Q&A - PSLE Science

[Dharma]

ks2me:

tianzhu:

Hi
Will hot water kill a leaf?

Sorry my simple brain tells me hot water(i assume boiling hot?) will cook the leaf right? :? cook = dead?

But I do appreciate everything has to be explained in very scientific terms in science, just wondering if children use their common sense first before going into the scientific explanation.

Dear ks2me

You have brought up a very good point. A primary school kid is expected to understand the question in simple and straightforward manner to get the correct answer.

You're correct to assume that hot water means boiling hot water and when you place a freshly cut leaf in hot water, air inside the leaf will be removed, noticed by the bubbles coming out of the stomata. As the dissolved air wil be removed by the hot water, the leaf will die as it cannot photosynthesize.

Personally, I feel that the question should have eliminated the possibility of air in the atmosphere to get re-dissolved in the hot water. Anyway, I guess that it is just a school paper, and maybe extra effort was not taken to ensure there was no ambiguity.

[tianzhu]

Dharma:

a) Point X

b) At Point V, kinetic energy will be greater than the gravitational potential energy while at Point Y the gravitational potential energy will be greater than the kinetic energy

Hi Dharma
I guess this question is from the topic P6, Energy. You may like to explain your answer for (a) and (b).Do you take the rotation of wheel into consideration? It’s obvious the the GPE at point Y is greater than point V, but I am unsure about the comparison of KE between point V and Y

[corneyAmber]

Dharma:

Dear ks2me

You have brought up a very good point. A primary school kid is expected to understand the question in simple and straightforward manner to get the correct answer.

You're correct to assume that hot water means boiling hot water and when you place a freshly cut leaf in hot water, air inside the leaf will be removed, noticed by the bubbles coming out of the stomata. As the dissolved air wil be removed by the hot water, the leaf will die as it cannot photosynthesize.

Personally, I feel that the question should have eliminated the possibility of air in the atmosphere to get re-dissolved in the hot water. Anyway, I guess that it is just a school paper, and maybe extra effort was not taken to ensure there was no ambiguity.

Thanks Dharma for the clear explanation and I agree that teachers should try harder at eliminating ambiguity for questions. I am sometimes concerned that our children buried their heads too deeply into a scientific explanation that they lose their simple senses about science which is lovely. That can make life very complicated when people ask simple questions to them. I hope my child will not answer me at great lengths next time when I ask her a simple question, \"Have you cooked the vegetables?\"

[Dharma]

tianzhu:

Dharma:

a) Point X

b) At Point V, kinetic energy will be greater than the gravitational potential energy while at Point Y the gravitational potential energy will be greater than the kinetic energy

Hi Dharma
I guess this question is from the topic P6, Energy. You may like to explain your answer for (a) and (b).Do you take the rotation of wheel into consideration? It’s obvious the the GPE at point Y is greater than point V, but I am unsure about the comparison of KE between point V and Y

Hi Tianzhu,

a) The highest most point X, has the greatest GPE, therefore KPE has to be lowest based on Law of Conservation of Energy. GPE is dependent on the height of the object above ground. The greater the height of an object above ground, the greater is its GPE.

b) I don't think you are asked to make a comparison of KE between points V and Y but I think that KE at point V to be higher than that at point Y. This is because, to move the carriage, it is given a huge push at point U giving it a high initial velocity; therefore high KE. This high KE will propel the carriage upwards along the track. As it goes upwards, its some KE will be used overcome the gravitational force while the remaining KE will be converted to heat and sound energy. When the carriage is at its highest point at X, its KE is very low, almost zero. As the carriage moves down the track, it will gain KE until it reaches point U again.

Therefore KE at point V will greater than at point Y.

[tianzhu]

Hi Dharma,

Let's have a summary of your thoughts.
[quote]Boiling hot water will kill the leaf as the dissolved air in the water will be removed, preventing the leafs ability to make food.[/quote]Hot water killed the leaf; therefore it cannot carry out life processes.
[quote]Personally, I feel that the question should have eliminated the possibility of air in the atmosphere to get re-dissolved in the hot water. Anyway, I guess that it is just a school paper, and maybe extra effort was not taken to ensure there was no ambiguity.[/quote]The presence of re-dissolved air is immaterial as the leaf had been killed. So the layer of oil is of little relevance.
[quote]Here, we have a leaf has been freshly cut, air (oxygen) that is still in the leaf from its process of photosynthesis will be forced out of the stomata as air bubbles into the hot water.[/quote]Now the gem where the answer lies. I think we should go along this line.

[tianzhu]

Hi Dharma

[quote]I don't think you are asked to make a comparison of KE between points V and Y[/quote]You are right.
We should only compare the GPE and PE at the localised points V and Y.
This is a question relating to the conservation of energy,where it changes from one form to another.

[tianzhu]

Thank you for your help.


Q31
http://www.wendykoh.com/08/primary5-rgssa2-science.pdf

At first look, I wanted to attempt this question using the concepts of different characteristics of material. However, the answer sheet suggested that this question is from the topic cells.It goes along the line that the plastic bag gives a shape to the paper bag.I am still trying to understand the solution.

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[tianzhu]

[quote]I am absolutely sorry for leading you on the wrong path ...my thoughts were somewhat distorted then !!!.[/quote]
Hi Dharma,
I appreciate your responses to my questions. Your answers came across as sincere, insightful and penetrating. One can feel the passion of a caring father who is concerned about the progress of his child. I know that much effort had been put in.
We may not arrive at the perfect answers, but through the exchanges of ideas and thoughts, one can pick up some useful tips. Even if we do make mistake, we make it with good intention.
Kids,particularly those without tuition, will definitely gain from your active participation. It’ll certainly inspire me to search for more questions.

[ksmama]

Hi, I think the paper bag question is both a test of materials and structure of a cell.

a) Set up A consists of a paper bag. Paper is thin and and not strong. It is thus unable to withstand the increase in pressure when air is pump into it and ends up bursting. On the other hand in set B, the paper bag is placed in a plastic bag. Plastic is a strong material and is able to support and protect the paper bag inside it. When air is pumped into this bag, the increased pressure inside the paper bag, the bag is cushioned and supported by the plastic bag outside it and therefore, it does not burst.

b) the animal cell has only a thin cell membrane as its covering. It is not strong and is not able to withstand the increase in pressure when water is pumped into it and ends up bursting. The plant cell, on the other hand, has an inner cell membrane, supported by an outer rigid cell wall. When water is pumped into the plant cell, the cell wall will protect and support the cell when its inner pressure increases and thus prevents it from bursting

[tianzhu]

Thank you for your help.

http://www.wendykoh.com/08/primary6-nanyangca1-science.pdf
The answer(ws) to the last part of the question is Potential energy --> Electrical Energy.Please help to explain why.
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