Q&A - PSLE Science

[atutor2001]

Verysuperkiasu:

Dear Atutor,


Some time ago, you mentioned volume of water decreases when temp decreases until about 4 deg, vol of water actually expands (even b4 solidifying).

How about melting process - where a bottle of ice is taken out of a freezer to melt. The question asked to observe
(a) the level of melted water
(b) the outer surface of the bottle. - (I think we can handle this part)

for (a) - melting process, ie, gaining heat - temp rises, does the volume of water increase or decrease? and why?

Thanking you in advance.

Hi Verysuperkiasu

My apology for bringing in the 4 degree to 0 degree part which can be confusing. My recommendation is to ignore it because the expansion of the volume from 4 degree to 0 degree is very very small, only 0.01%. However, when water changes from liquid at 0 degree to ice at 0 degree, the expansion of the volume is big, about 9%. Comparing the volume of ice to the volume of the liquid state at room temperature, the expansion is also about 8.9%.

Therefore, for primary level, we can just note that :
Comparison of volume of Liquid Water from 0 degree to Liquid Water at 27 degree - contracts by about 0.3%
Comparison of volume of Ice at 0 degree to Liquid Water at 0 degree - expands by about 9%
Comparison of volume of Ice at 0 degree to Liquid Water at 27 degree - expands by about 8.9%

So when ice melts, the water level in the bottle will decrease.

[Nebbermind]

ms_lim:

Dear atutor2001,

Can you elaborate ans (3) as the graph shows amount of undigested food vs type of organ? Do you think (2) is a better ans?

I thought it's (2) too coz shouldn't it be 'digested food' vs 'time' graph?

[Nebbermind]

cimman:

at point B would be the time that the door was opened as there is a drop in temperature. At point C, would be the time the door was closed, since there is a temperature increase now. The question would be: is baking considered started when the dough was first put into the oven or when the door was closed ?

By your explanation, I take it the baking only starts at C coz if door is not close, the temp will continue to drop and baking cannot occur.

[Alchemist]

ms_lim:

atutor2001:

[quote=\"ms_lim\"]Please help to explain Qn #25 from Pei Chun 2011 Prelim, Ans key is 2 :


http://i55.tinypic.com/ehu0x.jpg\">

If I follow the language used in the options very strictly, my answer will also be (2) because option (4) says that the candle is not strong enough - does not make sense. I am presuming that what they mean is that the \"flame of the candle\" is not strong enough.

The reason why (2) was not pick is because paper is a poor conductor of heat so it takes time to transfer heat to the water. Similarly, water is also a poor conductor of heat and will not remove heat from the paper very quickly. There is also no mention of the temperature of the water so I presume that it must be at room temperature. Therefore, heat transfer from the point on the paper (where the flame is applied) to the water will be very slow and the paper should get burnt. A more likely reason why it takes a while for it to get burnt should be that the flame of the candle is not strong enough.

(This question is quite similar to the one whereby a piece of paper is wrapped around a rod with one half made of wood and the other half made of iron. The paper wrapped around the wooden half will get burnt when a flame is brought to the centre of the rod before the iron part)

Dear atutor2001,

Thank your for explaining. Is the ans (2)? Personally I think this is a bad question. Please correct me if I am wrong.[/quote]
I remember my son struggling with this as well. He answered (3) - he said that since air is a poor conductor of heat - it will not conduct the heat away to the paper as quickly. Which would lead to the paper cup not catching fire immediately. - was not sure how to argue against that argument either.

[Alchemist]

For the below OE - for part 1 - my son's answer reads \"the higher the temperature the more air bubbles will appear in the beaker and vice versa\". However I feel this is an observation - not hypothesis. The answer key says the hypothesis is that \"the higher the temperature the higher the rate of photosynthesis.' But my son insists they were told to use the variables provided to them to give the answers.


Would appreciate views on this. http://i53.tinypic.com/auyvd.jpg\">

[Verysuperkiasu]

atutor2001:

Therefore, for primary level, we can just note that :
Comparison of volume of Liquid Water from 0 degree to Liquid Water at 27 degree - contracts by about 0.3%
Comparison of volume of Ice at 0 degree to Liquid Water at 0 degree - expands by about 9%
Comparison of volume of Ice at 0 degree to Liquid Water at 27 degree - expands by about 8.9%

So when ice melts, the water level in the bottle will decrease.

Thank u for clarifying!

[atutor2001]

ms_lim:

atutor2001:

[quote=\"ms_lim\"]Please help to explain Qn #25 from Pei Chun 2011 Prelim, Ans key is 2 :


http://i55.tinypic.com/ehu0x.jpg\">

If I follow the language used in the options very strictly, my answer will also be (2) because option (4) says that the candle is not strong enough - does not make sense. I am presuming that what they mean is that the \"flame of the candle\" is not strong enough.

The reason why (2) was not pick is because paper is a poor conductor of heat so it takes time to transfer heat to the water. Similarly, water is also a poor conductor of heat and will not remove heat from the paper very quickly. There is also no mention of the temperature of the water so I presume that it must be at room temperature. Therefore, heat transfer from the point on the paper (where the flame is applied) to the water will be very slow and the paper should get burnt. A more likely reason why it takes a while for it to get burnt should be that the flame of the candle is not strong enough.

(This question is quite similar to the one whereby a piece of paper is wrapped around a rod with one half made of wood and the other half made of iron. The paper wrapped around the wooden half will get burnt when a flame is brought to the centre of the rod before the iron part)

Dear atutor2001,

Thank your for explaining. Is the ans (2)? Personally I think this is a bad question. Please correct me if I am wrong.[/quote]Ya I also think it is a bad question. I will pick (4) as my answer (assuming that they mean to say that the flame is not strong enough) because the water being a poor conductor itself and the water is also at the same temperature as the paper, can offer little in helping to remove heat from the paper.

[atutor2001]

ms_lim:

atutor2001:

[quote=\"P5G\"]Please help.


http://i53.tinypic.com/syxqwy.jpg\">

TIA.

My answer is (3) because as time goes by, more food will be digested.

Dear atutor2001,

Can you elaborate ans (3) as the graph shows amount of undigested food vs type of organ? Do you think (2) is a better ans?[/quote]Sorry I meant to put (2) but got the number mixed up.

[atutor2001]

cimman:

atutor2001:

[quote=\"P5G\"]Please help.


http://i54.tinypic.com/e3ji0.gif\">

TIA.

My answer is (3) C where there is a slight decrease in the temperature caused by the opening of the oven door which the time actual baking started.

at point B would be the time that the door was opened as there is a drop in temperature. At point C, would be the time the door was closed, since there is a temperature increase now. The question would be: is baking considered started when the dough was first put into the oven or when the door was closed ?[/quote]Thank you for the correction. In my opinion I would take it that baking has started when the door is closed.

[atutor2001]

vidhyalog:

For the below OE - for part 1 - my son's answer reads \"the higher the temperature the more air bubbles will appear in the beaker and vice versa\". However I feel this is an observation - not hypothesis. The answer key says the hypothesis is that \"the higher the temperature the higher the rate of photosynthesis.' But my son insists they were told to use the variables provided to them to give the answers.


Would appreciate views on this. http://i53.tinypic.com/auyvd.jpg\">

In my opinion, your son's answer is ok because it is a hypothesis. To check if the proposed hypothesis is ok, I would ask myself whether if the given experimental set-up can be used to \"verify\" your son's statement. If yes, then it is a valid hypothesis.