Q&A - PSLE Science

[PlasmaStorm]

ChiefKiasu:

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?\"

There are two answers I can think of:
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

[tianzhu]

Hi

Some lighter moments after PSLE
Best wishes

http://www.youtube.com/user/hdcyt?blend=1&ob=4

[Tang]

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.

[Full.010894Cream]

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:

[Tang]

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.

[atutor2001]

Hi Tang


I agree with your answer for Q10.

For Q26, I think the question cannot be answered. We need the following information :
1. mass of metal block & ice,
2. specific heat capacity of metal block,
3. latent heat of fusion for ice,
4. final temp. of both set up.

However, I think the intention of the question is just to test the concept that greater temperature difference means greater heat flow which is why their answer is (2).

[tutormum]

I also agree with Tang’s answer for Q10. Don’t forget answer sheet can be wrong so I always go for what I think it’s best. For question 26, I think it’s cos metal is a good conductor of heat and at a temperature of 90, it has more heat to lose (greater difference in temp between the metal block and water compared to difference in temp between the ice and water) so the water in beaker A will gain more heat at a quicker pace.

[tianzhu]

Hi


In P4 Science, students learn that heat flows from a higher region to a lower region until both of them have the same temperature. This process is called conduction.

For Q10, we need to focus only on option (1) and (3).
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.

As for Q26, based on what that has been learned in P4 science, heat flows from a hotter object to a colder object. A greater difference in temperature, therefore greater heat gain.

Best wishes

[glchua]

Muffins:

endingteen:

[quote=\"ChiefKiasu\"]
To answer this question, ask another question:
\"What is the boiling point of water in space, where there is zero atmospheric pressure?\"

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.

Hi ChiefKiasu,
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...[/quote]Water does evaporate at any temperature. Even some solids have a vapour pressure.

All atoms and molecules vibrate and hence have some form of kinetic energy (above the temperature of absolute zero).

The water molecules in liquid water are constantly bumping around. Some may get bumped so hard that they leave the surface of the liquid water and become vapour.

Under normal circumstances, the water molecule that had left might return and recondense after it loses its energy. That happens readily in a closed system.

By definition, boiling point of a liquid is the temperature at which the vapour pressure equals to external pressure.

Chief is being nasty here :lol: Your question can only be answered by considering the phase diagram of water as the pressure is already close to zero.

[Full.010894Cream]

tianzhu:

Hi


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.


Best wishes

Ah, but we have no info on what is the temp of the air. For all we know, the air temp may be greater than 40degC