Q&A - PSLE Science

[atutor2001]

P5G:

MGS Science P5 CA1 2009
Q6: When water is heated to 100 degree C, bubbles can be seen rising through the water. What do these bubbles contain?

(1) Hot water vapour
(2) Hot water droplets
(3) Air dissolved in the water
(4) Only oxygen is released from the hot water

Worksheet's answer is (3) and my answer is (1).
Should the bubbles be hot water vapour or air dissolved in the water?
How do I decide which should be the answer? What are the key factors?

For this question, are we referring to 'before boiling' or 'boiling'?

Discussion at Road-to-PSLE

Bubbles:
atutor has emphasised on ‘when’ versus ‘while’ and decided that for ‘when’, the answer is hot water vapour.
To add to the confusion, I would like to emphasise on ‘to’ (when water is heated to 100 deg C) versus ‘at’ (When water is heated at 100 deg C), so for ‘to’, my answer is air dissolved in the water.
If the question has been ‘at’, my answer will be hot water vapour.

To bubbles:
Heated to 100 degree C meaning water has reached boiling point which is 100 degree C. At about 80 degree, dissolved air has been expelled, so the answer is water vapour.

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Any further comments.

Hi P5G

Thanks for the follow up. It is becoming a question on English rather than science.

Based on my experience with cooking books, if the instruction is :

\"When the water is heated to 100 degree C, put the meat in.\"
I will put the meat into the water only when the water reaches 100 degree.

\"While the water is being heated to 100 degree C, put the meat in.\"
I will put the meat into the water progressively during the heating process and will stop when the water is 100 degree.

\"When the water is heated at 100 degree C, put the meat in.\"
I will measure the temperature of the flame. When the temperature of the flame is 100 degree, I will put in the meat regardless of the temperature of the water. (e.g. \"The cake should be baked/heated at 100 degree\" - we set the temperature of the oven at 100 degree)

Hope my analogy is right. So far my cooking is ok :lol:

[starlight1968sg]

I have a question to ask:

We have a cup of pure water and a cup of salt solution. During a hot day, which one will evaporate faster?

[atutor2001]

starlight1968sg:

I have a question to ask:

We have a cup of pure water and a cup of salt solution. During a hot day, which one will evaporate faster?

My guess is that pure water will evaporate faster.

I think the explanation is beyond the Pr sch syllabus.

[trytry]

starlight1968sg:

I have a question to ask:

We have a cup of pure water and a cup of salt solution. During a hot day, which one will evaporate faster?

Pure water will evaporate first.
Comparing pure water and salt water, salt water has a higher boling point, and therefore takes longer time to boil.

(not necessary to know)
This is caused by the solute (salt) in the salt water, higher level of energy is required to convert the solute component to gaseous state.

[starlight1968sg]

trytry:

Pure water will evaporate first.

Comparing pure water and salt water, salt water has a higher boling point, and therefore takes longer time to boil.
(not necessary to know)
This is caused by the solute (salt) in the salt water, higher level of energy is required to convert the solute component to gaseous state.

Thanks trytry.
With these 2 cups in the freezer, the pure water will freeze faster than the salt solution?

[CoffeeCat]

starlight1968sg:

trytry:

Pure water will evaporate first.

Comparing pure water and salt water, salt water has a higher boling point, and therefore takes longer time to boil.
(not necessary to know)
This is caused by the solute (salt) in the salt water, higher level of energy is required to convert the solute component to gaseous state.

Thanks trytry.
With these 2 cups in the freezer, the pure water will freeze faster than the salt solution?

nope the presence of salt will lower the freezing point & raise the boiling point. That's why not all sea water freeze at colder areas.

[starlight1968sg]

CoffeeCat:

nope the presence of salt will lower the freezing point & raise the boiling point. That's why not all sea water freeze at colder areas.

A bit confusing here; does the pure water take a shorter time to freeze than the salt solution since you mentioned that the freezing point of the salt solution is lower?

This translates to pure water freezes faster than the salt solution.

[CoffeeCat]

yea pure water takes a shorter time to freeze. assuming all else equal.

[James Ang]

atutor2001:

starlight1968sg:

I have a question to ask:

We have a cup of pure water and a cup of salt solution. During a hot day, which one will evaporate faster?

My guess is that pure water will evaporate faster.

I think the explanation is beyond the Pr sch syllabus.

I wonder if the question is a real question or hypothetical question.

Students at P5 are taught that the rate of evaporation is affected by 4 factors, namely (and summarised as W.H.A.T);

W - Wind
H - Humidity
A - Area of exposed surface
T - Temperature

So the mentioned variable such as type of water (whether pure or containing salt) is not part of the 4 factors listed.

In secondary syllabus, in addition to the 4 factors, the rate of evaporation is also affected by another factor - Pressure of atmosphere. Still the syllabus does not mention about the concentration of a solution although there is a fact that impurities like salt lowers the melting and increases the boiling point of a solution, but that should be mixed up with evaporation.

[atutor2001]

To explain why pure water will evaporate faster than salt solution, we need to understand "vapour pressure" which I can’t remember if it is covered in JC or secondary.