Q&A - PSLE Science
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emerald:
Hi I'm a P6 student. What caused the motion could be because of the brownian motion. This is the random movements of tiny particles called atoms.Very impressed with the answers given here. Maybe someone could help ans this observation by my girl during mealtime :
Why does pepper(powdered) when added to a bowl of soy sauce float and 'move' on the surface before they finally 'settle down'?
:? What caused the 'move'?
From: PlasmaStorm
Fomerly: kohjl Jr
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PlasmaStorm:
Most past papers from Top school, put egg york as the answer and egg white as an option which was considered not correct.Hi I'm a P6 student and for question 1 I think it is option 1. My mother (kohjl) taught me that if you want to remove the oxygen from the water, I should boil it.
For question 2, I was not very sure so I researched and saw this:
The egg white serves to protect the egg yolk and provide additional nutrition for the growing embryo. It is composed of several proteins, primarily ovalbumin (54%) dissolved in water.
So I guess the answer is both the egg white and the egg yolk.
From: PlasamaStorm
(Formerly: kohjl Jr ) -
ChiefKiasu:
the answer is because there is such a thing latent heat of vapourisation which is the amount of energy required to be overcomed in order for water to change from its liquid state to gaseous state and vice versa. it is easier for water to be heated up from liquid to gas because you are heating it and heat is a form of energy. when water is heated to water vapour there is no external agent applied to the vapour to make it lose energy to become a liquid.Full.Cream:
My kids were asking me similar qn, ie, how come water is in gaseous state in the air when it's not 100degC. How to answer this?
To answer this question, ask another question:
\"What is the boiling point of water in space, where there is zero atmospheric pressure?\" -
sorry i just realised i quoted the wrong post. i am answering full cream’s post.
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endingteen:
the answer is because there is such a thing latent heat of vapourisation which is the amount of energy required to be overcomed in order for water to change from its liquid state to gaseous state and vice versa. it is easier for water to be heated up from liquid to gas because you are heating it and heat is a form of energy. when water is heated to water vapour there is no external agent applied to the vapour to make it lose energy to become a liquid.[/quote]Hi ChiefKiasu,ChiefKiasu:
[quote=\"Full.Cream\"]My kids were asking me similar qn, ie, how come water is in gaseous state in the air when it's not 100degC. How to answer this?
To answer this question, ask another question:
\"What is the boiling point of water in space, where there is zero atmospheric pressure?\"
You are going into advanced statistics of atoms and vapours and temperatres in space... Let's leave it to Earth!
Hi Full.Cream,
This is because water can evaporate at ANY temperature, not only boiling. The definition of boiling is \"the application of heat to change something from a liquid to a gas.\" The definition of evaporation is \"vapourization: the process of becoming a vapour\". This is why they say the temperature of water vapour ALWAYS varies. It never evaporates at a constant temperature... -
ChiefKiasu:
There are two answers I can think of:Full.Cream:
My kids were asking me similar qn, ie, how come water is in gaseous state in the air when it's not 100degC. How to answer this?
To answer this question, ask another question:
\"What is the boiling point of water in space, where there is zero atmospheric pressure?\"
1) The diff between liquid and gaseous state is that the liquid is tightly packed together by intermolecular forces but less so if gas. So the gas has no definite volume. If you leave a bowl of water on the table, the water will start to evaporate as it has heat. Based on what my eldest brother and his secondary textbook taught me, heat is an illusion. it is actually kinetic energy. Heat causes the molecules in the substance, in this case water, to vibrate. When the water molecules vibrates, the vibration causes intermolecular forces causes the water molecules to vibrate and destroy the intermolecular forces within it, causing it to turn into a gas.
2)If you put water into a vaccum (which is where nothing is, not even air), it will turn into gas. The atmosphereic pressure helps the intermolecular forces to hold the molecules together keeping it in the liquid stage. BUT when there is no atmosphereic pressure, the intermolecular forces are too weak to hold the water molecules together. thus it boils.
(My 2nd reason was derived from what ChiefKiasu said. Thanks!)
Hope this helps!
From: PlasmaStorm
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Hi
Some lighter moments after PSLE
Best wishes
http://www.youtube.com/user/hdcyt?blend=1&ob=4 -
Extracted postings from cbox.
http://www.wendykoh.com/08/primary5-chijca1-science.pdf
Q10. WS answer (3). My answer (1). I thought more milk will absorb more heat from the hot water at the same temperature.
For (2) and (4), the milk loses heat to the water. So not correct.
For (1) and (3), the initial temperatures of the milk and water are the same. As there is more milk in (1), the final temperature of the milk and water will be lower for (1). So the water loses more heat. Hence the milk gains more heat. (1) should be correct.
[For (1), as the milk gains heat from the water at the surface where they are in contact, the heat gained by the milk travels to the milk above the water level. Hence the milk for (1) is always at a lower temperature than (3). With a greater temperature difference, the heat gains by (1) will also be faster than (3) for the same given time. Hence (1) is correct.]
Q26. How could we determine which one has the greatest heat gain?
WS answer (2). Why not (1)?
(3) and (4) losed heat, hence definitely not correct.
With regard to (1) and (2), should temperature difference be the only factor to decide which gain the most heat.
Any comment on Q10 and please help on Q26. -
Hey Tang, that was a great analysis for Q10 and I agree with you that the fuller glass will gain more heat.
My only doubt is whether that was the intended qn. Or do they really mean \"greater rise in temp\"? This is P5 after all.
As for Q26....I give up!! :oops: -
Full.Cream:
Hey Tang, that was a great analysis for Q10 and I agree with you that the fuller glass will gain more heat.
My only doubt is whether that was the intended qn. Or do they really mean \"greater rise in temp\"? This is P5 after all.
As for Q26....I give up!! :oops:
Q26 seems to be unfair.
Eg. if the final temperature of the water in beaker A is 27 degree C and that in beaker B is 22 degree C, then the ice gains more heat than the water in beaker A. So temperature difference cannot be the only factor, the volume of water and mass / volume of metal block and ice will affect which gain more heat.
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