Chemistry multiple choice question help! Watch

Little Ant
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In June 2010 (edexcel) chemistry unit 2 there was this question:

'Compound X is a white solid. On heating this compound, a colourless, acidic gas is
the only gaseous product.A flame test is carried out on the solid residue and a reddishflame is observed.

Compound X is
A calcium nitrate.
B calcium carbonate.
C magnesium carbonate.
D strontium nitrate.'

The answer is B but why? I don't understand why an acidic gas is produced?
Could anyone explain why? Thanks so much!
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ScienceFreak94
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In June 2010 (edexcel) chemistry unit 2 there was this question:

'Compound X is a white solid. On heating this compound, a colourless, acidic gas is
the only gaseous product.A flame test is carried out on the solid residue and a reddishflame is observed.

Compound X is
A calcium nitrate.
B calcium carbonate.
C magnesium carbonate.
D strontium nitrate.'

The answer is B but why? I don't understand why an acidic gas is produced?
Could anyone explain why? Thanks so much!
The gas produced is carbon dioxide, which is slightly acidic. The flame test results in a red flame because of the solid residue which is calcium oxide (contains a calcium cation)
The equation for this reaction is CaCO3 ---> CaO +CO2

It can't be any of the other choices because the nitrates don't decompose to give co2 and it can't be magnesium carbonate because magnesium cations do not give off a coloured flame in a flame test.

Hope this helps.
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Little Ant
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(Original post by ScienceFreak94)
The gas produced is carbon dioxide, which is slightly acidic. The flame test results in a red flame because of the solid residue which is calcium oxide (contains a calcium cation)
The equation for this reaction is CaCO3 ---> CaO +CO2

It can't be any of the other choices because the nitrates don't decompose to give co2 and it can't be magnesium carbonate because magnesium cations do not give off a coloured flame in a flame test.

Hope this helps.
Indeed it does! I didn't realise that CO2 was slightly acidic :P Thanks!
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Pilz
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Jesus christ, how is anyone meant to know that CO2 is acidic?
It's just these little things that really piss me off in edexcel chemistry exams.
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charco
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(Original post by Pilz)
Jesus christ, how is anyone meant to know that CO2 is acidic?
It's just these little things that really piss me off in edexcel chemistry exams.
It's a non-metal oxide - this means either acidic or neutral

But the only common neutral oxides are CO and N2O ...
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Saracen's Fez
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(Original post by Pilz)
Jesus christ, how is anyone meant to know that CO2 is acidic?
It's just these little things that really piss me off in edexcel chemistry exams.
For WJEC it's specifically part of the syllabus (though it's CH1). We had to cover carbonate-hydrogencarbonate equilibrium in seawater.
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