Please help.

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'?


Q21: Which of the following statements about yoghurt making are true? You may refer to the link for the picture given:
Link

(A): Bacteria are added
(B ) Fermentation of milk takes place
(C): Oxygen is needed
(D): Yeast cells are added

(1) A and B only
(2) B and D only
(3) A, B and C only
(4) B, C and D only

Worksheet's answer is (1) and my answer is (3).
Please advise if oxygen is needed ?

TIA.

Hi there,

Both answers provided are correct.

Question 1)
For the first, we must realise that water is not just H2O but also contains gasses such as air that have been dissolved in water at lower temperatures (they are soluble in water). When the water is heated, you will notice bubbles starting to form at the point where the water is being heated even before 100 degrees C. Normally this is at the bottom of the jar / container. As this happens the dissolved Air will become less soluble and since gasses are less dense than liquid they rise up through the water.

Note: You will notice that these bubbles start to appear before the water is actually boiling. They could in fact, start to appear around 80 degrees C. Once the water temperature reaches 100 degrees C it converts to a gaseous state - water vapour.

So what is water vapour? Water vapor is water in its gaseous state-instead of liquid or solid (ice).

Question 2)
Oxygen is NOT required to make Yoghurt. Thus the answer is 1).

The process of creating Yoghurt is called an "Anaerobic Process" which means it does not require air to occur. As an example, you may purchase "Yoghurt-Making" machines that create an air tight seal which do not allow oxygen in.

Please help.

Q43
b) Name the two stages that the butterfly can be harmful to plants.
c) Describe how this is so.

Worksheet's answer
b) Larva and Adult
c) They feed actively on the leaves leaving the plant with few or no leaves to make food for itself.

Is it true that adult butterfly feeds actively on leaves? I thought butterfly feeds on sap and nectar. Any comments?

TIA.

hi, this is what I found from the internet:

Feeding

The larval or caterpillar stage and the adult butterfly have very different food preferences, largely due to the differences in their mouth parts. Both types of foods must be available in order for the butterfly to complete its life cycle.

Caterpillars are very particular about what they eat, which is why the female butterfly lays her eggs only on certain plants. She instinctively knows what plants will serve as suitable food for the hungry caterpillars that hatch from her eggs. Caterpillars don't move much and may spend their entire lives on the same plant or even the same leaf! Their primary goal is to eat as much as they can so that they become large enough to pupate. Caterpillars have chewing mouth parts, called mandibles, which enable them to eat leaves and other plant parts. Some caterpillars are considered pests because of the damage they do to crops. Caterpillars do not need to drink additional water because they get all they need from the plants they eat.

Adult butterflies are also selective about what they eat. Unlike caterpillars, butterflies can roam about and look for suitable food over a much broader territory. In most cases, adult butterflies are able to feed only on various liquids. They drink through a tube-like tongue called a proboscis. It uncoils to sip liquid food, and then coils up again into a spiral when the butterfly is not feeding. Most butterflies prefer flower nectar, but others may feed on the liquids found in rotting fruit, in ooze from trees, and in animal dung. Butterflies prefer to feed in sunny areas protected from wind.

I don't think butterflies feed on leaves..

Please help.

If adult stage is not harmful to plant, which stage is besides larval stage?

TIA

from what I know, the larval stage which is harmful to the plants. This is the stage whereby it is a caterpilla and feed on leaves. As for pupa stage, it is actually in the process of transforming into a butterfly.

For 43a, I think the answer is only the larva stage and no other stages. The answer the adult stage is wrong.

Details of discussions on Bubbles

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'?

------------------------------------------------------------------------
Comments as given by Mr Chris Roger from Onsponge.com

Answer provided is correct.

We must realise that water is not just H2O but also contains gasses such as air that have been dissolved in water at lower temperatures (they are soluble in water). When the water is heated, you will notice bubbles starting to form at the point where the water is being heated even before 100 degrees C. Normally this is at the bottom of the jar / container. As this happens the dissolved Air will become less soluble and since gasses are less dense than liquid they rise up through the water.

Note: You will notice that these bubbles start to appear before the water is actually boiling. They could in fact, start to appear around 80 degrees C. Once the water temperature reaches 100 degrees C it converts to a gaseous state - water vapour.

So what is water vapour? Water vapour is water in its gaseous state-instead of liquid or solid (ice).

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Discussions at Kiasuparents.com

Mr Tianzhu,
Your answer is correct. It is water vapour. A similar type of question has been discussed earlier in this thread.


Mr James Ang,
I reasoned that the correct answer is (3) and not (1) for the below reasons;

1. Water vapour escape from the surface of the water rather than from the bottom of the container. (kinetic particle theory - where the active particles leave the surface of the liquid and become gas, leaving the remaining particles to have less average energy per particle)
2. Water contains dissolved air which is necessary for the survival of living things that live in water. Boiling will expel the dissolved air from the water. That's why water animals cannot live in boiled water that has cooled down as the water does not contain dissolved oxygen.

I am sure the above reasons are convincing and scientific for you.


Mr Tianzhu,
Let’s us revisit this question again.

My answer is same as you .It is water vapour. James has a different answer.
Have you checked with your science teacher? For the benefits of PSLE 2010 kiasu kids, please let us know the correct answer.


P5G
My personal notes after discussions with my Science teacher.

Before boiling -- Air bubbles (air dissolved in the water)

During boiling - Water bubbles (water in the gaseous state, that is water vapour)

When water is heated to 100 degree C -- it refers to before boiling. So Worksheet answer is correct.
(Time when water is below 100 degree C to 100 degree C)

Confusion - 100 degree C.

