Recently, a parent requested for me to discuss the question below which requires students to apply Heat Energy concepts to the topic of Water Cycle.

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Questions that require students to apply concepts from multiple topics are increasingly common in primary school Science examination papers.

As such, your child needs to have a good grasp of the concepts from all topics and a strong analytical mind in order to tackle questions effectively.

Please read on to learn more about our thought process & answering techniques for the following question!

 

Question

 

What Your Child Needs To Know

Questions on Water Cycle typically involve the concepts learnt in Heat Energy, such as:

  • Gain heat from _________ / Lose heat to _________
  • Heat processes (Evaporation and Condensation)
  • Hot water concept and cold water concept

Although questions testing on the topic of Water Cycle usually only take up a small fraction of the marks in Open-Ended Section (2 to 4 marks), it is often tested in examinations among the different schools.

As such, it may be beneficial for students to be able to identify such questions and apply their relevant concepts to formulate a sound and comprehensive answer.

 

Thought Process

Let’s take a look at Part (a):

“He observed water droplets dripping from the plastic sheet into the bottle. Explain how the water droplets were formed on the plastic sheet in the bottle.”

This is one of the nine common Water Cycle questions taught to our students.

Firstly, we have to consider the items mentioned in the question – ice cubes, plastic sheet and hot water.

The presence of the ice cubes causes the plastic sheet to become cooler than the surrounding air.

Since heat travels from a hotter to a cooler region, we would expect the warmer water vapour from the air in the bottle to lose heat to the cooler plastic sheet and condense to form water droplets.

Thus, the answer for Part (a) would be:

The plastic sheet lost heat to the cooler ice cube and decreased in temperature. The hot water in the bottle evaporated to form water vapour. The warmer water vapour then rose and came into contact with the cooler inner surface of the plastic sheet, lost heat to it and condensed to form water droplets.

 

Now, let’s have a look at Part (b):

“Roy repeated the experiment and replaced the hot water with cold water at 5˚C. This time, he did not observe any water droplets forming inside the bottle. Give a reason for his observation.”

By replacing the hot water with cold water, the rate of evaporation of the water decreases.

Thus, the amount of water that will gain heat from the surrounding air to evaporate to form water vapour decreases.

Therefore, there would not be any water vapour that would lose heat to the plastic sheet to condense and form water droplets.

Hence, the answer to Part (b) is as follows:

At 5˚C, the rate of evaporation of water is very slow. Hence, little water gains heat from the surrounding air to evaporate to form water vapour. There would not be any water vapour that would lose heat to the cooler inner surface of the plastic sheet to condense to form tiny water droplets.

 

On a side note…

We would observe water droplets forming on the cooler outer surface of the bottle instead.

The warmer water vapour from the surrounding air that comes into contact with the cooler outer surface of the bottle would lose heat to it and condense to form water droplets.

This is because the WARMER bottle loses heat to the COOLER cold water to decrease in temperature.