Water out, water back — answer key

1. XQ1 · Practice

Model answer. The double arrow ⇌ shows that the reaction can go both ways: hydrated copper(II) sulfate can lose its water, and the anhydrous compound can take water back. The other equations have one arrow, →: they go one way.

A full answer includes

• The copper(II) sulfate equation, written with ⇌.

Also accept

• hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

Watch for

• “An equation with water in it is reversible.” — The arrow tells you, not the substances: ⇌ means the reaction can go both ways.

2. XQ2 · Practice

Model answer. Heating drives the water of crystallisation out of the blue crystals: the forward reaction. The water leaves as steam and condenses to drops near the cool mouth of the tube, and white anhydrous copper(II) sulfate is left.

A full answer includes

• (a) White.
• (b) Water (drops of liquid).
• (c) Anhydrous copper(II) sulfate.

Also accept

• White; water; anhydrous copper(II) sulfate

Watch for

• “The crystals melt.” — They don't turn into a liquid: they lose their water and crumble to a white powder.

3. XQ3 · Practice

Options as numbered on the pupil sheet: 1 — pink; 2 — blue; 3 — white.

Model answer. Copper(II) sulfate: blue crystals when hydrated, white powder when anhydrous. Cobalt(II) chloride: pink when hydrated, blue when anhydrous. Blue appears twice, so check which compound you mean.

A full answer includes

• Hydrated copper(II) sulfate: blue; anhydrous copper(II) sulfate: white; hydrated cobalt(II) chloride: pink; anhydrous cobalt(II) chloride: blue.

Also accept

• Blue, white, pink, blue

Watch for

• “Both anhydrous compounds are white.” — Anhydrous copper(II) sulfate is white, but anhydrous cobalt(II) chloride is blue.

4. XQ4 · Practice

Model answer. (a) The white powder turns blue, and the basin gets warm: the reaction gives out energy. (b) Water joins the anhydrous copper(II) sulfate to make blue hydrated copper(II) sulfate: the reverse of the reaction that heating drives.

A full answer includes

• (a) The powder turns blue; the basin gets warm.
• (b) The reverse reaction: hydrated copper(II) sulfate is made.

Also accept

• Turns blue and gets warm; reverse; hydrated copper(II) sulfate

Watch for

• “Adding water dissolves the powder into a blue solution, so it's a physical change.” — A few drops of water join the powder's crystals and make blue hydrated crystals, a new substance, giving out energy.

5. XQ5 · Practice

Model answer. White anhydrous copper(II) sulfate has a different formula and colour from the blue hydrated crystals, and energy is taken in one way and given out the other. Both directions make a new substance, so both are chemical changes. Being able to go back makes the reaction reversible, not physical.

A full answer includes

• No: a new substance forms each way, so both directions are chemical changes.

Also accept

• No: a new substance forms each way

Watch for

• “Heating blue copper(II) sulfate is a physical change, because water turns it back.” — A new substance forms each way: white anhydrous copper(II) sulfate has a different formula and colour from the blue crystals, and energy is taken in or given out. Both directions are chemical changes.

6. XQ6 · Practice

Model answer. (a) Heating drives out the six water molecules of each formula unit: CoCl₂·6H₂O ⇌ CoCl₂ + 6H₂O. (b) The hydrated compound on the left is pink; the anhydrous compound on the right is blue.

A full answer includes

• (a) CoCl₂·6H₂O ⇌ CoCl₂ + 6H₂O.
• (b) Left: pink; right: blue.

Also accept

• CoCl₂·6H₂O ⇌ CoCl₂ + 6H₂O; pink ⇌ blue

Watch for

• “Hydrated cobalt(II) chloride is blue.” — It's the other way round: hydrated is pink, anhydrous is blue. Blue cobalt chloride paper turns pink with water.

7. XQ7 · Practice

Model answer. Use dry cobalt chloride paper, which is blue, and touch it with a drop of the liquid, holding it with tweezers. If the liquid contains water, the paper turns pink: the anhydrous cobalt(II) chloride takes up water and becomes hydrated.

A full answer includes

• Put a drop of the liquid on dry (blue) cobalt chloride paper.
• It turns from blue to pink if water is present.

Also accept

• Blue paper; turns pink

Watch for

• “The paper turns blue if there is water.” — Dry paper is already blue. Water turns it pink.

8. XT1 · Apply

Model answer. (a) Water from the damp air has reached the blue anhydrous cobalt(II) chloride and made the pink hydrated compound, so the sachet is full of water and no longer drying the shoes. (b) Heating drives the water out again, the forward reaction, and the crystals turn blue: the sachet can be used again.

A full answer includes

• (a) The crystals have taken up water: the cobalt(II) chloride is now hydrated (and the sachet can't take up more).
• (b) Heat it gently, for example in a warm oven, to drive the water out: it turns blue again.

Also accept

• Took up water, hydrated; heat to drive the water out

Watch for

• “The crystals have gone off and must be thrown away.” — The change is reversible: heating drives the water out, and they work again.