Q&A - P3 Science

[atutor2001]

Full.Cream:

when salt is added to the ice, the melting point of the ice is lowered, ie, it goes below 0 deg C.

That being said, I believe the ice/salt mixture will turn into water faster but I don;t think the temp will go down.

In order for the temp to go down, heat must be removed from the mixture. But from the pic, heat seems to be added to the ice since the water in the test tube gets colder.

Did I go wrong somewhere?

This is a question that is beyond primary level which I don't understand why schools have included in their paper - maybe trying to impress.

The \"layman\" explanation is as follows :

1. When salt is added, the Freezing point of those parts of the ice that are in contact with the salt will be lowered. So, those portions of the ice cannot remain as solid and \"must melt\".

2. In order for ice to change to liquid, heat must be gained (latent heat). Interestingly, those parts of the ice in contact with the salt, instead of gaining heat from external sources, somehow are able to draw on its \"own heat\" to change from solid to liquid phase.

3. Having used its own heat to melt and become salt solution, its temperature will decrease - becoming negative. Therefore, the melted solution will be less than zero degree (can be as low as -14 degree C if I remember correctly).

Sorry if I confuse you. But the explanation using science terms is even more confusing.

[Full.010894Cream]

Thanks! missed the latent heat part. Is this (latent heat) expected of P4?


if the temp of the ice goes below 0 deg C, can the water in the testtube freeze and its temp continue to drop below 0 deg C too?

[atutor2001]

Full.Cream:

Thanks!


if the temp of the ice goes below 0 deg C, can the water in the testtube freeze and its temp continue to drop below 0 deg C?

The thermometer is inserted in the centre of the ice in the test tube. So it would be quite impossible for the temperature of the centre of the ice to become less than zero degree based on the experimental set up. (exposed to surrounding air at room temperature)

However, if the test tube is placed in a freezer maintained at -20 degree C for sometime, than the temperature of the ice will finally be at -20 degree C.

The temperature of any object will lose or gain heat to/from its surrounding until the temperatures of the object and the surrounding are the same. When the temperatures are equal, heat will stop flowing.

[Tang]

Full.Cream:

when salt is added to the ice, the melting point of the ice is lowered, ie, it goes below 0 deg C.


That being said, I believe the ice/salt mixture will turn into water faster but I don;t think the temp will go down.

In order for the temp to go down, heat must be removed from the mixture. But from the pic, heat seems to be added to the ice since the water in the test tube gets colder.

Did I go wrong somewhere?

All I could remember from my P4 Science experiment is that the water in the test tube will freeze.

From internet search, there are 2 relevant articles.

Extract from first article http://www.saburchill.com/physics/mpi/mpi009.html:

The ice is still at 0°C above its new melting point. So it absorbs heat energy from the surroundings, as it melts, lowering the temperature until it freezes again. In this way the freezing point of ice can be lowered down to -20°C.

With my limited knowledge, I don't quite understand the unlined words - absorbs heat energy from the surroundings yet temperature drops.

Extract from the second article http://www.rohmhaas.com/assets/attachments/about_us/locations/springhouse/salt_ice.pdf

Most people think that salt heats the surface or raises the temperature of the ice, but the reverse is true. Salt actually mixes with water to lower the temperature at the surface of the ice and, at the same time, the salt lowers the point at which water will freeze – the freezing point. The effect of salt on ice is called “freezing point depression.”

It does not explain how the temperature drops. It mentions freezing point depression. Not sure what it is!

[Tang]

tianzhu:

Hi

The key point is A heating element is used to maintain the temperature of the steel sheet at 100 deg celsius.

Condensation takes place when steam touches a cooler surface. Therefore option (1) is correct.

Hi uncle Tianzhu,

Since the steel sheet was maintained at 100 degree C, why it expanded then?

[tianzhu]

Hi youngman


In P4 science, students learned that metal expands when heated. Imagine the piece of steel sheet being heated from room temperature to 100 degree Celsius, from the basic concept, expansion occurs.

The fact that the steel sheet is maintained at 100 degrees Celsius is to rule out condensation. It is to test students’ understanding on the concept of condensation.

[tianzhu]

Hi Tang


Do you have a sibling in P4?

I can’t find this paper in misskoh.com.

I think gullet is included.

[Tang]

tianzhu:

Hi Tang


Do you have a sibling in P4?

I can't find this paper in misskoh.com.

I think gullet is included.

My sister is in P4.


Managed to get this URL for earlier thread:

http://www.wendykoh.com/08/primary4-taonansa2-science.pdf


I remember that in P3, we were told that digestion only occurs in the mouth, stomach and small intestine.

So why do we need to include gullet?

Since we include Gullet, why don't we also include Large intestine (G) and Rectum (J)?

Thanks.

[atutor2001]

Tang:

All I could remember from my P4 Science experiment is that the water in the test tube will freeze.

From internet search, there are 2 relevant articles.

Extract from first article http://www.saburchill.com/physics/mpi/mpi009.html:

The ice is still at 0°C above its new melting point. So it absorbs heat energy from the surroundings, as it melts, lowering the temperature until it freezes again. In this way the freezing point of ice can be lowered down to -20°C.

With my limited knowledge, I don't quite understand the unlined words - absorbs heat energy from the surroundings yet temperature drops.

Extract from the second article http://www.rohmhaas.com/assets/attachments/about_us/locations/springhouse/salt_ice.pdf

Most people think that salt heats the surface or raises the temperature of the ice, but the reverse is true. Salt actually mixes with water to lower the temperature at the surface of the ice and, at the same time, the salt lowers the point at which water will freeze – the freezing point. The effect of salt on ice is called “freezing point depression.”

It does not explain how the temperature drops. It mentions freezing point depression. Not sure what it is!

Hi Tang, allow me to try and explain :

(1) Under normal circumstances, when ice and water are in contact, the molecules on the surface of the ice escape into the water (melting), and the molecules of water are captured on the surface of the ice (freezing).

(2) At 0°C, the rate of freezing is the same as the rate of melting. As a result, the amount of ice and the amount of water won't change on average. The ice and water are said to be in dynamic equilibrium with each other.

(3) When salt is added, it will disrupt the equilibrium. The salt will dissolve in the water but it cannot \"freeze\" onto the surface of the ice.

(4) Therefore, there are now fewer water molecules on the liquid side because the salt molecule will take up the place of some of the water. As a result, the number of water molecules freezing into ice decreases. However, the rate of melting from the ice surface remains unchanged. Therefore, the rate of melting from the ice will be faster then the rate of freezing back of water into ice.

(5) Before salt was added, the rates of melting and freezing are equal. As such, the heat needed for the ice to melt is provided by the heat released by the water when it freezes back into ice.

(6) Now, with the rate of freezing being lower than the rate of melting, the heat released from \"freezing\" is not enough to support the rate of melting. So, the heat for melting must come from the salt solution and the ice. This cause the temperature of the ice & salt solution to decrease.

(7) With heat being taken from the salt solution, more water molecules will have less energy and more water will freeze back into ice.

(8) The process continues until a new equilibrium is reached, with the temperature of the ice being \"depressed\".

[tianzhu]

Hi Tang


This question asked for the organs involved in the digestion of food.

You are correct to point out digestion only occurs in the mouth, stomach and small intestine. This is what students learned from primary science.
Think about this, without the gullet to bring the food from the mouth to the stomach, can our digestive system works? Digestion of food ends at the small intestine.

Although the large intestine and anus are parts of our digestive system, they are not involved in the digestive process, hence is beyond the scope of this question.

Hope this helps.