Q&A - PSLE Science
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ADoc:
Thank you for your clear explanation, ADoc!
Hi there! A quick explanation would be:
The white mist/smoke/fog is in fact condensed water vapour from the surrounding air.
Perhaps a useful way to make yout kid understand without having to explain sublimation process would be:
(1) dry ice is solidified carbon dioxide
(2) it is very cold
(3) when dry ice sublimes, the carbon dioxide vapour (gaseous) is also very cold
(4) the surrounding water vapour in the air (which is at a higher temperature) loses heat to the carbon dioxide vapour since heat flows from regions of higher temperature to regions of lower temperature
(5) water vapour cools and condenses to form the white mist/cloud that we see when dry ice sublimes
Of cos the mechanics is a little more complicated than above explanation, but these should be sufficient to \"cure\" our kids' curiosity and \"why why why\"...
hope this is useful. cheers!
Initially, the confusion my child has is this - If dry ice goes straight to gaseous (which should be invisible) then why we still see the mist which suggests liquid state. Correct me if I'm wrong - that means to say the mist (liquid) we see is from the condensed water vapour from the air and is not the gaseous state of dry ice. Am I right? -
Yup, you’ re right, I supposed!
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May I know what is transpiration? I had forgotten and tomorrow is my Science examination!
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Verysuperkiasu:
hi! That's right, just as we can't observe CO2 in the air mixture around us.
Thank you for your clear explanation, ADoc!
Initially, the confusion my child has is this - If dry ice goes straight to gaseous (which should be invisible) then why we still see the mist which suggests liquid state. Correct me if I'm wrong - that means to say the mist (liquid) we see is from the condensed water vapour from the air and is not the gaseous state of dry ice. Am I right?
Just to elaborate further. Some kids may have played with dry ice and they observe that \"white bubbles\" are formed when water is added to dry ice. And so wrongfully conclude that carbon dioxide is \"white\".
cheers!
Yes, the bubbles are the CO2 (gaseous) that is released. But the \"white outer surface\" is actually condensed water vapour. Think of it as an invisible ball with condensation on its surface. The ball itself is invisible or transparent but due to the condensation, it's as though the ball is \"white\". -
ADoc:
Thank you for clarifying our confusion....:)
hi! That's right, just as we can't observe CO2 in the air mixture around us.Verysuperkiasu:
Thank you for your clear explanation, ADoc!
Initially, the confusion my child has is this - If dry ice goes straight to gaseous (which should be invisible) then why we still see the mist which suggests liquid state. Correct me if I'm wrong - that means to say the mist (liquid) we see is from the condensed water vapour from the air and is not the gaseous state of dry ice. Am I right?
Just to elaborate further. Some kids may have played with dry ice and they observe that \"white bubbles\" are formed when water is added to dry ice. And so wrongfully conclude that carbon dioxide is \"white\".
cheers!
Yes, the bubbles are the CO2 (gaseous) that is released. But the \"white outer surface\" is actually condensed water vapour. Think of it as an invisible ball with condensation on its surface. The ball itself is invisible or transparent but due to the condensation, it's as though the ball is \"white\". -
ADoc:
while I readily agree that the white vapour in the air is from the water vapour, I'm still trying to figure out where does the water vapour come from if the dry ice is inside the water.
Yes, the bubbles are the CO2 (gaseous) that is released. But the \"white outer surface\" is actually condensed water vapour. Think of it as an invisible ball with condensation on its surface. The ball itself is invisible or transparent but due to the condensation, it's as though the ball is \"white\". -
![](\"http://i49.tinypic.com/2ur8rrq.jpg\")
http://i49.tinypic.com/2ur8rrq.jpg\"> Can I ask if anyone know the answer to this question. The answer is 4 but I think it's wrong.
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mom-of-2:
mom-of-2:
The answer is 1 -
Hi,
Can help with the following question:
In an Archery lesson, John was holding a bow and arrow and trying to hit the bull's eye. Name the force(s) that is/are acting on the moving arrow towards the bull's eye.
My DD's answer is gravitational force and air resistance.
Answer sheet is gravity and friction.
Can anyone cfm whether friction is correct and my DD's answer on air resistance is correct or wrong?
TIA -
fightingmom:
The problem is \"what is air resistance?\", i.e. is air resistance a type of force? I recalled forces being classified into push, pull, magnetic, elastic spring, gravitational.... but no air resistance force.Hi,
Can help with the following question:
In an Archery lesson, John was holding a bow and arrow and trying to hit the bull's eye. Name the force(s) that is/are acting on the moving arrow towards the bull's eye.
My DD's answer is gravitational force and air resistance.
Answer sheet is gravity and friction.
Can anyone cfm whether friction is correct and my DD's answer on air resistance is correct or wrong?
TIA
Therefore, probably the syllabus does not recognise air resistance as a type of force. The most kiasu answer will be.
\"Air resistance caused by frictional force.\" I remembered science is \"positive marking\" so as long as there is no major conceptual error, once keyword is there, mark has to be given.
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