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PSLE SCIENCE MIST QUESTION
January 5, 2019
DETAILED SOLUTION TO PSLE SCIENCE TYS QUESTION(question adapted due to copyright issue)
For some buildings, water mist systems are used to cool the surrounding air, and tiny water droplets are produced in the form of mist, as shown in the figure
Explain how such a system is able to lower the temperature of the surrounding air and give a reason for your answer. 
If you were to add fans to the water mist system, how would it help to cool the surrounding air more effectively?
How to tackle PART A?
First off, we identify the topic in question. Based on the keywords used which is mist, and temperature, the student has to identify that this is a multi-topic question, and topics involved are water cycle and heat, specifically, on the concepts of the 3 states of water and temperature.
Next, recall the information learnt from these topics. Water has three states, solid, liquid, gas, and different processes convert water into its individual forms.
So, the mist is used to lower the temperature of the surrounding air. The student has to be able to recall that water is a cooling substance, and its different forms can act to cool environments. Based on the question, the mist form seems gaseous at some point, so the student must recall that evaporation results in gas, and during evaporation, water absorbs heat and changes into water vapor.
Now, we piece the clues together and form our solution.
The mist system will release mist, which are tiny water droplets, into the surroundings. The water droplets absorb heat from the surroundings and they evaporate. This cools the surrounding air, and the temperature of the surrounding air is lowered.
How to tackle PART B?
Part B is what we like to unofficially call a creative question. These sort of open ended questions are follow-up questions to part A, meaning, if you have failed to understand part A, you will not be able to do part B, and lose 3 whole marks. These questions are typically designed such that the answers cannot be found in the textbook, and to make the student think deeply about the subject matter. So how do we go about doing this?
By looking at the figure, we can see that there is now a fan behind the mist system. So what is the fan doing? The student must be able to identify that fans generate wind, and wind typically blows things around. But this is a layman term, we have to explain it in scientific terms.
So, what does the act of wind blowing things around cause? It causes things to move further away. Based on part A, the student has to realize that the purpose of the mist system is to cool the surroundings. So what does blowing the mist further away achieve? At this point, the student has to come to the conclusion, that blowing it further away covers more ground, and cools more people! Now we piece the clues together and form our solution.
The fans blow the mist further away to other nearby places, and this cools the other places as well, because now the mist covers a larger surface area.
Stay tuned for more solutions to popular promo and TYS questions!
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