Like you, I was curious to know where the rainwater came from. The good news is that this is what we will be tackling in this article!
We will be analysing a question on the topic of the Water Cycle from the 2012 First Toa Payoh School (FTPS) P6 SA2 Examination Paper and learning how rain is formed by looking at the water cycle demonstrated in a simple experiment.
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Let’s Take A Look At This Question
Source: First Toa Payoh School – 2012 P6 SA2 Examination Paper [Q41]
Thought Process
To compare the two diagrams, we need to understand what happens in the water cycle first.
Let’s talk about the natural water cycle first and take reference from water bodies.
Water bodies include rivers, seas, lakes, and oceans. What happens to the water in water bodies? The water in water bodies gains heat from the sun to evaporate, and then it forms water vapour.
For the water bodies to become water vapour, the heat process of evaporation takes place. After that, we can see that water vapour would form clouds.
☁️ What Are Clouds Made Of? ☁️
Clouds are made of tiny water droplets.
How does the water vapour become water droplets? There is a change in state. That means there is also a heat process involved. The warmer water vapour loses heat to the cooler surrounding air and condenses to form clouds.
As more condensation occurs, the clouds formed become bigger and heavier. Eventually, they fall to the ground as rain.
Rain is also liquid so there is no change in state. There is no heat process occurring.
When the rain falls back into the water bodies, the same thing occurs. No change in state and no heat process.
We’re going to compare this to the diagram of the setup. What do you think will happen to the hot water over time? The hot water will evaporate.
When the hot water evaporates to form water vapour, warmer water vapour will rise and come into contact with the plastic sheet, which is a cooler surface.
The warmer water vapour will then lose heat to the plastic sheet and condense to form water droplets. That’s what’s happening inside the set-up as well.
Let’s Analyse Part (A)
The first question is asking about clouds. Which part of the set-up will represent clouds in the actual water cycle? Clouds are made of tiny water droplets formed by condensation.
In the set-up above, we can see that condensation took place and it also forms water droplets. The water droplets formed in the set-up must represent the clouds in the natural water cycle.
What about the water from water bodies? They will undergo evaporation and form water vapour.
The same thing happens in the set-up. The hot water undergoes evaporation to form water vapour.
Suggested Answer For Part (A)
Clouds: water droplets
Water from water bodies: hot water
Let’s Analyse Part (B)
“Why did Jenny make use of ice cubes in the set-up?”
When we place ice cubes on the plastic sheet, the plastic sheet is going to become cooler.
What is the heat transfer involved here? Does the plastic sheet gain heat or lose heat to the ice? The plastic sheet loses heat to the ice cubes to become cooler.
How does having a cooler plastic sheet affect the rate of condensation? When you have a cooler plastic sheet, will the warmer water vapour lose heat to the plastic sheet faster or slower? It will lose heat to the plastic sheet faster.
Therefore, condensation takes place faster, and this results in more water droplets.
A cooler surface would mean that warmer water vapour will lose heat to the plastic sheet faster, and thus condense faster.
We will observe more water droplets in the end.
Suggested Answer For Part (B)
The plastic sheet loses heat to the ice cubes to decrease in temperature or become cooler. When the hot water evaporates to form water vapour, the warmer water vapour comes into contact with the cooler underside of the plastic sheet, loses heat faster to it, and condenses faster to form more water droplets.
Let’s Analyse Part (C)
Source: First Toa Payoh School – 2012 P6 SA2 Examination Paper [Q41]
When all the ice cubes have melted, the plastic sheet will become warmer because the plastic sheet will gain heat from the warmer surrounding air.
This is the opposite of what we discussed in part (b). If the plastic sheet becomes warmer, will the warmer water vapour lose heat to the plastic sheet faster or slower? This time, water vapour will lose heat to the plastic sheet at a slower rate.
Therefore, we will have a slower rate of condensation.
In this case, what can we say about the amount of water droplets formed? There will be fewer water droplets formed.
Suggested Answer For Part (C)
The amount of water droplets formed in the beaker would decrease.
Let’s Analyse Part (D)
For part (d), we need to explain our answer for (c). We’re going to follow the same thought process that we did earlier in part (b).
Suggested Answer For Part (C)
When all the ice cubes have melted, the plastic sheet no longer loses heat to it to stay cool. Instead, the plastic sheet gains heat from the warmer water vapour in the surrounding air to increase in temperature. Thus, the warmer water vapour that comes into contact with the cooler underside of the plastic sheet will lose heat slower and condense to form fewer water droplets.
Conclusion
I have illustrated how rain is formed using a simple experiment that you can try at home with the supervision of an adult. I hope that after reading this blog post, you have gained a better understanding of how rain is formed during the water cycle.
Check out our other articles on Water Cycle and continue to keep a lookout for more articles! 🙂
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