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# New GCSE style Question Watch

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1. Hi everyone!

I found out this new gases style question, and I'm quite confused
Here's the question

A student fills a test tube with 30g of water. He heats up the water so that it begins to boil and collects all of the water vapour produced via a tube into the bung of the test tube. After the beaker has cooled he finds that the mass of the water in the beaker is now 20g. State the mass of the water vapour the student collected. Explain your answer.

As I said I was quite confused about this. How can it end up being 20g when conservation of mass takes place.
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Also, I have my first physics exam on Tuesday
2. (Original post by bengaltiger_)
Hi everyone!

I found out this new gases style question, and I'm quite confused
Here's the question

A student fills a test tube with 30g of water. He heats up the water so that it begins to boil and collects all of the water vapour produced via a tube into the bung of the test tube. After the beaker has cooled he finds that the mass of the water in the beaker is now 20g. State the mass of the water vapour the student collected. Explain your answer.

As I said I was quite confused about this. How can it end up being 20g when conservation of mass takes place.
Spoiler:
Show
Also, I have my first physics exam on Tuesday
He collected 10 g of water vapour... 20 grams is left behind, so 10 grams must be water vapour?
3. (Original post by Boss_Rhythm)
He collected 10 g of water vapour... 20 grams is left behind, so 10 grams must be water vapour?
But why didnt the water vapour condense back to a liquid?
4. Not sure whether he's boiling in the test tube or boiling in the beaker tbh... is there a diagram?

anyway the mass of the water vapour + mass of the water left in the boiling vessel must equal the original mass of water, that's the key idea.
5. (Original post by bengaltiger_)
But why didnt the water vapour condense back to a liquid?
That wouldn't affect its mass? If you have 10g of water vapour, that'll condense into 10g of liquid.

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6. (Original post by kingaaran)
That wouldn't affect its mass? If you have 10g of water vapour, that'll condense into 10g of liquid.

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I get that, but what I'm trying to say is why didn't the water vapour condense?

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7. (Original post by bengaltiger_)
I get that, but what I'm trying to say is why didn't the water vapour condense?

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Well it was vapour when he was collecting it, it's probably condensed to the same mass of liquid by now.
8. (Original post by bengaltiger_)
I get that, but what I'm trying to say is why didn't the water vapour condense?

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As someone else said, if the question goes onto tell you that the water vapour condenses, they may as well detail how they assembled the apparatus, how they then took it apart, the energy given off by the change of state and I think you get the point; it's not relevant to the question they're asking.

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Updated: September 27, 2016
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