Q&A - PSLE Science

[ChiefKiasu]

...It may not be appropriate to use use steam in this instance.The buns may be hot, but it's not at boiling point.In this example, we are talking about hot buns placed in a hood to prevent contact with insects/flies...

Thanks tianzhu. You are right, I should not have used the word steam. There is indeed a big difference between steam and water vapour. In fact, steam is NOT white mist that comes out of the kettle spout when water boils at 100C. It is a completely invisible gas. The white mist is really made up of tiny droplets of liquid water.

The proper answer should be:
"The heat in the hot paos causes water in the paos to evaporate into the atmosphere as water vapour. This water vapour condenses into water droplets when it comes into contact with the cooler underside of the plastic hood."

[tianzhu]

The proper answer should be:
"The heat in the hot paos causes water in the paos to evaporate into the atmosphere as water vapour. This water vapour condenses into water droplets when it comes into contact with the cooler underside of the plastic hood."

This is the puzzling part, where does the water in the paos come from?
lizawa spoke about surrounding air absorbing heat from the hot paos, this may be more easier to comprehend.

As for hot coffee, we could say that water vapour from the coffee condensed to form water droplets.I think it's to reasonable to use steam as in the case of rice cooking.

[ChiefKiasu]

This is the puzzling part, where does the water in the paos come from?
lizawa spoke about surrounding air absorbing heat from the hot paos, this may be more easier to comprehend.

As for hot coffee, we could say that water vapour from the coffee condensed to form water droplets.I think it's to reasonable to use steam as in the case of rice cooking.

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?

However, the droplets that form on the outside the glass of a cold drink is water vapor from the air. That is because the temperature is so low that it precipitates the water vapor despite the Earth's atmospheric pressure that allows water to remain in a gaseous state under its boiling point of 100C.

[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.

[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]

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.

[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]

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).