CAIE A Level Chemistry 9701 · Group 17

Group 17: explain question

Explain4 marksStandard

A student added concentrated sulfuric acid to separate solid samples of sodium chloride, sodium bromide and sodium iodide. The results are shown.

Halide	Observations with conc. $\ce{H2SO4}$	Result of adding aqueous $\ce{AgNO3}$ to the solution after reaction	Result of adding aqueous $\ce{NH3}$ to the precipitate
$\ce{NaCl}$	Steamy fumes ( $\ce{HCl}$ )	White precipitate	Precipitate dissolves in dilute $\ce{NH3}$
$\ce{NaBr}$	Steamy fumes ( $\ce{HBr}$ ) and brown fumes ( $\ce{Br2}$ )	Cream precipitate	Precipitate dissolves in concentrated $\ce{NH3}$
$\ce{NaI}$	Steamy fumes ( $\ce{HI}$ ) and purple fumes ( $\ce{I2}$ )	Yellow precipitate	Precipitate does not dissolve in $\ce{NH3}$

Explain these observations. Include balanced chemical equations for the reactions with concentrated sulfuric acid.

How this question is marked

4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.

Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.

Group 17: explain question

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