Q&A - PSLE Science

[ChiefKiasu]

tianzhu:

No. The water must come from the pao. Imagine this: heat an iron bar and place in under a hood like the paos. Do you think water droplets will form?

...Sometimes, being more adventurous in answering questions can be a bit risky.Should it be more cautious to adopt the safer approach by just saying warm water vapour condensed on the cooler surface to form water droplets.

tianzhu, I don't think it is about being more or less adventurous. It is a question of what is more right. The water condensing on the underside of the hood must come from somewhere external to the system. It is true that it came from water vapor, but what triggered the spontaneous condensation in that situation?

The air (or atmosphere) is like a piece of cloth. It absorbs water (evaporation), and if you squeeze it, the water escapes (condensation). The amount of water in the air is called humidity. The more water-logged the \"cloth\" is, higher the humidity, the harder it is to add more water (evaporation) external to it, and the easier is it to lose water (condensation) from it.

Condensation only happens on surfaces cooler than the temperature of the water vapor. If you introduce an object cooler than room temperature, water vapour will condense on the object. However, if you introduce a hot object into an environment, it will reduce the relative humidity in that environment, allowing the air to increase its rate of absorption of moisture, if any. If no additional moisture is present, and the object and its surroundings cool to room temperature, the humidity simply returns to its previous state without condensation.

That is why I say that it is critical to say that the moisture must come from the buns if you want a proper answer.

[tianzhu]

Maybe, we can apply some finishing touches to adapt our answers to different scenarios.


1)\tHot paos/buns — Hot water vapour from the paos/buns condensed on the cooler surface of the hood to form water droplets.

2)\tHot coffee — Hot water vapour from the coffee condensed on the cooler surface of the hood to form water droplets.

3)\tRice cooking — Steam produced during the cooking of rice condensed on the cooler surface of the lid of the rice cooker to form water droplets.

[tianzhu]

Water

Sally filled one foam cup with cold water. She then filled another foam cup with the same amount of very hot water. Immediately she placed both cups in the freezer. What Freezes First, Hot Water or Cold Water? Explain.

[tianzhu]

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[tianzhu]

Solar System and Simple Machines are not in the new syllabus for P5 2009.Something sweet to hear for those taking PSLE science in 2010.

[suiyuan]

When the water boils at 100 degree C, bubbles can be seen. What is inside these bubbles? (1) Air (2) Steam (3) Water (4) Water vapour

[csc]

When water boils, it is changed into steam. The answer is steam.

[Vanilla Cake]

csc:

When water boils, it is changed into steam. The answer is steam.

Hi csc,

Thks for yr reply. Pls kindly explain. Why is it not air ?

[csc]

Hi Vanilla Cake,


Upon heating a beaker of water, some bubbles will be seen escaping from the water - those bubbles contain AIR.

If we continue heating the beaker of water until it starts to boil - the water should be bubbling and larger bubbles are seen escaping from it. These bubbles now contain STEAM as water is changed into steam at boiling point.

This is what I gathered a few years back when my son asked the same question in Pr 4. Hope it helps.

[ApronMama]

csc:

Hi Vanilla Cake,


Upon heating a beaker of water, some bubbles will be seen escaping from the water - those bubbles contain AIR.

If we continue heating the beaker of water until it starts to boil - the water should be bubbling and larger bubbles are seen escaping from it. These bubbles now contain STEAM as water is changed into steam at boiling point.

This is what I gathered a few years back when my son asked the same question in Pr 4. Hope it helps.

Just to be sure : steam is not air at boiling point but water droplet (liquid)form when water vapour at boiling point condense due to heat lose when contacting the surrounding room temperature. (correct me if I am wrong)