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chemistry igcse acapprinted 134 · PDF 134
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The answer is C, because heating the pink hydrated cobalt (II) chloride will cause it to revert to its previous blue
anhydrous form. A will not work because an acid cannot directly do so, B will not work because the water is
bonded to the cobalt (II) chloride molecules, so it is impossible to physically separate them, and D will not work
either for the same reasons.
2. Which statement about hydrated cobalt(II) chloride is correct?
A It turns blue when it is heated.
B It turns blue when water is added to it.
C It turns pink when water is added to it.
D It turns white when it is heated.
The correct answer is A, because heating hydrated cobalt(II) chloride would yield anhydrous cobalt(II) chloride,
which is blue. Adding water to it would have no effect, as it is already hydrated, so B and C are incorrect. Heating
it cannot turn it white, as that would be the case for copper(II) sulfate.
3 Which reaction is not a reversible reaction?
A combustion of alkanes
B hydration of anhydrous copper(II) sulfate
C melting lead(II) bromide
D thermal decomposition of hydrated cobalt(II) chloride
The correct answer is A, combustion of alkanes, because alkanes cannot be formed again by the combination of
their products, carbon dioxide and water. B and D cannot be the answers because the hydration and thermal
decomposition of both copper (II) sulfate and cobalt (II) chloride is a reversible reaction. C is also not the answer
because lead (II) bromide can be cooled to receive once more its solid state, making this too a reversible reaction.
4 The equation shows a reaction that is reversed by changing the conditions
.
forward reaction
CuSO4.5H2O → CuSO4 + 5H2O
How can the forward reaction be reversed? .
By
adding
water
By adding
heat
A
✓
✓
B
✓
✗
C
✗
✓