Q&A - P3 Science

[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.

[Full.010894Cream]

Fantastic explanation! Thanx, TZ & Tang!!

[Tang]

Thanks. Atutor2001 and uncle Tianzhu.

[Lynn2]

Hi atutor2001


Thanks for the details explanation but I do not know how to simplify it and relate it to a p4 kid. Can you break down as in what is the co relation between salt, melting point, freezing point, ice and water?

Honestly, I am abit lost too...thank you.

atutor2001:

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\".