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New GCSE style Question

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

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



He collected 10 g of water vapour... 20 grams is left behind, so 10 grams must be water vapour?
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?
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.
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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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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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.
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.
(edited 7 years ago)
Original post by Boss_Rhythm
He collected 10 g of water vapour... 20 grams is left behind, so 10 grams must be water vapour?


thank you i needed that too
Original post by Samir Mazumder
thank you i needed that too


There has to be 10 grams of water since the water being boiled, the pure water vapour travelled into the test tube. So technically 10 grams of pure water vapour was collected and the rest mightn't even be water. Or maybe only 10 grams were collected because the student didn't let the water in the beaker boil completely.

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