Hope that my personal notes are accurate and correct and are useful to all.


Mr Tianzhu,
You’ve brought up a very interesting point regarding the definition of when.

We are given the understanding that
When water is heated to 100 degree C -- it refers to before boiling.

This means water has not reached its boiling point of 100 degree C according to your discussion with your teacher.
Should we interpret it in this way? I am not so sure, maybe, some members who are more proficient in English may help to clarify?


P5G
The water is heated to 100 degree C, its boiling point.

According to my teacher to see water bubbles, we need to continue to supply heat. The heat will be used to change water from liquid to gaseous state. It also explains why the temperature remains at 100 degree during boiling.


Mr Tianzhu,
You’ve now clarified that water has reached its boiling point of 100 degree C .Based on one of your earlier statement,

During boiling - Water bubbles (water in the gaseous state, that is water vapour)

I am wondering why water vapour is not the answer.

This is an interesting article about misconceptions about boiling water.
www.csun.edu/~ms4288/SED%20690/Misconceptions.pdf


P5G
My Science teacher actually drew a Temperature - Time graph to explain.

The graph starts from Point A (Room temperature at Time 0) to point B (Boiling point at time 10 minute) (as an example). It then remains horizontal at 100 degree C till Point C for another 5 minute.

She explained that from

Point A to B, the bubbles in the water are Air bubbles.

Point B to C, the bubbles in the water are Water bubbles.

When water is heated to 100 degree -- refers to any Point between AB to the Point B -- so Air bubbles (answer as given in worksheet).


Additional information discussed.

From Point A to B
- Heat supplied raises the temperature of water.
- Evaporation takes place only at the surface of water, so bubbles seen in the water are not water bubbles but air bubbles.

From Point B to C
- Heat supplied does not raise the temperature of water.
- Heat is used to change water from liquid state to gaseous state, so bubbles seen in the water are water vapour.

Hope that it is now clearer.


Pecalis,
I thought water at 100 deg C is called steam? Water can be changed to water vapour at any temp during evaporation but at 100 Deg C, it is NOT water vapour but steam?


Mr Tianzhu,
When water is heated from A to B in 10 minutes, it reaches its boiling point of 100 degree C. From primary science, we learnt that water changes from liquid to gas (steam).The temperature of boiling water remains at 100 degree C until all water has turned into water vapour.

Your question on bubbles seen when boiling is not an easy one. If you do a google search, if will find many interesting articles discussing misconceptions on boiling water bubbles.


Ms atutor2001,
Maybe we can change the content of the sentence so that we can get a clearer understanding :

"When the liquid was heated to xx degree C, it turned red".

Compare the above sentence to this one :

"While the liquid was heated to xx degree C, it turned red".

To me, the use of the word "When" means that the "event" happens only at xx degree C. If we want to describe the process from 0 to xx degree C, the word "while" should be used. My opinion is that your answer (1) hot water vapour, should be the correct one.

By the way, all the dissolved air in the water can be expelled before the water reaches 100 degree C. Therefore, if we observe carefully the heating process, we will notice that the water actually stopped bubbling a few seconds before it reaches 100 degree C as all the dissolved air has already been expelled and the water "cannot boil yet".

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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.

------------------------------------------------------------------------
Any further comments.

Wow. That shows dedication !

IMO it is still 3, despite all the points of view primarily because: I interpret it as the time before boiling; the bubbles are rising through the water rather than from the surface as James as pointed out, and by definition water vapour is water in a gaseous form. From this we should be able to infer that something else is causing the bubbles.

The only additional advise I can offer, considering I am just a parent as it relates to science, is that it is only worth 2 marks Regardless, let me pass the question off to an "English Expert" for their opinion on the wording.

The response from the "English expert" ....

In summary, he wouldn't analyse the words in isolation because it detracts from the intent of the science being tested. He has broken down the question like this:
- the experiment (when water is heated to).
- the observation (bubbles seen rising)
- inferences/conclusions (the question asks for one possible inference)

On the specific words ...
- "when" is taken to refer to the experiment duration rather than a specific point in time
- "to" refers to the end-point, from presumably room temperature to 100 degrees C.
- "bubbles can be seen rising" refers to the observation during the [entire] experiment.
- using "at" instead of "to", would refer to having reached the 100 deg, but I don't see it of relevance here

His response, based on the question's intention is number 3.

Note: He also used to teach Sec 1 science many moons ago and this question was raised back then.

Question 1)
For the first, we must realise that water is not just H2O but also contains gasses such as air that have been dissolved in water at lower temperatures (they are soluble in water). When the water is heated, you will notice bubbles starting to form at the point where the water is being heated even before 100 degrees C. Normally this is at the bottom of the jar / container. As this happens the dissolved Air will become less soluble and since gasses are less dense than liquid they rise up through the water.

Note: You will notice that these bubbles start to appear before the water is actually boiling. They could in fact, start to appear around 80 degrees C. Once the water temperature reaches 100 degrees C it converts to a gaseous state - water vapour.

Hi Chris

It seems to me that your earlier explanation differs from your present one.

I’d think that it’s of relevance if the water reaches 100 degree C or not. If water had reached 100 degree C as interpreted by the English expert, then wouldn’t the answer be water vapour as explained in your earlier post.

This is not an easy question; otherwise there would not be so many articles on boiling water bubbles in cyberspace. But I must say I am very impressed by the diligence of P5G in her pursuit of the correct answer.

Best wishes