Q&A - PSLE Science

[lizawa]

tianzhu:

ChiefKiasu:

[quote=\"tianzhu\"]Evaporation and Condensation


Your loving mother put some hot Tiong Bahru Paos (buns) in a transparent plastic hood. After a few minutes , water droplets are observed to be formed on the inner surface of the hood. Is it right to say that hot water vapours from the paos (buns) condensed into water droplets when they touched the cooler interior surface.

Likewise for a cup of hot coffee, placed under a hood, is it right to say that hot water vapours from the coffee condensed to form water droplets. Similarly for rice cooking, it is more appropriate to use steam or hot water vapours when explaining about condensation at the inner lid of the rice cooker.

The heated water in the paos is transformed to the gaseous state as steam. When steam gets in contact with cool surfaces such as the plastic hood, it cools and transforms back into the liquid state. This forms the water droplets on the underside of the plastic hood covering the hot paos.

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.[/quote]When the buns are heated, they have a higher temperature than the surrounding air. The heat will travel from the hotter object (bun) to the colder surrounding air. (concept of heat conduction). This will cause the water vapor to gain heat. When the water vapour touches the underside of the plastic hood (which is cooler), it will lose its heat and condenses, hence forming water droplets.

[tianzhu]

Respiratory/circulatory systems


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.

What is the more appropriate answer?

[lizawa]

tianzhu:

Respiratory/circulatory systems


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.

What is the more appropriate answer?

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

NB : No mark is given for \"running\" if the explanation is incorrect. This is what I understand from the science teacher with regards to how strict the marking for the Science paper open ended question is.

BTW : Tianzhu, it will be good if you can also provide some \"model answers\" from the teacher for all these questions. Good reference for us. Thanks !

[tianzhu]

Hi lizawa


Questions concerning chestnuts and hair dryer are conceived through observation of some regular activities. As for the question on breathing/heart beats, there are varied explanations and I am still searching for the most suitable one.

I try to keep updated by reading some assessment books, so that I can help my son when he faces difficulties.

I put up this thread to create more awareness about Primary Science questions, and to serve as a platform for discussion on the ways to tackle them. Hopefully, through brainstorming and piggy-back on each other’s view, we are able to arrive at some fruitful answers.

[tianzhu]

Hi lizawa,


I am still trying hard to understand the relationships between circulatory and respiratory systems, and to put in the relevant information to secure precious marks.You are right to say that marking is stringent.

Two popular types of questions are why there are increase in breathing rates and heart beats during exercises.

Fortunately, there is still time, and hopefully we can have the most appropriate before PSLE Science exam.

[caroline3sg]

tianzhu

Personally I think assessement books are not enough. You need to get guide books. I usually get many types.

I am impressed with ChiefKS’s & Lizawa’s answers to the questions that you have put up. We may know the answer but to put them in proper and convincing way is not easy.

With regards to the zero mark for running …, this answer was given in P4 syllabus, while Lizawa’s answer is based on P5 syllabus, which I think MOE / Science syllabus should not be half here and half there.

[caroline3sg]

Regarding the mould on bread question, this was 2007 PSLE question. It appeared in ST Forum to MOE.

[lizawa]

caroline3sg:

tianzhu

Personally I think assessement books are not enough. You need to get guide books. I usually get many types.

I am impressed with ChiefKS's & Lizawa's answers to the questions that you have put up. We may know the answer but to put them in proper and convincing way is not easy.

With regards to the zero mark for running ......., this answer was given in P4 syllabus, while Lizawa's answer is based on P5 syllabus, which I think MOE / Science syllabus should not be half here and half there.

Hi Caroline,

I am also not 100% sure if my answers is 100% accurate. But we learn along the way, so that we can guide our children better.

Agree that guide books are good reference. I also like those assessment books with a summary of key points before each topic. And those with summary tables or mind maps are my favorites. Very useful for revision just before the exams.

[caroline3sg]

Lizawa

Yes I have bought those books since P3. In fact I use guide book to teach as text. Sch texts are too shallow.

[ChiefKiasu]

tianzhu:

...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.\"