Q&A - PSLE Science

[Dharma]

tianzhu:

Please click at link to see question Q37, ACS 2007 SA2.

http://www.wendykoh.com/07/primary6-acs-science.pdf

Can a freshly plucked leaf carry out life processes when immersed in hot water?

I don't think it able to for too long. Hot water contains low levels of dissoved oxygen and will be insufficient for respiration.

a) Bubbles are formed as exchange of gases takes place through the stomata of the leaves as the leaf photosynthesizes ...ASSUMING...there is light energy. The bubbles are oxygen that is produced during the process of photsynthesis.

b) There are more stomata on the underside of the leaf which leads to more number of bubbles seen there.

[tianzhu]

Dharma:

a) Bubbles are formed as exchange of gases takes place through the stomata of the leaves as the leaf photosynthesizes ...ASSUMING...there is light energy. The bubbles are oxygen that is produced during the process of photsynthesis.

Hi
Will hot water kill a leaf?

[Dharma]

tianzhu:

Dharma:

a) Bubbles are formed as exchange of gases takes place through the stomata of the leaves as the leaf photosynthesizes ...ASSUMING...there is light energy. The bubbles are oxygen that is produced during the process of photsynthesis.

Hi
Will hot water kill a leaf?

Tianzhu,

Boiling hot water will kill the leaf as the dissolved air in the water will be removed, preventing the leafs ability to make food.

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.

Back to your question on whether hot water will kill the leaf ...

Here, the question just says hot water but hot water will lose heat if left to cool. Furthermore, there is nothing to prevent the oxygen in the air to re-dissolve in the water to provide life to the leaf. Maybe, if a layer of oil was poured on the surface of the hot water, we can be more sure ... I guess it all depends how fast the dissolved air is removed when leaf is placed in the hot water as compared to the rate at which air gets re-dissolved in water again.

[corneyAmber]

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.

[tianzhu]

Thank you for your time.Another question about rides in theme park.

http://farm4.static.flickr.com/3409/3446771458_b9ae1ca727_o.jpg\">

[Dharma]

tianzhu:

Thank you for your time.Another question about rides in theme park.

http://farm4.static.flickr.com/3409/3446771458_b9ae1ca727_o.jpg\">

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

[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.