Q&A - PSLE Science

[Full.010894Cream]

amazing how many (professional) manhours have been spent on this milk qn which carries only 2 marks!!

[tianzhu]

atutor2001:

The problem with Q10 is that the \"end point\" which we need as reference to decide which \"milk\" have gained the most heat was not clearly stated.

Hi atutor2001

In answering this question, it‘s of relevance to remind ourselves of the limited knowledge that primary students have on the topic of heat.

For Q10, for a fair test to find out the heat transfer, I assume that a cup of milk at 27 degree Celsius is immersed in a container of water at 40 degree Celsius. The variable factor is the volume of milk .Using the concept of heat transfer as taught in P4, heat is transferred from the hot water to that portion of milk immersed in the hot water. For the same period of time, the heat gain is the same for both options. However, in option (1), for the portion of milk above the water, heat is lost to the surrounding air.

One may argue about the temperature of the surrounding, again, for a primary student, I think it’s reasonable for a student to think that such an experiment is carried out in a laboratory/classroom condition.

Best wishes

[Tang]

atutor2001:

tianzhu:

......For both options, heat is transferred from the hot water to the milk through the point of contact which is the portion of glass immersed under the water. However for option (1), heat is also lost from the portion of milk above the water level to the surrounding air.
Therefore, it’s logical to say that that he milk in option (3) gained the most heat.

Hi tianzhu

The problem with Q10 is that the \"end point\" which we need as reference to decide which \"milk\" have gained the most heat was not clearly stated.

However, we can set 2 reasonable end points :
1. At the point where the temp. of the milk and the hot water are equal
2. At the point where the temp. of the milk, water and surrounding air are equal.

End point 1
Since there is more milk, the final temperature of the milk of option (1), when the temp of milk = temp of water of option, will be lower than option (3). However, which \"milk\" will gain more heat cannot be determined because though the mass of milk in (1) is more, its temperature change is less than (3).

Since the final temperature of (1) is lower than (3), the heat lost by water in (1) is higher than (3). As the heat lost is gained by the water, so water in (1) gains more heat. Isn't it?

End point 2
When the final temp of milk = temp of water of = temp of air, option (1) will have gained more heat since it has greater mass than (3).

So I think (1) is the more appropriate answer.

Regards

[Tang]

tianzhu:

atutor2001:

The problem with Q10 is that the \"end point\" which we need as reference to decide which \"milk\" have gained the most heat was not clearly stated.

Hi atutor2001

In answering this question, it‘s of relevance to remind ourselves of the limited knowledge that primary students have on the topic of heat.

For Q10, for a fair test to find out the heat transfer, I assume that a cup of milk at 27 degree Celsius is immersed in a container of water at 40 degree Celsius. The variable factor is the volume of milk .Using the concept of heat transfer as taught in P4, heat is transferred from the hot water to that portion of milk immersed in the hot water. For the same period of time, the heat gain is the same for both options. However, in option (1), for the portion of milk above the water, heat is lost to the surrounding air.

One may argue about the temperature of the surrounding, again, for a primary student, I think it’s reasonable for a student to think that such an experiment is carried out in a laboratory/classroom condition.

Best wishes

The one with more milk gains more heat, so answer is (1). The reason given by the one who posted the question at cbox. Isn't that simple enough?

For the last sentence, I meant \"Isn't the reasoning given by the poster simple enough?\"

[tianzhu]

Hi


A forum is a place where members interact constructively for the benefits of the community.

Members who live happily together are like musical instruments playing in harmony. One may have different viewpoint but should know how to disagree politely.

Best wishes

[atutor2001]

tianzhu:

The variable factor is the volume of milk .Using the concept of heat transfer as taught in P4, heat is transferred from the hot water to that portion of milk immersed in the hot water. For the same period of time, the heat gain is the same for both options. However, in option (1), for the portion of milk above the water, heat is lost to the surrounding air.

Hi tianzhu

Thanks for sharing your view.

Since for the same period of time, the heat gain is the same for both options, I changed my focus to the path of heat flow from the hot water.

In option (1), above the immersed portion of milk is \"milk\"
In option (3), above the immersed portion of milk is \"air\"

In option (1) heat flow from hot water to the \"immersed portion\" to the layer of milk on top than to glass/air.
For option (3), eat low from hot water to the \"immersed portion\" to the surrounding air on top.

So from this perspective will it be correct to say that at any point in time from the commencement of the experiment, option (1) will retain more \"heat from the hot water\" than (3) since heat flow from hot water to external air needs to go through a longer path in (1) than (3).

Regards

[tianzhu]

Hi atutor2001


I am not sure if my interpretation of Q10 is correct or not but this is what I have in mind when I read the question.

Picture two cups of different volumes of milk at 27 degrees Celsius being immersed in a container of hot water at 40 degrees Celsius. Heat transfer takes place at the same time , but the resultant heat gain in option(1) will be lesser as some heat is lost to the surrounding air through the exposed portion of the milk.

Best wishes

http://farm3.static.flickr.com/2481/4080190900_3a4e92d1be.jpg\">

[atutor2001]

Hi tianzhu


Your illustration is so wonderful. It shows your interpretation very clearly and I think your interpretation will definitely be correct if the temp. difference between the surrounding air and the milk is big.

Thank you

Regards

[tianzhu]

glchua:

Not true. Water needs a lot of heat to increase 1 degree of temperature. Compared with iron, it is about 10 times more! Energy required to pull the orderly molecules from a solid state to a messy and energetic liquid state is very high too.

Hi

Thank you for your view. I am trying to look at this question from the concepts on heat learned in P4.

What would our primary kids think when they first read this question? Formula such as Q =mass x specific heat x temperature difference, specific heat capacities of water and iron and latent heat of fusion are not taught yet.

In Q26, one has to compare the heat gained by the water from the hot iron as compared to the heat gained by the melting cube of ice, bearing in mind that the beaker of water has greater mass than the cube of ice.
Primary students learn that heat flows from a higher region to a lower region. Hence, in applying this principle, a bigger temperature difference, greater heat gain for the water in the beaker(option 2).

This is how I would have answered this question. Of course my answer may not be right.

Best wishes

[tianzhu]

Hi


There are many questions from past year papers on measuring photosynthesis rates by counting the number of bubbles. This essay on floating leaf disk explores another way of measuring photosynthesis.

Happy reading

Best wishes.
http://www.elbiology.com/labtools/Leafdisk.html