Q&A - P3 Science

[ChiefKiasu]

tianzhu:

ET-PH:

Thanks Tianzhu and Muffins, I couldn't tell my dd the answer when I went through this paper with her previously. After looking at your replies (though not sure I am correct), I have explained to her that bottle X being in contact with the cold water first, contracts, pushing the ink towards Y. When the coldness propagates to the air in bottle X, the air in bottle X contracts and causes the ink to move back towards X.

Hi muffins

Have your teacher taught you how to explain why the ink drop starts to move slightly in one direction first before moving in the other way?
Have you started your Chemistry lessons?

Best wishes

Glass has one of the lowest coefficient of thermal expansion, which means that it hardly expands when heated. This is also the reason why glass bottles crack quickly when directly heated with an open flame.

It is safe to say that we should not consider the case when the glass contracts due to the cold water, because the rate of contraction of the air in the bottle will far exceed the loss of volume due to the glass.

[tianzhu]

Hi Chiefkiasu


If I can recall correctly, there was a question on two flasks with an ink droplets in a tube linking them in one of my boy’s ws in P4/P5.

Heat was applied to flask A, and the answer given was the ink droplet moves slightly to the left first before moving to the right towards flask B.I think the kids are taught this in schools, as muffins had already stated this(move slightly) in one of his earlier post. The reason is that the heat reaches the flask first.

Maybe as we continue our journey, we may be able to offer some reasonable explanation.

Best wishes

[tianzhu]

buds:

I foresee having the need to consult you every now and then

Hi buds

Well, by the time your ah girl reaches P6, much of PSLE science will be erased from my old memory.

I hope I can still be of any help to you.

Best wishes

[ChiefKiasu]

tianzhu:

Hi Chiefkiasu


If I can recall correctly, there was a question on two flasks with an ink droplets in a tube linking them in one of my boy’s ws in P4/P5.

Heat was applied to flask A, and the answer given was the ink droplet moves slightly to the left first before moving to the right towards flask B.I think the kids are taught this in schools, as muffins had already stated this(move slightly) in one of his earlier post. The reason is that the heat reaches the flask first.

Maybe as we continue our journey, we may be able to offer some reasonable explanation.

Best wishes

It's the same concept. Whether the ink drop moves one way before the other depends on the relative thermal expansion efficiency of the container and the gas, and also the conduction of heat of the material's container. If the material allows heat to be conducted through it before it expands, then the rate of expansion of the gas will negate any perceived effects of the expansion of the container. So if a lead container (which has one of the greatest coefficiency of thermal expansion) is used instead of the flask, then I will agree with the observation that the ink drop will move one way and then the other.

[tianzhu]

Hi Chiefkiasu


At this moment. I am unable to offer reasonable explanation on the slight movement of liquid drop based on knowledge acquired in primary science.

As stated earlier, the primary textbook did not offer much explanation. I also don’t know how much is expected out of kids in answering this question.

However, if you read some assessment books, the answer is the ink drop moves slightly to the left first as heat reaches the flask first.

Here are some replies from muffins’s earlier post.
[quote]hi tutormum, first, the cold water will make a little of the bottle contract, making the droplet move towards Y. Then,l after about 1 minute, the air inside X will contract, causing the droplet to move towards X.[/quote][quote]Actually, I have carried out this experiment and it is true that the ink droplet will move towards the right first, in this case.[/quote]Best wishes

[atutor2001]

ChiefKiasu:

..... If the material allows heat to be conducted through it before it expands, then the rate of expansion of the gas will negate any perceived effects of the expansion of the container. .........

Fully agreed with your analysis. However, all common materials do not behave this way. So all primary school answers for such questions (regardless it is heating or cooling) must state that the ink drop moves slightly in a certain direction before moving in the opposite direction.

[kitty2]

Please help with these questions,thanks


1) John had 2 similaar pieces of bread.he placed each of these pieces of bread in 2 identical jars,x and y.He added a few drops of water to the bread in jar x only.The jar x turned mouldy and jar y turned slightly mouldy.

a) Why did John make the jars airtight?
b) A 3rd piece of bread was heated in the oven.If this piece of bread was put in another airtight jar,what would John expect to see after 3 days?


2)Melissa wanted to find out the volume of the air in a container.She filled it to the brim with 250 cm cube of stones.

a) The most likely volume of the container is

1)less than 250 cm cube
2)is equal to 250 cm cube
3)more than 250 cm cube

[adhdadhd]

Try with best knowledge to answer,


1) John had 2 similar pieces of bread. He placed each of these pieces of bread in 2 identical jars, x and y. He added a few drops of water to the bread in jar x only. The jar x turned mouldy and jar y turned slightly mouldy.

a) Why did John make the jars airtight?
To make the experiment fair, all bread must be exposed to same amount of air.

b) A 3rd piece of bread was heated in the oven.If this piece of bread was put in another airtight jar,what would John expect to see after 3 days?
The bread in this jar will not turn mouldy.

[adhdadhd]

We have to put some common sense in this question,

it said "stones", and did not elaborate clearly… if you fill a container (with unknown shape given) with stones, these stones will not be able to fill the containter fully, so my best answer is 3.

2) Melissa wanted to find out the volume of the air in a container. She filled it to the brim with 250 cc of stones.

a) The most likely volume of the container is

1)less than 250 cc
2)is equal to 250 cc
3)more than 250 cc

[peggy]

My ans is also 3. There are air gaps in between the stones and air also takes up space. Hence, the vol shld be more than 250cc.