Q&A - PSLE Science

[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

[glchua]

PlasmaStorm:

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?\"

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 :D[/quote]Intermolecular forces are a constant for a set of molecules and a specific value of average intermolcular distance. It will not change with pressure.

[glchua]

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.

I agree with answer of (1) for Q10. But the question should indicate that the volume of water is more than that of the milk.

Someone mentioned about heat loss from the exposed milk. It will have minimum contribution due to the extra exposed surface area not significantly more than the surface of the milk.

My intuitive answer for Q26 is (2). Another poorly constructed question. I think atutor already mentioned the issues.

It is not a simple case of temperature difference. 3 important data are missing: specific heat of fusion for water, specific heat capacity of water and the specific heat capacity of the metal.

If you do the math, the contribution from the melting of ice needs 3 times more energy than to bring the melted ice (0oC) to 25oC. If the specific heat capacity of metals and water I pulled out from the web is not wrong, and I assume that the metal is iron, and both metal and ice have the same mass, and I further assume that the volume of water is far in excess to the volume of ice and metal, then the heat gain by the ice is actually more than the heat loss by the metal. A really terribly wrong question.

[glchua]

tianzhu:

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

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.

[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).

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:

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.

Best wishes

For (1), it is precisely because heat is also transferred to the portion of milk above the water level, so the temperature of the milk in (1) is always lower than that in (3). S the rate of heat transfer from water to milk in (1) is faster, hence the milk in (1) gains more heat than (3). The amount of heat lost from the milk to the surroundings may not be substantial since air is a poor conductor of heat and the exposed surface area of milk to air is less than the contact surface area of milk with water.