Q&A - PSLE Science

[tianzhu]

No. The water must come from the pao. Imagine this: heat an iron bar and place in under a hood like the paos. Do you think water droplets will form?


Hi, you brought up an interesting point about the hot iron bar.

Students are required to answer in the context given in the question.To show their understanding, it's better to adapt the answers to varying circumstances.

Sometimes, being more adventurous in answering questions can be a bit risky.Should it be more cautious to adopt the safer approach by just saying warm water vapour condensed on the cooler surface to form water droplets.

[csc]

tianzhu:

In this question, there was a table showing the composition of three air samples. The volume of each sample is 100 cubic centimeter.


Part B of the question asked which activity used up more oxygen, sleeping or running. Explain your answer. (1 mark)

The following answer was given zero marks. ----- Running. When we are running, our lungs need to take in more oxygen so that the heart can pump more blood containing oxygen and digested food to the various parts of the body to produce the amount of energy needed.

Running. More oxygen is needed to break down the digested food to produce more energy for the activity

Hi, just to take our discussion a bit further.

The question hinges on the keypoint which is oxygen.The original answer had correctly pointed out our lungs need to take in more oxygen.However, it went offside when it went on to explain about the heart pumping more blood.Yes the heart is pumping hard, but the answer is not given in the context of the question.This is what I suspect.

I think it may not be quite appropriate to say that more oxygen is needed to break down the digested food as food is digested by the digestive system.

I think the answer requires the student to make a clear distinction between breathing and heart rates, that's why zero marks are given when they are offside.

Just wondering if the zero marks was awarded because there was no COMPARISON made between using the oxygen for sleeping and running. There may be a need to explain why less oxygen is needed for sleeping. :idea:

[caroline3sg]

While oxygen is requested to breakdown digested food to release energy, it is the Circulatory System of which Blood Vessels CARRYING the digested food (ie glucose) to supply energy to the legs. At the same time, there is build up of carbon dioxide during running, the body needs to exchange carbon dioxide with oxygen, again oxygen is carried by the blood vessels.


I believe csc is correct to compare why sleeping requires lesser oxygen than running. I remember my girl’s worksheets correction, has to compare. Not just this topic, but also others like test tubes or pots of plants

[caroline3sg]

Regarding water in the paos vs using the word "steam", I strongly think it is steam from the boiling water (that is used to heat up the paos). The more you dwell too much on the correct word usage, the more you get lost.


White mist coming out of boiling kettle is steam. It is water vapour in gaseous state. Boiling at 100 degrees is called steam. There are water vapour around in the air at all times, invisible.

[csc]

caroline3sg:

Regarding water in the paos vs using the word \"steam\", I strongly think it is stream from the boiling water (that is used to heat up the paos). The more you dwell too much on the correct word usage, the more you get lost.


White mist coming out of boiling kettle is steam. It is water vapour in gaseous state. Boiling at 100 degrees is called steam. There are water vapour around in the air at all times, invisible.

Caroline,

It's a misconception that the white mist coming out of boiling water is steam.

It is in fact steam which has condensed into tiny droplets of water on contact with the surrounding air when the steam leaves the kettle spout.

Mist or fog is made up of tiny water droplets.

I remember this very clearly cos my son got this question wrong in his P4 exam. He was required to label the \"thing\" coming out of the kettle and he wrote steam which was marked incorrect.

[caroline3sg]

csc

So the thing coming out from boiling kettle is tiny water droplets (in gaseous state).

[tianzhu]

1)In an experiment, some students covered a pot of plant with a plastic bag (excluding the pot).Water droplets were observed to form on the inner surface of the plastic bag. Why?


2) A) In another experiment, some seeds were buried under some soil (depth of soil is covering the seeds is shallow).Can the seeds germinate? What happens when they are buried too deeply? B) Some seeds were put into a beaker of pond water covered with a layer of oil. Can the seeds germinate?

[ChiefKiasu]

tianzhu:

No. The water must come from the pao. Imagine this: heat an iron bar and place in under a hood like the paos. Do you think water droplets will form?

...Sometimes, being more adventurous in answering questions can be a bit risky.Should it be more cautious to adopt the safer approach by just saying warm water vapour condensed on the cooler surface to form water droplets.

tianzhu, I don't think it is about being more or less adventurous. It is a question of what is more right. The water condensing on the underside of the hood must come from somewhere external to the system. It is true that it came from water vapor, but what triggered the spontaneous condensation in that situation?

The air (or atmosphere) is like a piece of cloth. It absorbs water (evaporation), and if you squeeze it, the water escapes (condensation). The amount of water in the air is called humidity. The more water-logged the \"cloth\" is, higher the humidity, the harder it is to add more water (evaporation) external to it, and the easier is it to lose water (condensation) from it.

Condensation only happens on surfaces cooler than the temperature of the water vapor. If you introduce an object cooler than room temperature, water vapour will condense on the object. However, if you introduce a hot object into an environment, it will reduce the relative humidity in that environment, allowing the air to increase its rate of absorption of moisture, if any. If no additional moisture is present, and the object and its surroundings cool to room temperature, the humidity simply returns to its previous state without condensation.

That is why I say that it is critical to say that the moisture must come from the buns if you want a proper answer.

[tianzhu]

Maybe, we can apply some finishing touches to adapt our answers to different scenarios.


1)\tHot paos/buns — Hot water vapour from the paos/buns condensed on the cooler surface of the hood to form water droplets.

2)\tHot coffee — Hot water vapour from the coffee condensed on the cooler surface of the hood to form water droplets.

3)\tRice cooking — Steam produced during the cooking of rice condensed on the cooler surface of the lid of the rice cooker to form water droplets.

[tianzhu]

Water

Sally filled one foam cup with cold water. She then filled another foam cup with the same amount of very hot water. Immediately she placed both cups in the freezer. What Freezes First, Hot Water or Cold Water? Explain.